By now your greenhouse is winterized and thoughts have shifted to the temptations of spring, fed by the glossy catalogs that arrive daily in your mailbox. The catalog’s pages are filled with glorious photos of bedding plants, vegetables, fruits, perennials, and tropical plants all vying for a spot in your garden. This eclectic offering represents the vastness of the plant world and the important aspect of plant reproduction. Most of the world’s plants propagate themselves by seed, a sexual reproduction process that recombines genetic information to promote diversity and virility among the species of the world. Seed starting allows the gardener to partake in this ritualistic activity of the promotion of life. It is one of the most infectious and enjoyable aspects of the gardening experience.
Seed starting does not require an elaborate greenhouse setup and can be done from the windowsill of your kitchen. However if you do have a greenhouse, you are afforded certain luxuries to the craft that might not otherwise be available to someone lacking the setup. The practical considerations that greenhouse growers may take for granted are essential to the gardener with only a windowsill.
The practical considerations are as basic as those you might have learned in grade school.
The first of these is soil or medium. A variety of seed starting mediums are available for purchase at garden centers or home supply stores. Depending on the type of seed you are sowing you may be using anything from sphagnum moss to vermiculite to something in between, probably the safest bet. A good seed starting mix should contain ingredients that promote good drainage yet don’t allow the mixture to be robbed of moisture, air circulation (seeds need oxygen for respiration), and prevent hardening so seeds don’t become recessed in cement like seed mixture. As you’ve no doubt seen, seeds can be germinated in any variety of containers ranging from professional plastic mold flats to cut off milk cartons, to peat pots.
Second, most seeds need moisture to germinate. Upon sowing seeds, you should ensure that the medium is moist. It should not be so wet as to be able to wring water out of it. If the medium is moist before sowing, seeds may be less likely to be washed away during that first watering. Moisture is essential throughout the entire period of the life of the seed. Water is necessary to awake the seed to its new conditions or to break dormancy, a process called imbibition. It is important that watering is consistent and thorough and that the media not be allowed to dry out prior to or immediately after germination. If you are working in a greenhouse, the maintenance of stabile humidity is probably not much of a problem. However, for the windowsill gardener it may be difficult to maintain the moistness of the medium. To combat this it is common to cover the container with a plastic foil or sheet. This would need to be removed as soon as germination is observed to allow for air circulation.
The third of our practical considerations is temperature. Many seeds are genetically programmed to only germinate when soil temperatures are above a minimum limit. In general, 70-75 degrees is a common range of above minimum soil temperatures required for germination. This consideration may become a factor of concern for seeds being germinated from the windowsill as the fluctuation of temperatures from day to night may be dramatic enough to promote dormancy, not germination. The home gardener can use the top of a refrigerator or a space next to a heater or heat source instead if windowsill conditions are less than desirable. For the gardener willing to spend the extra dollar, heating mats can be purchased that provide a constant supply of bottom heat to your germinating seedlings. It is important to note, at this point, that not all species of plants you will encounter like nice toasty conditions at the outset. Many plants require their seeds to be stratified, that is put through periods of cold, moist conditions before the typical strategies are used. Many references publish germination tables that reference specific temperatures and conditions for a wide range of species.
The fourth, and most subjective of our considerations, is light. Many seeds do not require light to germinate. In general, those seeds that are planted at any depth greater that ¼” do not require light to germinate. Often this can be judged from the size of the seed. Larger seeds are planted deeper and do not require light for germination. Smaller seeds such as those of lettuce, grasses, and impatiens need light for germination. These seeds are called photoblastic seeds. They are usually sown on the surface or under a thin layer of inorganic substances like sand or crushed gravel. Light is obviously needed after germination in order to promote growth and development. GG
Kelly D Norris is a contributing editor for Garden & Greenhouse and farm manager for Rainbow Iris Farm.