|Do Some Seed Need Special Treatments to Germinate?|
Most annual and even perennial seed need no special treatment to germinate. However, some have evolved over the centuries to need certain specific conditions to start their growth cycle, such as the scarification caused by going through a bird’s digestive tract, or the heat and chemical changes resulting from a forest fire. Some need total darkness to germinate, and others need total light, some have a hard, impermeable seed coat, and some will die if they dry out at all. For a quick reference as to which seeds require special treatments, see What Seed Need for Good Germination. Of course, the individual listings of seed and their cultural requirements include any special treatments that a particular genus and species might require.
At this point, let us examine the different basic treatments needed by particular seed, including scarification, soaking, stratification, chilling, moisture maintenance, and storage conditions.
1. Scarification - Some seed have seed coats so hard that they cannot absorb moisture. Of course, this usually means that they store very well over a long period, but it also means they need to have their coats broken, scratched or mechanically altered so water can interact with the seed to trigger germination. This process is called scarification, and it can be done with a knife, sandpaper or a file. This should be undertaken with care so the seed is not injured, which can prevent germination or allow pathogens to attack the seed.
In general, you can expect seeds of the Fabaceaee, Malvaceae, Cannaceae, Geraniaceae, Convolvulaceae, Solnaceae, Chenopodiacae and Aeacaceae families to have hard seed coats. Some seed (Pelargonium, for example) are shipped from the grower already mechanically scarified, which makes it easier for the home gardener to get them to grow, but also means they do not store well over a long period. Some seed are too small to be easily handled and mechanically scarified; these should be soaked to soften their hard seed coat.
2. Soaking - The germination of some seeds is greatly aided by soaking, which will significantly reduce the time required for them to germinate. Some have a hard seed coat that is softened by soaking, some need to have chemical inhibitors leached out to start germination. Soaking for 24 hours is usually sufficient. If more time is needed for a particular plant, change the water once a day to provide needed aeration and to prevent the buildup of microorganisms.
Place the seed to be soaked in a container and add 5-6 times their volume of hot (190 degrees F.) water. Never boil seed in water, as this will injure and even kill them. Plant the seed immediately after soaking, and do not let them dry out before sowing.
3. Stratification - This is the process of exposing seed to cold, moist conditions before sowing to trigger the germination process. It is necessary for seed that have an immature or dormant embryo when harvested, which includes most perennials and woody plants. Some plants, such as Lettuce and Delphinium, become dormant when exposed to temperatures over 75 degrees F. for an extended period; these must be chilled to induce germination.
There are a couple of methods to stratify seed, depending on your preference and the size of the seed. You may go ahead and plant your seed in your chosen container filled with moistened medium, place the whole container in a plastic bag and put it in the refrigerator (for temperatures around 40° F.) or freezer (for temperatures of 32° F. or lower) for the required time.
An alternative is to mix your seed in 2 to 3 times their volume of moistened medium, place them in a plastic bag tied and then store them in your refrigerator or freezer. When the chilling period is over, do not separate the seeds from the medium, but sow them together.
Another method of stratifying your seed is to place them outside for their chilling period, provided your area has the right cool temperatures for a sufficient time to meet the seed’s requirements. You can sow your seed in beds or in flats in late fall or early winter and winter them outdoors. They will germinate when the weather warms in spring. If you use flats, place them on the north side of the house away from drying winds and sun, sink them into the ground to just below their top or place them in a coldframe.
You will find specific time and temperature recommendations for individual plants that require stratification in the Flower or Vegetable section. Do not refrigerate or freeze the seed while in the packet for stratification purposes, and do not cold treat seeds in water. They need contact with air as well as moisture for the chilling to be effective, and you may send your seed into a second dormancy or severely injure them if you don’t give them proper conditions!
4. Simple and Double Dormancy – In speaking of seed, dormancy refers to any conditions that keep the seed from germinating. This, of course, is quite different from the dormancy of perennials, woody plants or bulbs, for which dormancy refers to a resting period between active growth periods.
When only one special treatment is needed (usually stratification), this is referred to as simple dormancy. Some seed, however, need more than one special treatment to begin their growth cycle, and this is called double dormancy. For example, some Ilex, Taxus, Viburnum, and some Lilies and Tree Peonies need a warm period (68-86 degrees F.) of three months in which the root develops, followed by stratification for one to three months which triggers stem development, before emergence of the growing tip will occur.
5. Wet Shipment – Seed of some tropical and woody plants are shipped fresh in moistened sphagnum moss immediately after they are harvested and must be sown immediately. If the seed coat is allowed to dry out, absolutely nothing will cause these seed to germinate! These seed include Anthurium, Philodendron, Ginkgo and Clivia.
6. Special Treatments for Seed Storage – One of the most frequently asked questions that gardeners ask me is "How long can I store my seed and still have them viable?" Well, there are stories of Evening Primrose seed lasting 50 years and Lotus germinating after 1,000 years, but these are definitely the exception, and not the rule! Most annuals and vegetables will store well for two to three years, provided they are kept in a cool dry place. Moisture frequently triggers germination, so until you are ready to plant, it is important to keep your seed dry. If seed are kept in an unopened, moisture-resistant package and not subjected to abnormal temperatures, most will keep perfectly well for several years. Some vegetables, such as beets, cucumbers and radishes, will even keep for as long as ten years.
If your seed packet is already opened, store the seed in a dry, airtight container in a cool place. It helps to store the seed as cool as possible (refrigeration helps) for reduced temperatures will lengthen storage life, and this is especially true for seed of woody plants.
As you might expect, there are some seed that just do not follow the norm in what they require, and you should not try to store them for any length of time as they will not be viable no matter what you do. These include Asparagus species, Tanecetum coccinium (formerly known as Chrysanthemum coccinium or Pyrethrum), perennial Delphinium, Dimorphotheca, Geranium, Gerbera, Kochia, Magnolia, Passiflora, Potentilla, Salvia splendens and Impatiens, as well as onions. The Wet Shipment group mentioned above should also not be stored.
NASA Seeds in Space
"Plants vary in their heat stress tolerance, not only from species to species, but also from cultivar to cultivar. In addition, unusual seasons-fewer or more hot days than normal-will invariably affect results in your garden, as will extremely dry or humid conditions," Dr. H. Marc Cathey, with Linda Bellamy, Heat-zone Gardening, How to choose plants that thrive in your region’s warmest weather.