Seed Collection

Italic numbers in parentheses refer to Literature Cited.

Timing Collections

The timing of seed collection from wildland plant species is one of the most crucial and difficult steps in propagation. Collection of immature seeds results in low seed viability or dormancy. The danger in delaying collection is that the fruits of many wildland plants dehisce (fall from the seed head) very rapidly and seeds are lost if collection is delayed. Collecting seeds from the ground may be possible, but usually results in low-quality seeds and excessive cleaning costs.

Most crop plants bloom in sequence, beginning with the uppermost or central flower; therefore, these plants have a determinate inflorescence or flower arrangement. In contrast, many wildland plants have indeterminate inflorescences where the flower stalk continues to grow with prolonged flowering and many different stages of seed maturity on the same stalk. This makes uniform seed collection difficult. If the seed collector is able to selectively harvest only the ripe portions of the inflorescence, the indeterminate inflorescence is no great problem.

There is no substitute for experience in judging when to collect seeds of wildland species. To start a collection program for a species with which you have no previous experience is difficult. Guides to maturity can be obtained for regional floras. Such manuals usually provide a range in maturity (for ex-ample, late June or July). More detailed information can be obtained from the few specialty manuals or articles that deal with the collection of seeds of native plant species (5). The original "Woody Plant Seed Manual'' (1) and its more recent edition (23) provide guidelines for the woody species. The old and difficult-to-obtain bulletin "Collecting and Handling Seeds of Wild Plants" (17) is an excellent source of information for California collectors.

Considerable information is available concerning seed collection of the important browse species bitterbrush (Purshia tridentata). The seeds of this shrub are collected by personnel of land management agencies and professional seed collectors. Nord (19) pointed out that bitterbrush seed production may be forecast a year before the fruit develops. Because bitterbrush flowers on second-year twigs, good crops generally follow years of average or better moisture and when stem elongation averages at least 3 inches. For species that fruit on the current year's wood, this observation would not be valid.

Essentially, the novice seed collector must judge plant phenology or the sequence of plant development. Flowering is the first phenological stage of which the would-be seed collector must be cognizant. Flowering is obvious for many species with colorful petals, sepals, or bracts, but careful attention is required to note anthesis (shedding of pollen) with many grasses. After flowering, the sequence of phenology is as follows:

1. Soft-dough stage. This stage is indicated by the excretion of dough from seeds when squeezed between the thumb and forefinger. Seeds collected at this stage generally have low viability if they will germinate at all.
2. Hard-dough stage. The hard dough stage can be judged by biting the grain, once the milk or dough stage is completed. Once the seed is fully mature, it is usually too hard to bite. Seed collection should start with the transition from soft to hard dough. The time interval between soft and hard dough is a good indication of how soon to repeat the collection. With these first collections, the chance of obtaining plump, fully matured seeds can be increased by not stripping the seed from the plant, but rather by cutting considerable plant material. In some species, this method will continue the maturity process. Care must be taken to insure that the mass of plant material dries uniformly and does not mold.
3. Maturity. Obviously, the goal of wildland seed collectors is to harvest mature seeds. Unfortunately, maturity and seed dehiscence may occur at the same time. To make sure some seeds will be obtained, repeated collections are necessary. These collections extend from the latter part of the soft-dough stage until all seeds are lost. Each collection must be clearly labeled with the collection date, location, species, and stage of phenology based on physical appearance. Descriptive notes on associated plant or site factors that may aid in reidentification of the stage of maturity are valuable.

For large-scale collections of seeds of wildland species, seed-moisture curves will be valuable guides to proper timing of harvesting. Moisture is high in immature seeds, usually about 60 percent, but drops to about 10 percent as the plants mature (9). The seed-moisture curve for each species will show a characteristic shape because of differences in the slope or drying rate. The rate of seed moisture change varies with climatic conditions, but averages about 3 percent per day during the seed maturity period. With above-average hot weather, the slope of the curve temporarily increases; whereas, during cooler rainy weather, the curve flattens. For many crops, the seed-moisture curves are known and are used for guides to harvesting seeds. Methods of determining seed moisture are provided in the seed storage section of this manuscript.

Germination tests on each collection, made over a period of phenological development, provide the ultimate basis for the correct time to harvest. Remember that optimum germination may occur in the most mature seeds, whereas optimum seed yield may occur at an earlier stage of maturity before seeds are lost by shattering. Because of year-to-year variation in growing conditions, no method provides an absolutely accurate prediction of the specific date for seed collection.

The period of optimum seed collection can be extended by starting seed collection at low elevations and following maturation upslope. The same procedure can be applied to species that produce tillers that mature later than the main inflorescence.

Nord (19) found that 74 percent of the variability in date of seed ripening for bitterbrush was accounted for by variability in latitude and altitude. Through application of Hopkins' Bioclimatic Law (14), adding or subtracting one day for each 100 feet of elevation or 15 minutes latitude has led to a highly significant relationship between actual and theoretical seed ripening dates.

Often the seed collector can take advantage of microenvironmental differences at a given location to aid in collecting mature seeds. If seeds are immature on north-facing slopes, plants of the desired species growing on south slopes will generally be at a more advanced stage of maturity. Plants growing in swales or along drainage bottoms may produce more seeds than the same species on arid south slopes.

Areas burned in wildfires are excellent for seed collection for several seasons after burning. This is a result of the natural plant succession following burning and the dynamic reproductive response of many species to reduction in competition and nutrient changes brought on by the fire.

Seed Caches

Rodents, birds, and insects, especially ants, are voracious collectors of some seeds. For some species, for example, juniper (berries) and pinyon pine (nuts), the seed collector must race the natural predators in order to obtain any seeds unless protective bagging or screening is used. Some seeds, especially conifers and bitterbrush, can be obtained from rodent caches. Seeds from warm desert annuals that have ant-attracting glands can be recovered from the refuse dumps of ant nests. For some species of ants, the viable seeds are stored in the nest, and only chaff is left on the soil surface. The droppings of many animals contain viable seeds or seeds that have improved germinability after passing through the digestive tract. The difficulty with any of these collection methods from caches or droppings is that the quantity of seeds obtained is small and they are often contaminated with pathogens.

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