Overview

	References

	Appendix A

	Introduction

	Background

	Approach

	Objective

	Propagule Production

		Justification

		In-lake Production

		Planting the Containers

		Propagule Types

		Plant Growth Requirements

		Facilities for Off-site Production

		Culture Maintenance

Introduction

The following is a short introduction to the propagation and establishment of aquatic plants.

Background

Aquatic ecosystem restoration often involves the establishment or reestablishment of aquatic plant communities. Most reservoirs without aquatic plants suffer from poor water quality (high nutrients, poor visibility, etc.), weak fisheries (absence of cover and nursery habitats), and are susceptible to invasions by weedy exotic plant species. The role of plants in aquatic systems is significant. Aquatic plants provide valuable fish and wildlife habitat (Dibble et al. 1996), serve as a food source for waterfowl and aquatic mammals, improve water clarity and quality (James and Barko 1990), reduce rates of shoreline erosion and sediment resuspension (James and Barko 1995), and help prevent spread of nuisance exotic plants (Smart et al. 1994).

Three situations occur in large, multipurpose reservoirs that require restoration through establishment of native aquatic plants: an absence of vegetation, low species diversity, or infestation by nuisance exotic plants species such as hydrilla (Hydrilla verticillata). In the first two situations, restoration involves addition of desired species of aquatic plants, while in the latter we must additionally address control of the invasive exotic species.

Aquatic plant communities in natural lakes develop over periods of hundreds or thousands of years. The average age of 672 Corps of Engineers' reservoirs is 40 years, and over half of these reservoirs are younger than 37 years. In these manmade systems there has not been enough time for native aquatic plants to arrive and become established. Time is not the only limitation. The absence of propagule sources, harsh abiotic conditions, and biotic pressures all contribute to reducing the likelihood that aquatic plants will become established in a particular reservoir.

Reservoirs are often constructed in areas that lack natural lakes, and may be remote from populations of aquatic plants that could serve as sources of propagules. As a result, many reservoirs have no aquatic plant seed bank and receive only limited inputs of seed and other plant propagules. These reservoirs are often first colonized by nuisance exotic weeds, which are adapted for exploiting disturbed conditions (Smart and Doyle 1995). Once established, exotic weeds can prevent establishment of native plants, regardless of subsequent propagule availability.

Unfavorable abiotic conditions include excessive water level fluctuations, high turbidities, and shifting sediments. Small, young plants are especially vulnerable to changing water levels that may place them in water too deep or muddy to allow for adequate light penetration or so shallow as to expose them to turbulence or desiccation or cover them with sediments.

Biotic disturbances are caused by a number of aquatic and semi-aquatic organisms. Fish and other animals, which feed or "root" in sediments easily dislodge seedlings and other small, young plants. Also, herbivory by turtles, crayfish, insect larvae, muskrat, nutria, and beaver has been shown to be a significant factor affecting establishment and/or growth of submersed aquatic plant communities (Lodge 1991, Dick et al. 1995; Doyle and Smart 1995; Doyle et al. 1997). These animals are all highly mobile and many are widely distributed throughout river systems. Also, many of them are omnivorous, so their presence is not entirely dependent on the prior availability of plants. As a result of their mobility and widespread distribution, omnivores are generally present in sufficient numbers to prevent, or at least delay, natural establishment of aquatic vegetation. In some systems, grass carp have been used to control aquatic weed infestations, and their continued presence may prevent establishment of any aquatic plant species for many years (Smart et al. 1998).

Approach

To overcome these impediments to natural establishment, we have developed an approach for establishing " founder colonies". Founder colonies are small colonies of aquatic plants established in strategic locations within the reservoir. Once successfully established these colonies expand through vegetative and sexual reproduction into adjacent, unvegetated areas (Figure 1). These founder colonies then serve as propagule sources for natural colonization throughout the lake (Smart et al. 1996, 1998).

Figure 1

Objective

Successful establishment of founder colonies relies on planting of robust propagules (such as mature transplants) into protected sites (Smart et al. 1996, 1998). This handbook describes techniques for producing aquatic plant propagules and for planting these propagules in reservoirs.

