Seed Dormancy - December 28, 2005
Jeff Schalau, County Director, Associate Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County
Most gardeners collect, purchase, and/or trade seeds. We do this because we believe these seeds will germinate and produce plants with predetermined values such as flowers, shade, food, aesthetics, erosion control, etc. Most times, little or no thought is given to what is inside the seed and what it needs to succeed. Understanding some basic seed morphology (physical structure) and physiology (how it works: metabolism) should increase your success in germinating seeds and maximize the plant values you are seeking.
A seed is contains a plant embryo, some energy containing material, and a protective coating. In broad-leafed flowering plants, the energy is contained in the seed leaves (cotyledons). In grasses, the energy is stored in starchy tissue called endosperm. Cereal grains have been bred to contain large amounts of endosperm which is ground to produce food. Other seeds store energy in the form of fats and oils.
The seed coat varies widely across plant species, but its functions are to protect the seed from damage. Some seeds also have structures that help them disperse by wind, water, animals, birds, and insects. Think of the winged maple seed that flies like a helicopter or the foxtail that adheres all to well to animal fur and clothing.
In uncultivated ecosystems, seeds of native plants seem to “know” when to germinate. Some seeds mature further after separating from the mother plant. Once mature, they lie dormant until the right combination of aeration, moisture, temperature, and light triggers their germination. Germination is no guarantee of survival, but it is a necessary first step.
Morphological (physical) seed dormancy can be related to maturity of the seed (ex. immature embryo) or a seed coat that is impermeable to air and water. Physiological seed dormancy is more complex and research to better understand it continues. Examples of physiological seed dormancy are when germination of the plant is prevented by insolubility of the energy storage compounds, by the presence of germination inhibiting chemicals, or other interactions. Morphological and physiological factors can also interact to affect seed dormancy within a given plant species.
Stratification is one of the most commonly used methods for artificially breaking seed dormancy. In general, stratification exposes the seed to low temperatures (33 to 41 degrees F) under moist conditions for one to six months. As you probably have guessed, stratification simulates winter conditions at the soil surface. It works very well for woody species and perennial wildflowers such as penstemons. While research-based information is available for many plant species, a general method is to put seeds between layers of moist paper towel inside a partially sealed zip lock bag and place the bag in the refrigerator for two months. Make sure the paper towel stays moist but not wet and sow the seeds gently in pots or prepared soil.
Other plants have hard seed coats that physically prevent germination by simply not allowing moisture to enter the seed. For these species, a nick in the seed coat with a file or sandpaper will increase germination. Sometimes heat is also used, especially in fire-adapted species. These techniques are called scarification. Commercial producers often used sulfuric acid to scarify large quantities of seed.
Temperature is a primary factor controlling seed germination of many plants. Cool season annuals and perennials germinate best during periods of cool temperature. Conversely, warm season annuals and perennials respond to warm temperatures of summer. Putting this knowledge to work, you should always sow seeds of cool season plants in fall (spring will also work for cool season annuals). Cool season turf grasses will be most successful sown (or sodded) in late September or October. Warm season vegetable crops, such as beans and squash, should only be direct seeded after soil temperatures have warmed up in May.
Light has an effect on seed germination of some plants. For example, some varieties of lettuce require light to germinate. Planting seed too deeply results in poor germination. Some weed seeds also respond to light. Mulching and tilling are often used to decrease germination of these species.
The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at firstname.lastname@example.org and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: December 22, 2005
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