Seeds are wonderous things. The smallest dustlike fleck contains all the necessary material and energy to grow into a plant: one or a pair of rudimentary leaves that store food, a tiny stem, the beginnings of a root system, and the hint of a new leaf bud, which will become the seedling’s first true leaves.
For many years, I have been raising a variety of herbs from seed. For a fraction of what it would cost to buy plants, I can raise a virtual army of seedlings to satisfy my need for tea, flavoring, and landscaping herbs. I can also grow some unusual herbs that are available only as seeds. Many of the perennials that I start from seed are, I suspect, better adapted to my particular growing conditions than purchased plants or roots from another region would be.
I thrill to the initiation of growth and to witnessing the first stirrings of life in seeds I have planted by my own hand—a certain perkiness, a swelling and break in the seed coat, then the tiny leaves unfolding and springing toward the light. What a way to start the growing season.
In the beginning, however, I had little success with growing herbs from seed. Too often, the seeds vanished in the soil without a trace, and I didn’t know why. Norman C. Deno’s pathbreaking book Seed Germination, Theory and Practice, first published in 1991, provided a new approach. Deno, a retired chemistry professor, challenged the old notion that seeds need only warmth and a little moisture to germinate. Although this is true for about 50 percent of plants from temperate regions, including the most important food crops and common garden flowers, the other 50 percent, as well as exotics of all types, desert and alpine plants, and wildflowers, require different treatment if germination is to take place. Until Deno’s extensive germination trials, the requirements of many of these species were little known.
Seeds or Plants?
When grown from seed, rosemary, lavender, thyme, and mint species show variations in leaf, flower color, habit, even flavor. Some gardeners enjoy discovering these differences, but if you’re looking for consistency, grow the species from cuttings or roots or purchase established plants—don’t bother with seed. To obtain lush plants the first season from such slow growers as rosemary and lavender, buying established plants is a necessity.
Cultivars (cultivated varieties) of rosemary, lavender (except ‘Lady’), thymes, mints, artemisias, santolinas, some salvias, and origanums also must be propagated by means other than seed (such as cuttings) to ensure that their offspring resemble the parents in every way, as must French tarragon, which doesn’t produce viable seeds.
Seed strains are selections that come true from seed. Among these are ‘Lady’ lavender (also faster growing than the species), various salvias, monardas, and others on the chart in this article's image gallery.
Seeds of every plant species are chemically or metabolically alive, although vegetatively “on hold”: Each contains one or more mechanisms to prevent germination until after it has been dispersed from the mother plant and conditions are favorable for its survival.
The seeds of many familiar garden annuals and vegetables require a period of dry storage, typically from one growing season to another, to destroy germination inhibitors within them. After a season of drying, however, the seeds gradually begin to lose their viability, although some kinds will remain good for years.
Germination of seeds encased in fleshy fruits generally is prevented by chemical inhibitors in the pulp that diffuse into the seed. In other kinds of plants, the barrier is physical, such as a hard seed coat through which water cannot penetrate to initiate germination.
Many kinds of seed have a temperature requirement. The seeds of desert plants germinate only at cool temperatures, a survival mechanism that ensures that the seedlings will have the benefit of some moisture before scorching temperatures set in.
Methods of treating seeds to overcome delay mechanisms, called conditioning, have been known for some time in a general way but had not been worked out in detail for many species. Deno’s great contribution has been to pinpoint the optimum conditioning regimen for the seeds of some 2,500 species of plants. He doesn’t include many herbs, but using his methods as a guideline, I’ve worked out treatments for the herb seeds I wanted to germinate.
Conditioning the seeds of a given species may involve one or more of the following strategies.
• Dry storage. Seeds are harvested when ripe, dried, then stored in a dry place, usually for six months, or until it’s time to sow them.
• Moist storage. The seeds of some herbs, such as angelica and sweet cicely, germinate best when fresh. The fresh, ripe seeds can be held for months in a moist, folded high-strength paper towel placed in a loosely sealed, labeled plastic sandwich bag. Store the bag at the recommended storage temperature. Check the towel periodically to be sure that it is still moist and remove any rotted seeds to prevent fungal disease from spreading.
