In my garden, most of the annual herbs such as borage and love-in-a-mist reseed themselves without any intervention on my part. Many of the herbaceous (nonwoody) perennial herbs also tend to reseed with reckless abandon. I’m sure anyone who has grown lemon balm, feverfew, chives, elecampane, pennyroyal, fennel, or sweet violets knows only too well the multitudes of seedlings these plants can generate and the area they can claim as their own. The only reason I might collect seeds from such plants would be to give them away or to control their rampant spread. The woody perennial herbs such as lavender, rosemary, and the thymes are more restrained in self-seeding and produce far fewer offspring. I normally propagate these herbs vegetatively rather than by seed, however.
Some herbs don’t self-sow freely, some cast their seed far from the parent plant (even, like butterfly weed and dandelion, on the wind), and some are difficult to propagate by cuttings or root division. It is from these groups that I collect seed for propagation in later seasons.
General Harvest Methods
In late summer, I begin checking the garden for signs of seed development. The withering or dropping of flowers indicates that seeds have begun to form. After that milestone, I watch for flower stalks that have dried and turned brown and seedpods that have turned from green or yellowish brown to brown, gray, or black. The vast majority of herb seeds are brown or black when ready to harvest.
A reliable test of seed maturity is a light tap on the dry flower stalk. If any seeds rattle or are dislodged, they are ready for harvest. Also, watch for birds eating the seed heads (as they do reliably on my Agastache plants). This is an obvious indication not only that the seed may be mature (though some birds will eat green seeds) but that you’d better get out there and harvest it. If the seeds are small or contained in pods so that their maturity isn’t outwardly visible, as in the sages (Salvia spp.) or anise hyssop (Agastache foeniculum), I select a dry, brown flower stalk and remove some of its seeds; if they’re dark brown or black, it’s time to harvest them.
I harvest seeds late in the day after a few days of dry weather to ensure that all plant parts are dry. If the foliage or seed head is wet when picked, it will not dry quickly and is likely to mold. Few sights are more disappointing than a bag of seed heads that have turned to compost.
Cut the entire seed head or part of the flower stalk that contains seeds, avoiding any part of the plant that is still green, and place it in a large paper bag, cardboard box, or wooden bowl. Place only one kind of seed in each container, and label each with the name of the herb it contains.
Occasionally, I come across seed heads that are covered with aphids. I harvest them anyway but then place them in the freezer for a few days to kill the aphids. This doesn’t seem to harm the viability of the seed.
Collecting From Specific Plants
The first couple of years I grew pink gas plant (Dictamnus albus ‘Rubra’), I collected the seedpods in a small, open basket and was mystified when I later found the pods open but no seeds in the basket. I discovered that the seedpods pop open when they dry, and the seeds are expelled forcibly. I now collect gas plant seed when the pods begin to turn brown but before they’ve opened, and I put them in a closed paper bag. I can hear the seeds as they hit the sides of the bag. Some gardeners collect gas plant seeds by placing a paper bag or a piece of netting or sheer pantyhose over the immature seed heads while they’re still on the plant, and attaching it to the stem with a twist tie.
The ripe seedpods of butterfly weed (Asclepias tuberosa) split open to expose great multitudes of seeds lodged in a cottony mass for wind dispersal, much like dandelion seeds. When the first pod on a plant splits open, I harvest all the pods on that plant and, as with gas plant, put them in a closed container.
Seeds of many plants, including honeywort (Cerinthe spp.) and borage, mature along the flowering stem until hard frost kills the plant. Borage will easily self-sow, but our winters are too hard for the honeywort, and so after the plant has been in flower for a while, I inspect the lower ends of the flower stalks daily. I pick any mature black nutlets carefully to avoid disturbing the upper end of the stalk, which is still flowering and contains immature seeds.
After-Ripening and Drying
Few seeds will germinate if planted immediately after ripening on the plant. I therefore leave the seed heads I’ve collected in their containers a few weeks until the seeds have dried and ripened completely. After the seed coat has dried and hardened, the embryo slowly loses moisture and also undergoes chemical and other physiological changes. The seed needs to be kept in a dry, warm place with good air circulation; I prefer the garage, as any hitchhiking insects can escape without entering the house. If you’re pressed for time, you can remove the seeds from the dry pods or seed heads and clean them immediately after harvest, but then give them a few weeks of open-air drying before storing them in airtight containers.
The main danger in storing seeds in an unheated garage or barn is the high humidity that several days of rain can produce. Seeds and other plant parts will take up the moisture from the air and thus become more susceptible to disease. By the time the fall rains start (usually in early September in northwestern Oregon), I will have taken all my seeds inside the house and started the next tasks: checking them carefully for insects, then cleaning them.
