Many, if not most native plants' seeds are ready to harvest between June and October. Annuals and perennials will be ready for seed collection 2-5 weeks after peak bloom while shrubs and trees may take two months or longer for fruits and seed to mature.
Seed collecting requires you to familiarize yourself with the seeds themselves.
Rancho Santa Ana Botanic Garden
offers website photo collections of many common Western United States native plants and their seed along with numerous resources about native plant seed collections that are relevant regardless of locale. For additional regional resources, look online to find local native plant societies to see what resources they offer.
A good hand lens or magnifying glass is a useful tool for examining crushed seed heads and identifying seeds among the chaff.
Depending on the type of plant and the type of fruit produced, there are varying signs to look for to tell if the seed is ready to be harvested. In general, if the seed comes away from the plant freely, the seed capsule is dry and tan, or the fruit is pulled from the plant easily, then the seed is ripe and viable. Mature seeds are usually dark in color, firm and dry.
The flesh of pulpy fruits often becomes soft and changes from green or yellowish to reddish or blue-purple and detaches from the plant easily when ripe. Determine seed maturity by cutting open the fruit and examining seeds for firmness, fullness and dark color.
For plants that haven't been collected from before, start checking the pods early, rather than come back to the plant long after all the seeds have dropped. Many pods or capsules dehisce (come open and expel seeds) when ripe and mature at staggered intervals, making collection difficult. Once maturation begins, check it every few days to collect any newly matured seeds. Try inverting a paper sack over the immature seed and tying it off with string, or hold the pod together using a rubber band. Dry, well-ventilated areas quicken the opening of pods. Hot and dry weather can speed up seed maturity, while cool, moist weather can slow it down. It's a good idea to record weather conditions, as well as flowering, seed maturity and collection dates. This information will provide a useful reference guide for future collections.
Use fingers to shake or pry seeds from seed heads. It can be helpful to wear gloves. Sometimes it may be easier to separate the actual seed from the chaff(fine plant material) at the time of collection. In other cases, it may be preferred to use garden shears to clip seed heads from their stems for later separation of the seed from the chaff, flower parts and stems. In general, dry fruits are harvested into paper envelopes or bags while moist fruits are collected in plastic bags, buckets, etc.
In the case of native grasses, it is sometimes more efficient to use a fine- toothed metal hair comb to rake seeds from their stems than it is to strip them off by hand.
To collect a higher percentage of a population's genetic diversity, collect fewer seeds from more individuals as opposed to more seeds from fewer plants. For most situations, the more individuals contributing to a seed collection, the more useful it is. As is practical, sampling should be done randomly and evenly from throughout the population.
Harvested material should be placed loosely and not packed into collection bags. Harvested plant material can also be loosely wrapped in newspaper, spread out on paper or over screens to continue drying. Some fruits release their seed explosively; therefore their collection bags should be well-sealed and spread collections covered.
Post harvest care of collections includes protecting them from rodents and keeping them under moderate to cool temperatures until they can be cleaned, safely packaged and placed into storage.
Basic steps in seed cleaning include: threshing, screening, and winnowing.
Threshing can be accomplished by rubbing, banging and stomping. Then
screens that have slightly larger or smaller holes can be used to separate most of the largest and smallest plant materials.
Once seeds are "sifted", they'll be mostly clean but there may be a bit more chaff to deal with. A final step to remove a significant portion of any remaining chaff is to winnow the seed, allowing the wind to carry away the lighter chaff while leaving the heavier seed behind. The larger/heavier the seed, the easier this step is. Place the seed/chaff in a large shallow bowl. With a circular up-and- down motion, the chaff will be winnowed away.
For seeds that are borne in a fleshy fruit, the fruit can decay, but can also act as a germination inhibitor. Most moist fruits are easiest to clean shortly after harvest while they are moist. Care should be taken to keep them hydrated and as with any fresh fruit they can be maintained for extended periods under refrigeration.
First step in cleaning is to place all the fruit collected into a Ziploc bag, seal, and using fingers mash the fruit through the bag. Let the fruit decay for a few days, mashing the fruit periodically. After a few days to a week, the pulp will have decayed from a firm substance to a fairly watery one.
Dump the mass of pulp and seeds into a large bowl and fill 2/3 with clear water. Remove as much pulp from seed as possible by squeezing and rubbing the seeds. Carefully pour most of the water off - the seeds will mostly sink, while the lighter pulp will pour off. Refill the bowl, repeat several times. Clean seeds will fall to the bottom of the bowl.
If seed is going to be stored for any length of time, it must be completely dry or it will likely rot. Make sure the seed is dry and put seeds into paper envelopes in a dry, dark place.
Even when seeds are ripe and hard to the touch, they often still benefit from additional drying before being stored. This will prevent mold from developing and increase the seeds longevity. The best approach to drying seed is to leave it in an open bag or bin where they receive good air circulation. Hot locations, or areas that receive direct sunlight are not good for drying, and can kill otherwise viable seed.
Protected from rain and animals, often just a few weeks of additional drying at ambient temperatures are sufficient.
Collections should be labeled, preferably at the time of collection. Label collections with the date (including year), the botanical name of the plant, and location information.
Kept in a cool, dark place, seeds of most native plants can be stored for years with only a slight loss in viability.
The two most critical necessities for storing seeds are constant temperatures and low humidity. A temperature of 50 degrees Fahrenheit or less and a humidity of 50 percent or lower are ideal. Rule of thumb, temperature + humidity = 100.
Store seeds in the refrigerator, not the freezer, until you are ready to plant. Low temperature, humidity and light level protect seed longevity. If it is not practical to store seeds in your refrigerator, store them in any place that is cool, dark and dry, protecting them from insects as much as possible. Store the seeds in paper sacks to allow good air circulation and prevent molding. Do not store seeds in plastic bags or other nonbreathable containers unless they are air-dried thoroughly first. It is important to include basic information on labels, including date of collection, species name, and location of collection.
Scarification means scratching or nicking the seed coat. It's a good idea to scarify anything with a hard seed coat. The appropriate method depends on seed size.
For bigger seeds: if possible, use a stone grinder, holding the seed with needle-nose pliers and making sure not to let it heat up.
For smaller seeds: Dip in sulfuric acid. For thin coats, five minutes or less, while thicker coats may take up to 30 minutes. Be sure to rinse the acid off really well.
Other methods include:
* Lacerating seed with a sharp knife
* Nicking seed with nail clippers
* Roughing up the seed coating with sandpaper
* Freezing overnight, then adding seed to boiling water (cooled just a bit) in order to cause cracks
Important - scarifying increases the number of germinated seeds that are vulnerable to extreme weather conditions. Once scarified, most seeds will germinate quickly, and the seedling will require water.
Stratification involves putting seed in a plastic bag with growing media (vermiculite is suggested for smaller seeds, perlite for larger) and chilling it. This serves as a sort of simulated winter.
The amount of time a seed is in the cold depends on the plant:
* Perennials such as milkweeds, columbines and penstemons can be stratified from two to four weeks.
* Stratify trees and shrubs for 30 to 90 days.
Be sure to check bag daily and keep the media moist. Once germination occurs, sow the seed in soil.