This is a new technology and the state of the art is increasing at a relatively rapid rate. Many of the techniques we present here have not yet been rigorously tested in multiple systems. While we have used these methods with some success, they may not be universally applicable - there are large differences among plant species, among different regions of the country, and even among reservoirs. Annual variations in weather and hydrology are also likely to affect the outcome of any plant establishment efforts. The techniques we present here are certainly not the only methods that can be used for establishing aquatic vegetation; we are developing and evaluating many new techniques, at a number of reservoirs in several southern states. Likewise, we have thus far attempted to establish only a few species on a large scale or in multiple situations. Establishment of additional species is being evaluated. Some of these new techniques and new species will prove successful while others may not. Because the technology is in its infancy and is developing at a relatively rapid pace, this handbook is intended to be a preliminary technology transfer product. We intend to provide several extensive updates and revisions over the next few years as better information becomes available.

Propagule Production

Justification

Each restoration project will require many individuals of several aquatic plant species. Even on the scale of founder colony establishment, the number of plants required can be quite high. Because acquisition of large numbers of appropriate propagules in a timely manner can be difficult, we have begun developing methods for producing our own transplants and other propagules, tailored for each specific project.

Although commercial suppliers may, in some cases, be used to provide some of the plant materials needed for the restoration project, propagule production may be preferred for several reasons. Currently, only a limited selection of aquatic plant species (particularly submersed plant species) is readily available from commercial sources. Propagule types offered are also frequently unsuited to the demands of plant establishment in large water bodies. For the most part, stem fragments, seeds, root crowns, or dormant perennating organs (tubers, winterbuds) are sold commercially. These propagules are weak, and require near-ideal conditions for successful establishment. In the harsh environment of artificial reservoirs, most are destined to fail.

Additionally, such propagules are often only available at certain times of the year, very possibly at the wrong time of the year for a particular restoration project. One reason for this is geographic location of the commercial suppliers/collectors. As an example, northern suppliers generally must wait until spring thaws occur, which may be beyond the period for optimal establishment in southern reservoirs. In other cases propagules may be readily available in the spring, but hydrologic conditions (e.g. spring flooding) may dictate planting at a later date. Plant material, even dormant propagules, may not survive holding for extended periods.

Plant origin may be an issue as well. Although the same species may be found throughout the U.S., there may be considerable genetic variability among plants from different regions. These variations are likely due to differences in environmental (climatic or geological) conditions. A northern variety may not do well in southern climates. Finding source plants locally (or as locally as possible) is highly recommended.

This chapter is intended to be a guide for those who choose to produce their own propagules for lake restoration. Finding local plant stocks and cultivating desired species (and propagule types) may be the preferred technique for such projects. We cover general requirements and considerations for the culture of a variety of aquatic plants, including submersed, floating-leaved, and emergent growth forms. Specific information on several common North American species is given in Appendix A.

In-Lake Production

In-lake cultivation may be preferred for many projects, especially when available culture facilities are located some distance away from the project. Transportation of mature transplants over long distances can be logistically difficult and stressful to the plants. In these cases we recommend construction of plant production nurseries within the project water body.

A simple design that we use illustrates the basic components of an in-lake nursery (Figure 2). A large but movable container (such as a wading pool) for holding and stabilizing the pots, and a protective fence to prevent grazing (and other disturbances) is required. Pots are filled with lake sediments, planted with propagules (from local or other sources) and the plants are allowed to grow within the protection of the fencing. When plants are mature, they can be moved to designated sites and transplanted. The emptied pots can then be refilled with sediment substrate and a subsequent crop can be started to ensure a continued supply of mature transplants (or other propagules) throughout the growing season.

Figure 2

Planting the Containers

The general procedure for making potted aquatic plants (mature transplants) is as follows:


1. Fill pot to about 1/4 full (above the drainage perforations) with potting medium.
2. Add an appropriate dose of fertilizer.
3. Fill remaining 3/4 of the pot with medium.
4. Place pots in growing vessel (pond, tank, etc.)
5. Slowly fill growing vessel to 10 or 15 cm above the pot with clean water to allow the potting medium to become saturated. Apply pressure to consolidate the substrate, expressing any trapped air or water and eliminating voids. Add additional medium if necessary.
6. Allow filled pots to "cure" for one to several weeks - particularly if using a non-aquatic substrate. We generally observe an initial nutrient pulse as some nutrients and organics are released into the water. This is evidenced by brown staining of the water by humic materials or by the presence of an organic film on the water surface. Flush the water and refill several times if necessary.
7. Make an indentation in the center of the potting medium.
8. Plant the propagule, and "backfill" to ensure that the plant is anchored.
9. Fill the growing vessel to the desired cultivation depth with clean water.

