Sunday, February 7, 2010

Starting Vegetables and Flowers from Seed

Starting seeds indoors makes a lot of sense in our part of the country. Setting out sturdy seedlings in late May or June will give you a head start on the growing season. In addition, you will find many more plant varieties available in seed packets, than can ever be carried on nursery shelves. Most importantly, you will have the satisfaction of growing exactly what you want and controlling at that happens to your young plants.

Seed starting is not difficult, once you understand the process and gather the needed supplies. It needn't be expensive, but some initial investments will pay off. It's ideal to have an artificial light source, as our winter sun exposure is not adequate to raise most seedlings. Two side by side 48-inch double tube shop light fixtures suspended over a table or counter top will provide enough light for several flats of seedlings. The fixtures can be fitted with two "cool white" and two "warm light" fluorescent bulbs alternated, or with full spectrum bulbs.

Heat mats under the seed flats will warm the soil and promote growth, but they are not critical to success. Start simple and see if you can catch the seed-starting bug!


To avoid damping off and other disease problems, use a commercial soil-less medium for seed starting. It's worth spending a little extra money for a quality mix labeled specifically for starting seeds. Do not spend money on a soil that includes fertilizers; each type of plant has different needs and timing for fertilizer and one size does not fit all.


You may start seeds in any container or flat or insert you choose. Many seedlings benefit from at least one transplanting. In general, seeds germinate best in a shallow tray or in small individual pots.


Labeling begins before the first seed goes into the planting medium. Record the complete name on a wooden or plastic stick to label each pot or each section of an open flat. A stick-on label may be used for entire flats or containers of a single variety. Also, record the plant date and any critical instructions such as "needs light to germinate" or "do not fertilize". Save original seed packet for future reference.


Each plant has a characteristic pattern of germination. Some germinate in only a few days; others need weeks. Some need light to germinate; others must be covered by soil. Some should be sown shallowly, others buried more deeply. Ideal germination temperatures may vary. Always read your seed packet carefully so that you can handle the seeds appropriately.


Once your seeds have germinated, you may find that the new sprouts are very crowded. Though its tempting to let all of the new tiny shoots grow, you will have better success if you thin them by removing as many as necessary to leave each remaining seedling with some elbow room. Seedlings in tight competition for space, water ans soil nutrients may survive, but they will not grow into vibrant plants. To avoid disturbing the roots, snip off competing seedlings at soil level with small sewing or craft scissors. Pulling them out will compromise the remaining seedlings.

Fertilizing your Seedlings

Once you see the second set of true leaves, you can begin to fertilize your plants. The easiest way is to use a soluble fertilizer (5-5-5 or 5-10-5) that you mix with water. Dilute the fertilizer to one-third strength, and use this solution to water the plants once a week. Don't overdo it; more is NOT better. Too much fertilizer, especially nitrogen, can harm plants and inhibit fruit development. Be careful not to get fertilizer directly on stems or foliage since it can burn delicate plant parts. Never apply a more concentrated solution than is recommended on the label.


When using artificial lights, keep the light source about an inch above the tips of the plants. Seedlings grow at different rates, so it can be a challenge to adjust the lights appropriately. If plants are in separate pots, shorter ones can be propped up to match the height of taller ones, or pots can be arranged in order of height under a slanted fluorescent tube. Seedlings will thrive on 12-14 hours of bright natural or artificial light per day, and 6-8 hours of darkness. This cycle is critical for some flowering ornamentals, but less so for vegetables.


Check the seed packets for suggested germination temperatures, which will usually be in the 70's to low 80's F. Following germination, daytime temperatures should be lowered to about 60-65 degrees F and night time temperatures to about 50-55 degrees F.


When seedlings have developed their first pair of true leaves (following the cotyledon "leaves"), they are ready to transplant to a deeper container. Prepare the new container and soil first so that tender roots are exposed to the air for the shortest possible time. Pre-water the new soil to uniform dampness.

Prick out each seedling with a narrow dull knife or similar tool. If possible, tease the roots gently free as a "root ball" rather than pulling them out of their soil. Hold the plants by their first leaves or cradle them plant and roots in your hand. Do not handle by the stem, which is easily damaged.

Settle the seedling into its new home, arranging the roots so that they spread without twisting or crimping. Firm the soil gently to seat them securely in the new container.

Leggy Seedlings

If your seedlings are looking leggy, with long stems between the nodes, they are probably suffering from inadequate light, excessively high temperatures or overcrowding. Try to correct the problem to produce shorter, stockier bushier plants.

Hardening off

Hardening off your seedlings is critical to their survival in the outdoor garden.

Hardening off is the process of slowly allowing greenhouse or indoor grown plants to adapt to outdoor sunlight, wind or cooler temperatures. Overexposure without gradual adaptation may result in damaged foliage and even death of the plant.

A few weeks before the proposed planting date (mid to late May in southeast PA), stop fertilization and being placing the seedlings outdoors in a protected area for a few hours a day. Early morning sun or light shade is optimal for the first few days. Avoid windy conditions. Bring the plants back indoors after a few hours exposure. If the weather turns unseasonably cold or windy, put off the hardening process until it is safe to continue.

Gradually increase daily exposure until the plants are accustomed to being outdoors for a full day, and be tolerate nighttime temperatures. Pay attention to water needs, as young plants can dry very quickly in outdoor air. Beware also of outdoor pests. Squirrels, rabbits, chipmunks, cats and birds can damage tender new seedlings.

Special treatments

Scarifying is the process of scratching or nicking the surface of a seed to promote germination. Often scarifying is used in conjunction with soaking, as water at the wound site causes swelling and further breakdown of the hard seed coat.

Stratification is the process of chilling seeds to promote germination. This happens naturally to seeds of plants that grow in very cold climates, but can be mimicked with refrigeration.

Rinsing seeds before planting will benefit some plants like parsley, which contain compounds that inhibit germination. Rinse water must be discarded and not used for other seeds or plants.

Germination test

If you have seed packets that are a year old or older, a quick test for viability is worth doing. Seeds kept cool or dry should be viable for several years. Place 10 seeds from a packet on a moist paper towel, fold it up, place in a plastic bag and label it. One or two weeks later, open the paper towel and count the number of seeds that have germinated. If five or fewer seeds germinated, you may want to throw the packet of seeds out and purchase fresh seeds. If seven or fewer germinated, you may want to sow your seeds thicker than you normally would.

Seed Starting Calendar for 2010

Week of… Sew Seeds For…
Feb 21- Early March Hot peppers

Sweet peppers, basil, parsley
Tomatoes, cabbage
Cucumbers, squash, pumpkins
2-May Start hardening off seedlings for outdoor planting

Source: Seed Starting Workshop for Master Gardeners; Feb 1, 2007
Penn State Extension


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