Harvesting Native Seed

Proper timing of seed harvest and use of effective harvest techniques increases the likelihood of producing a profitable yield of viable seed that will be effective for restoration of native vegetation. Harvest is clearly a critical step in seed production. It is also one of the steps that is most susceptible to unintended selection leading to adaptation of wild species to production systems. Wild populations have variation in inherited traits related to seed harvest such as the timing of maturation and the degree to which seed shatters or is retained on the plant at maturity. Variation in these traits provides wild populations with the ability to adapt to changing conditions and to disperse and establish without human intervention. Loss of these traits within production populations is possible when all seed in a field is harvested at peak maturity (eliminating the genetics of earlier and later maturing individuals from the harvest) or when most of the harvested seed comes from individuals that retain seed longer (i.e. are less prone to shattering). The risk of selection is compounded as harvested seed is used in expanding fields and/or planting new fields over several generations. Keeping the risk of selection in mind and utilizing strategies to avoid it is key to ensuring that commercially produced seeds retain genetic diversity and the ability to grow, survive, and reproduce as wild plants in restoration sites.

Gauging Seed Maturity

Factors Affecting Seed Maturity and Dispersal

Gauging seed maturity is key to harvesting quality seed. Immature seed stores poorly, losing viability more quickly than mature seed. Determining the ideal time to harvest can be difficult. Within a population of native plants, there is genetic variation among individuals in the timing of seed ripening, and repeated harvests may be called for as seeds mature on different plants. In addition, seeds in different parts of an individual seedhead or inflorescence are often at different stages of ripeness. Seed maturity usually progresses from top to bottom of the seedhead in grasses and many prairie forbs. In forb species with flattened flower clusters, seed often matures first at the center of the inflorescence.  Mature seeds may be quickly dispersed by gravity, wind, water, or animals, so it’s important not to delay harvesting.  Some species forcefully eject seed at maturity (e.g., spiderworts, phlox, wild petunia, and violets) and require special harvesting strategies. 

Environmental factors also influence seed maturation. Cold, moist conditions tend to delay seed maturity while hot, dry conditions may hasten it. Latitude also affects ripening since many plants flower and set seed in response to photoperiod. Flowering and seed set may be delayed if plants are grown northward from their origin, or hastened if moved southward. If moved a great enough distance north or south of their origin (greater than 300 miles) they may fail to set mature seed. 

Gauging Seed Ripening in Grasses 

Seed ripening and timing of harvest varies by species, source of parent material, and environmental conditions. For example, cool-season grasses begin growth early in the growing season and consequently ripen earlier as compared to warm-season grasses. In grasses, there are roughly four stages of seed maturity: milk, soft dough, medium dough, and hard dough. Firm thumbnail pressure on the caryopsis, or grain, will help determine maturity. Grasses should be harvested at the hard dough stage, when firm thumbnail pressure slightly dents the caryopsis. Many grasses  ripen about a month after flowering, and some do not hold seed long after maturity. Test ripeness by firmly striking the seed head against your palm; if some shattering occurs, the seed is ready to harvest. If it shatters with only gentle striking, harvest immediately. In some species, such as sideoats grama, visual assessment of the stand provides an indicator of maturity: the stand is ready to harvest when about 10% of the plants have started to shed seed. 

Gauging Seed Maturity in Forbs

Generally, the seedhead itself or the stalk immediately below the seed head will begin to appear dry or discolored as the seed ripens. The leaves may also change color as the plant approaches dormancy. If the seed is easy to strip off or shatters out when the seed head is gently struck against the palm, seed ripening has begun. Some species have seed capsules that dry and split open when ripe, allowing seed to be released by wind, passing animals, or gravity. The spiderworts (Tradescantia), members of the day-flower family (Commelinaceae), are a special case: they drop seed from individual seed capsules as they ripen even while the bracts remain green and other flowers in the same cluster are in bud or blooming. Species with dispersal apparatus such as awns or hairs will appear dry and fluffy at maturity. It can be tricky to time the harvest of fluffy-seeded species that release their seed soon after maturity (many species in the aster family). Picking a few heads and examining the developing “seeds” (technically a type of fruit called cypselae or achenes) can help gauge approaching maturity. Seed that is nearly ripe will develop the mature color and separate easily from the receptacle. 