Propaule Types

Many commercial suppliers sell aquatic plant propagules. We generally do not recommend using these propagules for establishing plant colonies in lakes and reservoirs, but they are often adequate as starter materials for plant propagule production in tanks or ponds. Availability is often seasonal, and locality concerns should be carefully weighed. If local or regional populations of a particular species are known, we recommend harvesting from these populations to obtain starter propagules.

Stem fragments, daughter plants, root crowns, tubers or winterbuds, even seeds (usually dependent upon species) may be used as starter materials for aquatic plant cultures. We suggest planting more pots than needed for a project if other projects are planned in the future. After a culture of a particular species is established, it can be used as a source for the next generation of cultivation. This prevents excessive damage that might be inflicted on donor populations if repetitive annual collections were required.

Stem fragmenters. Many aquatic plant species spread vegetatively from stem fragments. These species have apical meristems at the terminal ends of the shoots. Select stem tips and cut to a length of 15 to 20 cm. When selecting material remember that the greater the density of leaves along the stem, the better, as most nodes can produce roots as well as leaves and branches. Plant the apical cutting about 8 to 10 cm deep in the potting medium. For faster development, several cuttings could be planted per pot. Established plants readily regenerate new meristematic tissues after cutting, so once the culture is actively growing, additional cuttings can be taken to plant additional pots.

Rosette plants. Some aquatic plant species grow in a rosette form, reproducing by producing daughter plants along stolons. To propagate these species small plants should be clipped from the parent and planted directly into pots. These plants have a basal meristem and care must be taken not to cover this growing tip when planting. A relatively dense, firm substrate is important for these species because they are buoyant and, without sufficient anchoring, are easily dislodged from the potting medium. A layer of coarse sand or fine gravel can be placed over the substrate to help anchor the plants. Once pots are established, additional plantings can be made by removing daughter plants as they appear on stolons or by dividing potted plants.

Dormant perennating plants. Many aquatic plants perennate by producing tubers or winterbuds that survive winter or dry periods in a dormant state. The use of tubers or winterbuds is an excellent way to start a culture. Dormant propagules can be collected, held in a dormant state by refrigeration (for up to six weeks), and then planted when desired. Tubers are buoyant, and should be planted about 5 cm deep and covered completely with potting medium and/or coarse sand. Extra pots (or larger containers) can be prepared and the plants allowed to grow and complete their annual life cycle to produce tubers that can be harvested and used either for restoration projects or to produce subsequent crops.

Annual species. Almost all of the aquatic plant species that we might use for restoration projects produce viable seed. However, the ease of propagation of most aquatic plants by other means, and our rather limited knowledge of seed storage and germination requirements, limit the usefulness of seed as a starting material for producing plant propagules. We have used seed- or spore-laden sediments (obtained from drained pond cultures) to start plants of several annual species such as southern naiad (Najas guadalupensis), slender pondweed (Potamogeton pusillus), horned pondweed (Zannichellia palustris), or muskgrass (Chara spp.). Seed are also the propagule of choice for American lotus (Nelumbo lutea).

Appendix A lists the types of propagules recommended as starter materials for producing transplants of selected aquatic plant species.

Plant Growth Requirements

Considerations

The key to growing any plant is to provide conditions that allow the plant to fulfill its need for nutrients and sustain a rate of photosynthesis sufficient to provide for respiration and growth. All plants (particularly aquatic plants) have a basic need for water and an environment that provides appropriate temperatures. Beyond these basic requirements, photosynthesis depends on adequate levels of light and a continual supply of inorganic carbon (dissolved carbon dioxide or bicarbonate), while nutrient uptake depends on a supply of critical nutrients. Table 1 indicates sites of nutrient uptake and photosynthesis for terrestrial and aquatic plants of different growth forms. These facts must be considered in the development of plant culture methods and facilities.

Unlike terrestrial plants, submersed aquatic plants conduct photosynthesis in a water environment (medium). This is important for several reasons.