The only seeds that I find at all difficult to extract are those of licorice. The two hard seeds are contained in a small pod that’s covered with stiff, fine bristles like tiny slivers. I used to open each pod individually by pressing my thumbnail down on the pod seam, hoping my thumb was callused enough to prevent penetration by a spine. I finally wised up and now place the tough pods between newspapers and walk on them with heavy boots first, which tends to rub off the bristles as well as break open some of the pods. The seeds, dark green when mature, are tough enough to withstand this treatment.
Seed is cleaned by separating it from the plant material (chaff) that was harvested with it. By the time I get around to cleaning my seed, much of it has already separated from the plant in handling and is lying on the bottom of the bag. In other cases, vigorously shaking the dried flower spike will separate the seeds from the plant. Sometimes it may be necessary to “milk” the seeds out with a gentle squeeze at the base of the pod. However, experience has taught me not to try to collect every single seed, just the ones that separate easily from the plant. Those that have been injured or have not fully developed may not separate easily and should be thrown away; the wound that occurs when an underdeveloped seed separates from the plant can be the first point of entry for fungal infection during storage.
Freeing large seeds from the chaff is easy enough; I just pick them out with a knife or tweezers. I pluck really large seeds, such as those from lovage, angelica, and sweet cicely, directly off the seed heads individually and avoid the issue of cleaning altogether.
For small seed, such as that of summer savory, winnowing is the easiest method for separating the chaff from the seed. There are many ways of doing this and a lot of room for creativity. Each year, we dedicate a board meeting of the Hardy Plant Society of Oregon to seed cleaning for our seed exchange. Members bring an amazing array of aluminum pie plates, knives, clippers, wooden bowls, colanders, cookie sheets, homemade screens, and magnifying glasses, and use them in many clever ways to extract the seed from the chaff. Some folks scoop small amounts of round seeds (basil and clary sage) with their chaff onto a tilted cookie sheet; the seeds roll down, and the chaff stays put. Screens can also be helpful in cleaning. Start with a mesh size just large enough to allow the seeds to fall through when they are brushed lightly across the screen, then use a slightly smaller mesh that will hold the seeds but allow smaller material to be brushed through.
No matter what ingenuity you bring to the process, though, seed cleaning can sometimes be tedious. I have no special tools for the job, just a pair of tweezers, lots of patience, and perhaps a captivating television show. I spread newspapers on a table, dump out small amounts of seed, and manually pick out the seeds, throwing the chaff into a bag beside me.
The container you choose for storing the cleaned seed should be relatively airtight. Baby food jars or other small, lidded jars are good for seed storage. I use plastic margarine tubs, and I write the name of the herb and the year on a piece of paper taped to the lid. I leave the lids off for a few days to ensure that any excess moisture is gone, and then I snap the lid on tight.
Check stored seeds periodically for mold and insect damage. Clumping of seeds when the container is slowly tilted and rotated may indicate mold. Other signs include a black, sooty color and perhaps a moldy smell. If you suspect mold, dump the seeds on a sheet of white paper, then pour them back into their container and look for black, downy dust on the paper. If there is any mold, throw away the entire container of seed.
Fine dust at the bottom of a container may indicate the presence of insects, and further examination is wise. Most storage pests are larvae that are large enough to see without a hand lens, and their webs are usually visible in a container of seed. If you find or suspect an infestation, freeze the seed for a couple of days to kill the insects. Dry ice can also be used to kill insects in stored seed. Simply drop a piece into the container, then replace the lid lightly. The insects either die from the cold or suffocate when the dry ice sublimes into carbon dioxide. Caution: Don’t screw the lid tightly on a jar containing dry ice, as the jar will explode.
The optimum storage temperature for seeds ranges from 35° to 65°F, and humidity should be low. (A refrigerator is an excellent place to store seeds if you have enough space.) Seed stored under these conditions can remain viable for at least 2 and sometimes as long as 15 years, although with every additional year in storage seed viability will decrease.
Seeds must never become completely dry: the tissues within the seed must retain at least a small amount of moisture to remain alive. Some seeds with hard coats are able to withstand desiccation to a moisture content as low as 5 per cent of their total weight, while others with fleshy reserves may tolerate desiccation only to 60 per cent. Seeds stored in a paper packet take up and lose moisture within a range of 5 to 20 per cent of their total weight in response to the humidity of the surrounding air; seeds in the open air take up and lose moisture even more rapidly. These frequent fluctuations can seriously impair seed viability.
Seed Types and Characteristics
Most herb seeds are small and dry and have a hard, dark brown or black seed coat. They typically are long-lived: seeds of sweet basil often are viable for seven years or more under proper storage conditions. Many such seeds, especially those of biennial and perennial herbs, may require a period of cold or of dry storage to induce dormancy before they will germinate.