Hand Harvesting Techniques

Hand harvesting is time- and labor-intensive and not practical for most large projects, but it is an important way to collect the seed of native species that are commercially unavailable and/or impractical to harvest mechanically. Such species may be low or high growing species, early or late ripening species, species with explosive seed capsules, or uncommon or patchy species in native prairie. There are also situations in a seed production system when hand harvesting may be needed, for instance, when collecting seed from early maturing individuals to ensure conservation of genetic diversity. Efficiency can be improved by keeping both hands free to harvest by simply fastening collection bags and containers around the waist with a bungee cord. Hand harvesting aprons are also available from some native seed businesses and garden suppliers. A plastic comb can be used to strip seed from grass stems. Scissors, clippers, or a hand sickle are useful for cutting tough stalks beneath seed heads.

Hand harvesting or combinations of mechanical and hand harvest approaches may be needed for species with explosive seed capsules or capsules that shed seed immediately upon ripening. Strategies for collecting explosive seeds include: checking and harvesting daily (Viola), bagging seedheads loosely with a tightly woven mesh or cloth bag (Phlox), vacuuming seed from landscape fabric (Phlox), or clipping heads when some capsules have opened and drying cut material on tarps or cloth bags indoors (Ruellia). For the spiderworts (Tradescantia), check seedheads regularly near the typical harvest date. They begin shedding seed from the bottom of a flower cluster even when most of the plant is still green and there are still a few buds opening at the top of the cluster. When some seed is being shed from most plants, plots can be swathed and the cut material collected and laid on a tarp in a building. Capsules will continue to mature and release seed as the cut material dries.

Hand harvesting is also useful for collecting seed from wild populations for use as foundation material for seed increase and seed production plots. A seed collection is only a sample of the seed available in a population. How and when the seed is collected and propagated influences the genetic potential of the resulting population. Annual variation in rainfall and temperature can affect total seed production, quality, maturity, and dormancy. To optimize the capture of genetic diversity present in a population, approximately equal amounts of seed should be collected from many widely spaced individuals (minimum of 30, preferably 50 or more) throughout a site and over multiple years. In larger, evenly distributed populations, walk a linear transect through the population, sampling seed at intervals (e.g., every ten steps), then sample along additional transects parallel to the first. When collections from multiple sites are used to create a pooled population in a production plot, attempt to equalize the contribution of seed from each site. 

Populations grown and re-grown in a production field can become adapted to site conditions and nursery management practices. Even though most prairie plants are perennial, seed production in plots tends to decline after 3-5 years for most species, and plots need to be replanted. Therefore, it is important to save seed from the original collections and/or the first production generation for replanting production fields. Hand harvesting of early maturing individuals in a production plot is a way to retain important genetic diversity within the production population. The rest of the plants in the production plot can then be mechanically harvested at peak maturity. Later maturing genotypes can be retained by leaving a portion of the plot uncut and harvesting it later (either mechanically or by hand).

Mechanical Harvesting Techniques

Combine harvest  

As stated above, not all seeds ripen at the same time; with any species, a determination has to be made as to when most seeds present are at or nearing maturity. Grasses are generally harvested at the hard dough stage with some exceptions. Test ripeness by firmly striking the seed head against your palm; if shattering occurs with only gentle striking, the stand should be combined immediately. Since most grasses ripen from the top down, some shattering of the tops of the seed heads may have already occurred. If a species’ seed shatters very easily, harvest in the early morning when humidity is high and wind speeds are less, as a strong wind can reduce the harvest significantly in a single afternoon. In grasses that shatter easily at maturity, windrowing or swathing during the medium- to hard-dough stage can be effective, since seed will after-ripen for several days after cutting. It is important to be sure no rains are in the forecast for the next few days after cutting. Swaths can then be picked up with a combine after the material has dried at the site for a few days. 

