Seed Propagation of Native Wildflowers
Maintaining Genetic Diversity in the Nursery
by Heather McCargo
Copyright ©1998. This article may not be reproduced in part or in whole without permission of the author.
Until recently, much of North America’s indigenous flora has been overlooked by horticulturists in favor of exotic plants from around the world. Because of this, many native species are new to cultivation and have not been altered genetically by domestication and breeding. Now that our native flora is being propagated by the nursery trade, we need to examine what domestication will mean to these plants and question whether this is the end that we hope to achieve. I believe that the nursery trade should maintain the genetic diversity inherent in wild native plants, and I will outline propagation methods that aim to do so.
Genetic diversity in wild plants is extremely important for several reasons. Wild ecosystems are in a constant state of flux because the world’s climate is always changing, even without human interference. Genetic diversity within a species provides the best opportunity for a species to evolve with changing conditions and survive over time. In the wild, most plants reproduce sexually. Sexual reproduction results in variation among individuals, and individuals differ in their abilities to cope with varied conditions.
Drought, excessive rain, cold, heat, pollution and disturbance will affect individuals differently. For our native flora to survive into the future, it will need genetic diversity to aid in the process of evolution.
The selective pressures on wild plants differ from those in cultivation. In the wild, most plants contribute to the future of the species by sexual reproduction. Flower and seed characteristics reflect a mode of fertilization and dispersal of a species, not a human being’s vision of beauty. Seed ripening and dispersal may happen rapidly, with all the seeds exploding from their pods at once, or more slowly over days or weeks. Germination can be even more variable, with some species germinating immediately while others germinate irregularly over weeks or months. The seeds of many wild species need a cold period to break dormancy, some taking two years or more for the first shoot to emerge. These reproductive strategies are advantageous for wild plants because offspring are dispersed over time to survive unfavorable growing conditions.
Horticulture tends to discourage or eliminate these characteristics because they are inconvenient for the plant propagator. This can happen intentionally through breeding to develop more rapid and uniform ripening and germination of seed, or unintentionally by favoring plants that reproduce and flourish in a nursery or garden environment different from the uncultivated landscape. Horticultural "fashion" can also alter genetic diversity in plants through selection of "superior clones" with traits such as compactness of growth or double flowers (which are usually sexually dysfunctional). These plants must be propagated vegetatively to reproduce the desired traits. Unfortunately, characteristics that are "ornamental" to humans may also have traits that make the plant less fit. The result is that variation in reproduction and growth is eliminated from the gene pool in the effort to produce uniform plants for sale as quickly as possible. This limited genetic base in cultivated plants increases the chances of negative recessive genes dominating the gene pool and weakening the species (inbreeding depression).
Much of the interest in and demand for native plants today is from ecologists and designers for habitat restoration and naturalistic landscaping. They hope that once the plants are established, they will grow and reproduce with minimal human interference.
To fill this niche as propagators, we need to think in terms of maintaining the wild traits and genetic diversity in the native plants produced by the nursery trade so that they have the best chance of surviving in a variety of environmental conditions. Horticultural practices that alter the reproduction or life cycle of native plants and vegetative propagation of "superior clones" may have a role for plants destined for ornamental gardens, but not for natural landscapes.
Nurseries interested in growing native plants for restoration and naturalistic landscaping can maintain genetic diversity in cultivation with a seed propagation system that does not require expensive or sophisticated facilities. Seeds can be germinated outdoors in beds or flats. Responding to the local environment, seeds sown outdoors will germinate when soil temperatures are optimum for each species, which can vary from the cool and frosty temperatures of early spring to the heat of midsummer. Seeds that need cold stratification are taken care of by the freeze and thaw of our cold temperate winters, and seeds that must be sown fresh will not be thrown off their natural cycle by the artificial climate of a greenhouse.