Table 1

1) The medium (in this case water) must be of sufficient clarity to transmit adequate light to the leaves. This is not usually a problem in air.

2) If the water contains nutrients, particularly phosphorus, excessive growth of algae can cause problems by reducing light penetration to the submersed macrophytes.

3) The medium must provide a continual supply of inorganic carbon. Again, this is not usually a problem in open-air environments, but the diffusion of carbon dioxide in water is slower than in air, and the concentration of carbon dioxide can be greatly reduced in water, particularly at pH levels greater than 8.3.

4) Algae will compete with submersed macrophytes for inorganic carbon.

Successful culture of rooted submersed aquatic plants will thus depend on our ability to provide adequate nutrients via the sediment to the plant roots and adequate levels of light and inorganic carbon via the water to the plant shoots (Smart and Barko 1985). Because non-rooted submersed aquatic plants must obtain all of these resources (nutrients, light, and carbon) via the water (Table 1 above), they are quite difficult to grow under artificial conditions due to competition with algae. For this reason we do not at this time recommend culture of Ceratophyllum demersum and prefer to collect plants of this species from existing natural populations.

Sediment substrate

Submersed aquatic plants will grow in a variety of substrate types, ranging from pure sand to heavy clays. However, for optimum production a fine-textured substrate with a low to moderate organic content (10-20%) is ideal for most species of submersed aquatic plants. Sandy substrates are unsuitable because they are generally infertile and added nutrients may diffuse into the water column, causing algal problems. Highly organic substrates can be inhibitory to plant growth (Barko and Smart 1983, 1986). When available, we recommend using fine-textured sediments from ponds or lakes in which aquatic plants are known to grow.

If the growth potential of sediments is in doubt, small-scale trails should be conducted to determine sediment suitability for supporting aquatic plant growth. Because suitable natural sediments may not always be available, the use of commercial potting soils or top soils may be necessary. For relatively small-scale efforts, bagged soils may be practical. In selecting a soil for aquatic use, generally the lowest priced product will be the most suitable as it will generally contain the fewest additives. Avoid the use of products that contain bulk additives such as peat, vermiculite, perlite, or sand. For large-scale projects, local top soils may be purchased in bulk after ensuring their suitability.

Fertilization

Submersed aquatic plants. For short-term (2 months or less) cultivation of submersed aquatic plants, an initial fertilization of the potting medium is usually sufficient. Often, addition of nitrogen is required to achieve optimum growth (Smart et al. 1995). Rates of 1 g nitrogen per liter of medium are sufficient to support growth during this period. Nitrogen should be added as an ammonium salt, not as nitrate or urea. Nitrate can be rapidly lost from anaerobic sediments by diffusion and denitrification. Ammonium ions, however, are readily adsorbed and held on the cation exchange surfaces of clay minerals and macroorganics.

Longer-term cultivation of submersed aquatic plants may require periodic fertilization or addition of other nutrients. Adding nitrogen as either ammonium or nitrate to the water column every few weeks can be used to sustain the growth of mature transplants. Excess levels of nitrogen can be inhibitory to the growth of submersed aquatic plants so concentrations should not exceed 1 or 2 mg/l.

Floating-leaved and emergent aquatic plants. Floating-leaved and emergent growth forms generally produce more biomass than do submersed forms and have proportionately greater demands for nutrients. For this reason, larger quantities of fertilizer should be added to the sediment substrate. Because these forms have their photosynthetic and carbon uptake surfaces in the air rather than the water, excessive algal growth generally does not interfere with their growth. In fact, once they develop a canopy of leaves, these plants may shade out algae. Long-term growth of cultures of these growth forms can be sustained by adding nutrients directly to the water without concern. Although these growth forms generally are not well-adapted to absorb nutrients from the water, transpiration drives a movement of water (and dissolved nutrients) into the root mass. For this reason, we prefer to use pots with ample "drain" holes, so that the roots will be in close association with the water.

Free-floating-leaved aquatic plants. Free-floating aquatic plants, having their photosynthetic and carbon uptake surfaces in the air and their nutrient uptake surfaces (roots) in the water are relatively easy to grow. A soluble, complete nutrient fertilizer can simply be added to the water as needed. While easy to grow, we do not recommend the use of free-floating aquatic plants for lake restoration as these can grow to excess under eutrophic conditions. Excessive growth of free-floating plants can completely cover the water surface and cause problems by interfering with the entry of sunlight and atmospheric oxygen.