A few herbs, such as angelica, lovage, sweet cicely, and parsley, produce moist seeds. The seeds tend to be large and have fleshy, spongy inner tissues because of their large storage reserves. Such seeds are short-lived: they tend to dry out over time; this process is accelerated under improper storage. Cold storage in the refrigerator or freezer not only helps maintain the viability of moist seeds but also induces dormancy in those that require it for germination.
An annual herb completes its life cycle in a growing season, which is typically a year. After flowering and setting seed, the original plant dies. Most annual seeds have no special requirements for germination; once ripe and dry, they will germinate if given sufficient warmth and moisture.
A biennial plant completes its life cycle in two growing seasons or years. Most biennial herbs belong to the parsley family (Umbelliferae). They include caraway, parsley, and angelica. Such plants should be grown either from fresh seed that has been dried thoroughly and then planted immediately in early fall or from seed that has been stored in the cold to ensure high germination.
Perennial plants live for more than two years. All perennial herbs produce dry, hard seed except for French tarragon, horseradish, and true peppermint, which do not produce viable seed and must be propagated vegetatively. Gardeners may become frustrated trying to propagate perennial herbs from seed because many perennial seeds contain chemical inhibitors, or dormant embryos, or have impermeable seed coats or other characteristics that prevent germination unless the seeds are specially treated.
I find it fascinating to see a seed develop into a living plant, but it’s extremely frustrating when I plant a large number of seeds and only a few germinate. I try to use my own collected seed as much as possible because I know its history and can attest to its parentage, but often I must rely on commercial seed companies, friends, and seed exchanges through garden societies and botanical gardens (see “Desperately Seeking Seeds?” on page 43). In all cases, unless I test the seed for viability, I have no idea whether it is alive, has been properly stored, and has met its dormancy requirements for germination.
Testing seed is not hard to do: just take a sample (perhaps two dozen seeds) and place it on a pad of wet tissue or moistened paper towel in a closed container and see how many germinate, and how quickly. However, not all seeds are alike; germination for some may depend on the presence or absence of light, the actual spectral quality of the light, and/or the temperature, including the fluctuation between night and day temperatures, and some seeds may require pretreatment in order to germinate.
Scarification. Some seed coats, such as those of hibiscus seeds, are initially almost impermeable to water or air. To promote germination, you must open or soften the seed coat by either nicking it with a knife or sanding it lightly with a file or sandpaper. Extreme care should be taken to cut through or abrade only the seed coat and not injure the embryo. As soon as the seed coat is penetrated in this way, the embryo is susceptible to fungal infection, and the seed must be planted immediately.
Soaking. The hard seed coats of herbs such as parsley need to be softened to allow adequate water uptake and air exchange. Placing such seed in hot (but not boiling) water and letting it stand for between 6 and 24 hours will not only soften the seed coat but will help leach out any chemical inhibitors, shortening the germination time. Sow the seed immediately after soaking.
Stratification. In seeds such as those of sea holly (Eryngium spp.), the moist cold of winter causes physiological changes that are necessary for germination. To mimic this cold period, soak the dry seeds in warm water (170°–210°F) for 12 to 24 hours. Sow them immediately into a moist planting medium in an airtight container (I often use resealable freezer bags). Place the container in the refrigerator or freezer for three to five weeks. I put sweet cicely seeds in the fridge and angelica seeds in the freezer, but either fridge or freezer will yield about the same result. Empty film canisters with their tight-fitting lids work very well for stratifying small amounts of seed.
Propagation and Parentage
As a commercial wholesale herb grower, I still regard growing herbs from seed with mixed emotions. There is no easier way to propagate annual herbs and most biennials. However, continued seed propagation of cultivars or hybrids, if not done selectively, can result in the eventual loss of important genetic qualities of the original parents. I recall reading in old herb books about a dwarf purple basil that I believe is now lost, and I know of a commercial grower whose Purple Ruffles basil mostly came up with green spots this year. The popular lavender cultivar Munstead has been propagated by seed for years and is probably far different from the original strain.
To maintain the characteristics of the parents, cultivars of perennial herbs should be vegetatively propagated. Those grown commercially from seed must be selected for varietal characteristics; seedlings that don’t measure up should be discarded. Many annuals can be propagated from cuttings, and some growers use this as a means of maintaining a variety.
I feel that we, as gardeners, have a responsibility to try to preserve “old-fashioned” plants, and I applaud seed foundations that are establishing genetic seed banks for heirloom plants in an attempt to perpetuate certain varieties so that we won’t be left with an odd lot of hybrid seedlings.
Andy Van Hevelingen is a writer and wholesale herb grower in Newberg, Oregon.