A Master Gardener's Journal

Seed Germination

by Cynthia Sabatini, Delaware County Master Gardener

The gardener needs to consider the germination needs of seeds, which have a bearing on how seeds are sown and the success of the process. The following elements affect germination: moisture, oxygen, temperature, light/darkness, seed starting mix and fertilization.

Once seeds are sown in the sowing medium, they need minimum moisture. They should not be waterlogged, as this will cause the seed to rot and cause oxygen in the soil to be replaced by water. Seeds need oxygen to carry out various metabolic processes required for germination. If containers are covered with the appropriate wrap (transparent or opaque, which is discussed below), they will usually not need to be watered again until after the seeds sprout.

Generally, annual and perennial flower seeds germinate best when the temperature consistently ranges between 60º and 80º F. (Seed packets often specify the appropriate range for a specific type of seed.) If temperatures are higher or lower than the optimal range, then the germination rate (the percentage of seeds that sprout) will be negatively affected. Please note that the temperature required refers to the soil, not the air temperature. (Soil temperature in the spring is usually lower than air temperature.) When seeds are started indoors, heating mats or cables can be used to maintain the proper temperature range. These heat sources may also be necessary when starting seeds in an unheated greenhouse, because temperatures must be maintained during the night. Seeds need higher temperatures to germinate than is required for seedlings to promote growth.

Some seeds need light and others need darkness to germinate. The seed packet should indicate whether light or darkness is necessary. Seeds requiring light should be pressed into the starting mix, but should not be covered with a layer of mix. Cover seed containers with a transparent wrap, such as plastic or Saran, to accommodate the requirement for light. Examples of seeds requiring darkness to sprout include violas, pansies, wallflowers and sweet williams. The depth at which a seed requiring darkness is planted depends upon the size of the seed. A rule of thumb is that most seeds should be planted between 1/8 to 1/4 inch deep. Large seeds, such as sunflower and morning glory, should not be planted deeper than ½ inch. Seed packets often will indicate the appropriate depth. If a gardener is to err, it is better to plant a seed less rather than more deeply.

Seeds should not be fertilized until they have germinated into seedlings. Seeds carry enough food (carbohydrates) to promote germination. This is another reason why the appropriate starting mix should be used. Such mixes are not rich in nutrients because seeds do not need an “enriched” environment in which to sprout.

Note from Marion: Check back soon for the next installment in this five part series on starting plants from seeds!

Do you have a gardening question? In Delaware County, call our Master Gardeners hortline at 610-690-2671 on Tuesdays and Thursdays from 9 AM-12 NOON.

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