Water quality requirements

While floating-leaved and emergent plants are not as particular, a reliable source of high quality water is required for growing submersed aquatic plants. Ideally, water should be clear and relatively nutrient-free (at least phosphorus-free). Clear water allows adequate light penetration. Under low light conditions, some plants will become leggy and produce weak root systems. Nutrient-rich water often leads to algal blooms in the culture, and these interfere with plant production by competing for light and inorganic carbon. The use of municipally treated water is not recommended, unless chlorine is first removed. Treated water also often contains relatively high levels of phosphorus.

For our tank cultures, we use lake water that has been "polished" or treated to acceptable quality. In one method, we use a vegetated pond to reduce turbidity and remove much of the dissolved phosphorus from the water column. We simply pump the water from a vegetated pond directly to our culture facilities. In a second method, we treat the water with aluminum sulfate (approximately 0.1kg per 1000 l) to flocculate clays and suspended material and to remove phosphorus by sorption onto precipitates. The resultant floc is allowed to settle and the clear water above can be pumped to culture tanks or, if the water is treated in place, the floc can be pumped out and discarded. For a large-scale plant production system we use a 1.5 m deep, lined water supply pond as a reservoir. Lake water is pumped into the pond, treated with aluminum sulfate, and mechanically filtered with sand filters (Dick et al. 1997). The liner (generally PVC or rubber) prevents nutrients and clay minerals from being released or suspended from the soil into the water column. This system provides an abundance of high-quality water.

Additional requirements for water that will be used to grow submersed aquatic plants include a source of inorganic carbon and a balanced chemical composition including calcium, magnesium, and potassium ions (Smart and Barko 1984, 1985). Periodic replacement of part of the water may be desirable to maintain favorable levels of alkalinity, dissolved inorganic carbon, and dissolved ions. Alternatively, additions of sodium or potassium bicarbonate and calcium (as either a sulfate or chloride salt) can be used to maintain adequate levels of these constituents. Aeration (see below) is also needed to maintain a steady supply of inorganic carbon.

Water circulation and mixing

In unlined, earthen ponds, sediment respiration provides an abundant and continuous supply of carbon dioxide to support the photosynthesis of submersed aquatic plants. However, in lined ponds or tanks, carbon dioxide availability may often be a factor limiting growth of submersed aquatic plants. (Floating-leaved and emergent species acquire carbon dioxide directly from the air). Consequently, aeration of tank cultures is recommended for submersed species. A regenerative blower/compressor aeration system is required to supply the air, and vigorous bubbling of atmospheric air through air stones usually provides adequate mixing in addition to supplying carbon dioxide.

Facilities for Off-site Production

Production of aquatic plant propagules requires adequate facilities, but these need not be complicated or expensive. Small ponds, tanks, or raceways may be used to grow aquatic plants. To minimize transportation costs, and the inevitable damage that occurs during transport of plant materials, production facilities should be as close as possible to the restoration site. In this regard, in-lake production, if possible, might be the most economical means of propagule production. In the following section we provide guidelines on the suitability of various facilities for plant production.

Containers

We recommend using commercial nursery pots with drain holes in the bottoms. As previously mentioned, these holes allow movement of dissolved nutrients into the sediment substrate where they can be taken up by the roots. Various sizes and shapes of commercial nursery pots are available. We have used both quart- (4" or 10-cm diameter) and gallon- (6" or 15-cm diameter) sized pots for growing a wide variety of aquatic plant species. Commercial-grade nursery pots can be reused several times.

Small ponds

Well-designed ponds offer excellent sites for culturing aquatic plants. Although any pond that has a reliable water source (and water depth) will suffice, those in which drainage and filling are easily accomplished serve best. This allows the plant grower to manipulate water levels for cultivation needs such as planting, weeding, fertilizing, and harvesting. Because the objective is to produce robust, potted transplants we want to (as much as possible) restrict growth to the containers. Open pond bottom sediments allow growth of endemic vegetation and encourage the "escape" of cultivated plants. Either of these situations is undesirable because plants growing "wild" in the pond reduce the growth of potted plants by competition and interfere with maintenance and harvesting operations. For these reasons, lined ponds are preferred over earthen ponds. However, construction of concrete pads on earthen pond bottoms also offers the same advantage.

