Seeds are dormant, embryonic plants developed from fertilized ovules. All seeds contain an embryonic stem, an embryonic root, and meristems. The seed is surrounded by a protective seed coat. In addition, monocot plants have a single large nutritious cotyledon, whereas dicots have two nutritious cotyledons. The cotyledons supply food to the seedling until it begins photosynthesis.
The most important factors for seed germination are a properly prepared seedbed, consistent moisture, and adequate temperature. Seeds from different plants have a wide range of temperatures and moisture conditions required to break dormancy. Dormancy occurs to protect the plant embryo from emergence under adverse conditions that could kill it while it is still young. The conditions can be quite specific and are provided with the seeds when they are purchased.
Temperature requirements range significantly; some plants require vernalization, whereas some will not germinate unless they have been involved in a fire, such as the seeds of the Ponderosa pine. Plants that require vernalization must be exposed to cold or freezing temperatures for a set length of time, followed by exposure to warm temperature. This happens naturally as the season progresses from winter to spring.
Pretreatments may be applied. Seeds are sometimes boiled in water to kill viruses or treated with pesticides to inhibit diseases that would kill the seedling. Seeds that need cold treatment can be put in a cooler or freezer or planted in potting soil and put outside over the winter. They can also be sown in the ground after the first frost so they will germinate in the spring.
Seeds of cool-season plants, such as peas and carrots, can be planted outdoors early in the spring. Warm-season plants like tomatoes and peppers are more sensitive to lower temperatures and can be started indoors (in a house, greenhouse, hotbed, or cold frame) and transplanted into the garden after the last average frost date. Cool-season vegetables are characterized as hardy or half-hardy and warm-season vegetables as tender or very tender based on their ability to tolerate frost.
Frost occurs when the temperature drops to freezing (32°F) or below and is visible as a white substance that needs to be scraped off of car windshields in the morning. It is frozen water vapor and can damage sensitive leaves. Frost pockets are regions where cool air settles, such as at the bottom of a hill, and are prone to frost earlier and later in the season then other areas. You can find out your last average frost date from the United States National Arboretum Web site listed in the Further Reading section of the book. Your local cooperative extension or a local garden center can also provide this information.
Seeds that are sown directly in the garden will need to be watered daily for at least several weeks unless there is sufficient rainfall to keep the ground moist. The seeds only need to be kept moist; heavy rainfall or irrigation may wash them away.
Seeds that are started indoors are planted in germination media in small pots or flats and provided with supplemental heat and light during late winter and early spring. Germination media is formulated from materials that absorb water and stay moist, such as peat moss, which is found in bogs and harvested for sale at nurseries. Some people are opposed to the harvest of peat moss because it takes a long time to grow in the wild, and will use other substances such as coir from coconuts or a well-ripened compost that has been passed through a finemesh screen.
Germination media may not provide many nutrients because it is assumed that the seedlings will be transplanted into a more nutritious soil or potting mix shortly after germination. If the pots or flats are placed inside a plastic bag or covered with glass, the plants remain moist and will not need to be misted. The plastic or glass creates a miniature greenhouse and retains the moisture from presoaked germination media. Commercial growers have dedicated germination rooms that supply the necessary heat and humidity and also supplemental light for seedlings that have developed their first true leaves and have started photosynthesizing.
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