A Master Gardener's Journal

Care of Seedlings

by Cynthia Sabatini, Delaware County Master Gardener

After seeds have germinated, the gardener will need to consider the following factors: thinning seedlings, and light, moisture, temperature and fertilization requirements.

The need to thin seedlings depends upon how closely the seeds were sown and how close together they are growing. For example, if two seeds are planted in one small container and both sprout, one seedling may either have to be sacrificed or moved in order to give the remaining seedling the necessary room to grow. If a seedling needs to be sacrificed, “offer up” the weaker of the two. The weaker seedling should be cut with small scissors where the stem meets the soil. Because many gardeners have a difficult time sacrificing seedlings, they can attempt to transplant the extra seedling to another container. Before removing the extra seedling from the original container, prepare its new site (container) and make an appropriately sized hole. The seedling should be removed with some soil around its roots by gently tugging and then holding the leaves when transporting the seedling to its new site. In the new site, the soil should be tamped around the roots and the seedling should be watered.

Light is one of the seedling’s most critical needs. If light is inadequate, a seedling will be leggy (because it is reaching for the light), pale in color and weak in stature. For seeds started indoors, sources of light include windowsills, “greenhouse” windows (sometimes found in kitchens in place of a standard window), fluorescent lights, and grow lights. Regular light bulbs (i.e., incandescent bulbs) are not good sources of light, as they do not provide the proper spectrum of light. Additionally, these bulbs emit mostly heat. Cool and warm fluorescent lights in a 1:1 ratio suspended about 1 inch above seedlings can be used to provide adequate light. The actual number of cool and warm fluorescent lights needed will depend upon the length of the fluorescent tubes and the total length and width of the seedling containers.

If a gardener wants a “turn key” lighting system, grow lights can be purchased. Some brand names include: Hydrofarm® Jump Start Fluorescent Grow Light System and Growers Supply Compact Lite Carts. If a windowsill is used to provide light, the gardener should turn the containers every day so that the seedlings do not bend to one side.

Seedlings need to be kept moister than seeds during their early stage of growth. This may necessitate watering the seedlings every day. In later stages of growth, allow the soil to dry out a bit before re-watering seedlings.

Seedlings have lower temperature requirements than the seeds from which they sprouted. For example, pansy and viola seeds require soil temperatures between 60º and 65º F to germinate. However, pansy and viola seedlings will grow well at soil temperatures between 50º and 55º F. As with seeds started indoors or in an unheated greenhouse, the appropriate temperature for seedlings can be ensured with heating mats or cables.

A seedling should be fertilized once or twice weekly after it has developed its first set of “true” leaves. (“True” leaves are different than the cotyledons or leaves that first appear after a seed has sprouted into a seedling.) After the first true leaves appear, fertilize with one-half strength fertilizer, such as Miracle Grow, Rapid Gro or fish emulsion. Be sure to read the directions on the fertilizer bag/container and dilute to make a one-half strength solution. After 3 or 4 sets of true leaves have formed, the seedling can be fertilized with full strength fertilizer.

Note from Marion: Check back soon for the final 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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