Separation of plant species within a lined pond can be critical for successful cultivation of many species. Cross-contamination by faster (or earlier) growing species can reduce production of slower (or later) growing plants. Because many aquatic plants spread vegetatively from fragments, care must be taken when selecting species for polyculture within a single pond. Isolating fragment-spreaders (or prolific seed producers) in their own pond is highly recommended. A second option is to construct enclosures for these species. A fine mesh shade cloth fencing will serve to prevent spread by fragmentation of all species of aquatic plants discussed in this manual.

Tanks

Tanks are excellent vessels for growing aquatic plants. The advantages of tank culture include accessibility, water quality management, and separation of species. Many sizes and shapes of fiberglass and plastic tanks are available commercially. While these are generally manufactured for the aquaculture of fish and invertebrates, some models are well suited for culturing aquatic plants. When selecting tanks, ensure that tank depth is suitable for the species of plants to be cultivated. Another consideration is tank dimension. Easy access is critical to good plant cultivation. A tank width of about 1 m may be the maximum for easy access to plants. For many species of submersed and floating-leaved plants, we recommend tanks in the range of 0.75 to 1.0 m deep by 1.0 m wide by appropriate length (5 or more m). For ease of operation tanks should be accessible from both sides. Shallow tanks (25 cm) are suitable for emergent species.

Construction of custom tanks may be desirable and cost-effective on many projects. For long-term cultivation, concrete vats can be made to size for specific plant types. Permanent plumbing, including filling and drainage piping, can be included in such structures. Less expensive, custom tanks can be constructed from available building materials (lumber or concrete blocks) and pond liner material.

Shelters

Greenhouses, hothouses, and cold frames can be incorporated into tank designs to extend the growing season for many plant species. Some degree of protection may be needed for plants in northern areas, where water in tanks can freeze solid. An advantage to moderating temperatures (and possibly photoperiod) is early season production of plants - i.e. mature transplants can be produced, ready for transplanting as soon as project conditions allow. Without temperature and/or light control, most native plants will remain dormant until temperatures rise in the spring, reducing the transplant window during a particular season.

Excessive solar heating can be a serious problem, especially in above ground tanks. Hot, sunny days may cause excessively high temperatures within tank cultures, and plants may suffer high mortality. Because excessive light can also damage submersed aquatic plants, we recommend covering tanks with a light grade (30-50% reduction of sunlight) shade cloth. This will reduce both light intensity and temperature.

We do not recommend shading for floating-leaved or emergent species.



Culture Maintenance



Weeds

As with any culture or crop, nuisance weeds or pests may cause problems. Pond sediments often contain seeds and spores of aquatic species that might interfere with production of desired species. Although expensive and labor-intensive, sediments can be heat sterilized to avoid or reduce this problem. Hand weeding will be required to remove unwanted plants, but this also is time consuming and labor intensive.

Inadequate separation of plant species in mixed pond or tank cultures can also lead to cross-contamination and "weed" infestations, especially where production of single-species transplants is critical. Growing monocultures in separate tanks will usually prevent cross-contamination. If contamination does occur, rigorous hand weeding will be necessary to correct the problem.

Algae

Excessive algal growth is always a concern with cultures of submersed aquatic plants. High concentrations of nutrients (especially phosphorus and nitrogen) in the water column will generally support excessive algal growth. Algae, whether growing in the water, on the water surface, or on the plants themselves, cause problems by reducing the growth of desired plants. Algae compete with macrophytes for light, nutrients, and inorganic carbon, and, because they are capable of rapid growth, they can quickly grow to problem levels. Once algae become well-established in a culture, they are difficult to control so prevention is prudent. As mentioned earlier, using low-nutrient water and avoiding excess fertilizer will usually prevent algal problems. Reduction of existing algal blooms will require exchanging the water with low-nutrient water and either hand removal of filamentous growths or filtering the water to remove phytoplankton ("green" water).

Grazing pests

Herbivore damage may become a problem in some situations. Pond and in-lake plant cultures must be protected from turtles, carp, waterfowl, muskrats, and some invertebrates. Protective devices are discussed in the following chapter. Aphids and caterpillars can reach nuisance proportions in tank cultures, and may require chemical control.