Many native plants, especially the herbaceous perennials, have a reputation for being difficult to germinate because of a lack of understanding of seed dormancy and germination requirements. While the reproduction biology of some native species, such as the lady slipper orchids (Cypripedium), is not well understood, many species are easy to germinate. I follow several procedures based on the germination requirements for various species:
1. No pretreatment required
Seeds that need no pretreatment can be stored dry in the refrigerator and sown outdoors in early spring. These include:
2. Cold stratification required
Seeds that need cold stratification to germinate can be sown outdoors in late fall. These include:
3. Fresh seed
Some seeds need to be sown immediately upon ripening. If the seeds are allowed to dry out, they usually will not germinate. The seeds of most of these species ripen from late spring through summer and germinate the following spring. These include:
Some species take two years to germinate, including:
In some instances, species that disperse their seeds quickly or whose seeds are carried off by ants are easiest to handle by letting the seed disperse into a growing bed and then later transplanting the seedlings into pots. These include:
4. Alternating warm and cold
Some species need a warm, moist period of approximately three months before cold stratification to germinate. In the Northeast, they can be sown outdoors in midsummer and will germinate the following year. In warmer climates the seeds can be sown in early fall (when the seed naturally ripens), and the mild temperatures will suffice as the initial warm period. These include:
After the seeds are sown, seed flats should be covered with a thin layer of coarse sand to help prevent the seeds from drying or being splashed out in the rain. If rodents are a problem, the flats can be covered with wire screen. Because some species can take a year or more to germinate, monitoring for weeds is important. Flats of species that take a long time to germinate can be located in the shade where weeds will be less of a problem. This outdoor method is simple, produces a lot of plants, and reduces the incidence of pest and disease problems, since air circulates better outdoors than in a greenhouse. While some species that are slow to germinate and grow, such as Trillium, may be artificially accelerated with hormones or tissue culture, this usually results in weak plants that will probably not adapt well to restoration projects.
Acquiring native seed for propagation can be challenging because seed of many species is not commercially available (especially local genetic stock) and wild seed collection requires skill and time. (The New England Wild Flower Society publishes a list of mail-order nurseries that sell native seed and nursery propagated plants. Call 508-877-7630.)
If seed of a desired species is not available from seed companies, you can purchase plants to grow for your own seed. Make sure that your stock plants are not all clones of one another and that you have enough plants for good cross-pollination and viable seed set.
(This number will vary from species to species.) The other option is for the propagator to collect seed in the wild. Because this is time consuming and takes an experienced collector to be successful for a large number of species, this is not a viable option for all nurseries, but for those wishing to specialize in native species of regional genetics, it may be necessary.
When collecting seed in the wild, the following methods should be followed to ensure that genetic diversity is maintained and wild plant populations are not being depleted:
1. Make sure that you have properly identified the species when it is in bloom and have checked with your state Natural Heritage Program to ensure that it is not listed as an endangered or protected species.
2. Get permission from the landowner before collecting seed.
3. Look for large, healthy populations of desired species and collect seed from a number of individuals. If possible, collect seed from several populations in the same region to minimize genetic drift or inbreeding depression. Make sure the seed is ripe before collecting, because immature seed often has low viability.
4. Never collect more than 5% of the seed in any population. If the species you want has poor seed set or a small number of individuals, do not collect the seed.
5. Research the germination requirements of each species before collection so that the seed is handled properly and is not wasted (see bibliography).
Despite the varied germination requirements of wild plants, it is not difficult to develop a schedule to handle the various species and to efficiently produce many native plants.
While a nursery heavily invested in greenhouses and propagation chambers may prefer to stay on its present course, low-tech outdoor seed propagation of native species does fit a growing market niche and requires no expensive facilities to get started. Every region of the country has a diversity of native wild plants that can be economically propagated and made available by innovative nurseries.
Art, Henry, Wildflower Gardeners Guide, Garden Way Publishers, 1987
Bird, Richard, The Propagation of Hardy Perennials, Batsford Ltd., London, 1993
Dirr, Michael A. and Charles W. Heuser, Reference Manual of Woody Plant Propagation,
Varsity Press, 1987
Phillips, Harry, Growing and Propagating Wild Flowers, University of North Carolina
Young, James A. and Cheryl G. Young, Collecting, Processing and Germinating Seeds of Wildland Plants, Timber press, 1986
About the author: Heather McCargo is a consultant in native plant propagation, design and restoration in Brooksville, Maine. She is the former Propagator at the Garden in the Woods, the botanical garden of the New England Wildflower Society in Framingham, Massachusetts.