Combines may require significant modifications to make them suitable for harvest of native grasses and forbs. Reducing or shutting off airflow in a combine is a must for native species with lightweight seeds. Species with fluffy pappus (e.g., asters and goldenrods) should be harvested when seed is mature but just prior to the dry-fluff stage. If the seed is dry-fluffy, the combine will become a super seed dispersal machine. Plugging of shaker sieves and augers is a constant issue, particularly with fluffy seed or seed with long awns. The long twisty awns of Canada wildrye can be a combine’s bane. De-awner bars can be installed into the cylinder surrounding the concaves to increase the threshing action of the concaves. Seed of tick trefoils (Desmodium species) is dispersed inside pods with sticky hairs. Some of the pods ball up in the combine, fail to pass through the sieves, and are ejected from the straw-walker. To prevent significant loss of seed, attach a bin or tarp to catch the seed balls. Alternatively, cut or swath the plot and collect the seed stalks, then pass them through a stationary combine, where the ejected material can be more easily captured. 

 

Seed strippers

Commercial seed strippers are available as handheld, implement pull-type, or tractor-mounted equipment. They all use a rotating brush or bristles to strip the seed from stems and stalks of plants. While perhaps not as efficient as combines, strippers can be used for the successive harvest of species that ripen gradually or at different times.  Handheld strippers and pull-types light enough to be pulled with an ATV allow harvest of otherwise inaccessible native sites.

Leaf blower vacuum units

A leaf blower-vac unit on the vacuum setting can be used to harvest fluffy seed such as species in the aster family that have a pappus (“parachute”). Some suggest that the impeller blades of the machine need to be removed to avoid damaging the seed. However, we carried out a trial using an unmodified blower/shredder vacuum to harvest seed of prairie groundsel (Packera plattensis) and observed no visible damage to the seeds (cypselae) under a dissecting microscope. Because vacuuming removes seeds at the dry-fluff stage, immature seed can remain on the plants to ripen for later harvest. This should increase the yield of viable seed from each plant and reduce the risk of unintentionally selecting for a narrower seed ripening window. An additional benefit of this harvest method is that it removes much of the pappus from the seed at the harvest step, reducing the time needed in the seed cleaning lab. It would be interesting to try scaling up this technique using a modified pull-behind leaf vacuum.

Ethics of Harvesting from Remnant Sites 

Repeated, annual harvest of seed from remnant prairies for the commercial market is not encouraged.  First, seed production and quality from wild populations are not as high as from production plots, and second, it provides an incentive to manage remnants for maximal seed production.  Manipulation of a remnant prairie to maximize seed production – such as whole-site, repeated annual burns, herbicide treatments, or fertilizing – is inappropriate and unethical. A remnant prairie is a diverse, biotic community (both above and below ground) of microbes, fungi, plants, and vertebrate and invertebrate animals interacting in complex relationships.  Management applied indiscriminately and repeatedly is detrimental to some of these associations. Burning should be limited to a portion of a remnant any given year, and each portion should be burned on rotation and at different times of year, at varying intervals of time.  Bulk harvest from remnants may be appropriate when the seed is intended for planting on adjacent or nearby land to buffer and expand the native prairie. Finally, mechanical harvest in remnant sites should include a careful inspection and cleaning of equipment (including vehicles) to avoid introducing exotic/invasive species that may lead to the degradation of the remnant. 

 Be mindful of other ethical considerations when collecting seed from prairie remnants.  Federal and state endangered and threatened species cannot be collected without proper permits. Removal of any plant or plant part from preserves, natural areas, and parks may be restricted. Check with the proper agency before harvesting seed in these areas. Harvesting from roadsides may also be restricted in some states and counties. Contact the county engineer’s office or state department of transportation before harvesting from county and state roadsides. Obtain permission from the landowner before collecting seed on private property. Some seed producers lease native prairies to compensate landowners for excluding grazing over the summer so that seed can be harvested from the site.