• Exposure to warm (room temperature: 70°F), cool (refrigerator temperature: 40°F), freezing, or alternating warm and cool temperatures. Most seeds require warmth, but some also need a period of prechilling (moist chilling) or freezing to break down their chemical inhibitors. (This strategy is also known as stratification because it once involved layering seeds in sand outdoors.) Using the paper-towel method, you can stratify seeds indoors in the refrigerator or freezer. Alternatively, you can sow them outdoors in the fall and let nature take care of them; the seeds will germinate whenever temperatures are right, usually in the spring after a winter in the soil. Simply refrigerating or freezing the dry seeds in their packets does nothing to speed germination.
• Exposure to light. Many seeds will germinate whether exposed to light or kept in darkness, whereas others, such as those of feverfew and salvias, must have light to germinate. Gently press these seeds into the surface of moistened soil and place containers in indirect light. Most stratified seeds should be exposed to light in this way as soon as they are removed from cold storage.
A few kinds of seeds—those of salad burnet, fennel, and violas, for example—need a period of darkness before they will germinate. Cover these seeds with soil and lay a sheet of newspaper or black plastic on top of the container. Remove it as soon as the seedlings begin emerging from the soil.
• Soaking or chipping. The first step in germination is the entrance of water into the seed. (Even dead seeds will absorb water.) Soaking seeds with hard seed coats opens fissures in the coat so that water can enter. Place seeds in a cup and pour boiling water over them to cover (it won’t hurt them). After 12 to 24 hours, prick any seeds that haven’t swollen and resoak these in fresh boiling water. Plant swollen seeds immediately, before they have had time to dry out. Drilling a hole in the seed coat, chipping it with a sharp knife, or rubbing it lightly with sandpaper are other ways of letting water into the seed. Avoid the seed’s hilum, the small dent where it was attached to a pod. Large seeds may be held in a vise while you work on them.
The trick now is to choose the right conditioning regimen (seeds may possess more than one delay mechanism) and carry it out correctly. The seeds of even closely related plants may require different conditioning schedules. Fortunately, many seeds will germinate with less than perfect conditioning, but germination rates may be lower and the resulting seedlings less robust than when raised under the most favorable circumstances. Some recommended conditioning techniques may turn out not to be crucial. Lavender and dianthus, for instance, will germinate whether or not they are prechilled. Sowing these seeds shallowly or on the soil’s surface to satisfy a light requirement may be more critical. There still remains a good deal of mystery at the heart of seed germination.
Six years ago, inspired by Deno’s book, I began putting my new knowledge of germination into practice. Before sowing a single seed, however, I drew up a list of the herbs I wanted to raise indoors; these included slow-growing or reputedly difficult-to-germinate annuals, biennials, or perennials, as well as any I thought needed a head start in our short growing season. I made a chart with headings for herb name, when to start, conditioning method, sowing temperature, sowing depth, and expected and actual germination time. The chart, which I prepare anew each season, makes the actual sowing much easier and is useful later on when I evaluate the performance of my seeds. (My chart of the seeds I’ve sown is in this article's image gallery.)
Finally, the magic moment: time to sow the seeds! I took Styrofoam cups with a drainage hole poked in the bottom and filled them with well-moistened Pro-Mix, a commercial seeding formula. I sowed most seeds shallowly, either on the soil surface, pressing them in or just covered with soil. Mixing tiny or dustlike seeds with a little sharp sand enabled me to distribute them more evenly on the soil surface.
After placing the cups on a tray and wrapping it loosely in clear plastic, I set the tray over a plastic-covered heating pad set on “medium” (commercial heating mats work well, but I didn’t have one) and beneath a single 20-watt fluorescent tube, which provided a little light and heat in the dim, cool room. I kept the light on 12 to 14 hours a day and left the heating pad on until germination occurred.
The results of my first sowing were beyond all expectations. Everything I planted came up fast and abundantly—far faster than the seed packets and standard references had led me to expect. My only problem was what to do with all the seedlings. I attribute my success to my attention to conditioning techniques, sowing the seeds shallowly, and provision of steady bottom heat. Although I still sow some seeds outdoors (see the chart in the image gallery for some sure-fire successes outdoors), I find that germination is more rapid and rates are higher indoors, where I have more control over growing conditions. Speedy germination is of great benefit in a cool summer climate where every extra day in a plant’s life counts. More than anything else, I’ve come to realize that sowing seeds is a gardening craft to be mastered, not something to be taken for granted.