1. The Right Temperature

Seedlings of different species require different temperatures for best growth, though most will grow successfully at normal room temperature of 60 to 70 degrees Fahrenheit. Those that need cooler growing temperatures are so noted in the Encyclopedia section of this book, and many tropicals will be happy with temperatures as high as 75 degrees F. In some species, the seedling prefers a temperature different from the germination temperature. Having a waterproof seedling heat mat with a thermostat is ideal for providing the optimal temperature for different species. Read your seed packet, and consult the listings in the Plant Reference section, under Vegetables or Flowers for the needs of the specific plants you want to grow.

2. The Right Amount of Light

Once the seedlings have emerged, be sure they have the light they need. Newly sown seed flats should not be in direct sunlight, which is often too hot or drying. However, once the seedlings have emerged, they will need additional light to develop into strong, healthy plants. In dim light they will grow too tall, with weak, spindly stems. Fluorescent grow-lamps or greenhouses are a big help, but a sunny windowsill will work well also.

3. Air Circulation

Seedlings also need air circulation for good health, so avoid locations that are too closed-in. On the other hand, you need to avoid cold drafts for warmth-loving kinds. In any case, be sure to remove the clear plastic cover from the flat as soon as emergence occurs, to encourage good circulation of air.

4. Water

Watering is also important. Once you’ve sown your seed, check the moisture of your growing medium every day. Moist medium is dark brown, becoming lighter in color as it dries. Touch the surface; if it feels dry, water from the bottom with water at room temperature. Allow water to soak up until the surface becomes moist, but not so heavily that the medium stays soggy. Aim for a constant, moderate degree of moisture. Germinating seed actually require less water than seedlings, but the surface of the growing mix should not be allowed to dry out completely either during germination or when the small seedlings are emerging. When your seedlings have their first or second true leaf showing, you can reduce watering to every second or third day, rather than daily. 

    Watch seedling stems carefully. If some of them fall down, the base turning brown and collapsing even though the leaves still look good, this may indicate "damping off". This is a disease that is encouraged by soil that is too wet, and by poor ventilation. Reduce watering and increase ventilation! A fungicidal drench may also help. On the other hand, if your seedlings’ leaves shrivel and droop, or if their stems bend over along their entire length but are not discolored, they are probably too dry. Soak the flat thoroughly.

5. Nutrients

Feeding your seedlings encourages optimal growth and health. Unless your seed starting mix is pre-fertilized, fertilize with a weak solution once a week once the true leaves have appeared. A weekly dose of watered down fertilizer is better than feeding with full strength solution once a month, as growth will be more even and burning of young plants will be avoided. When bottom watering young seedlings, mix the fertilizer into the water. I recommend 1/4 teaspoon of seedling fertilizer per gallon of room-temperature water. As the seedlings mature, you can increase the strength of the dosage to 1/2 the strength of the recommended amount on the label.

Your seedlings should be a healthy, strong green color. Yellowish green may indicate more nitrogen is needed, so fertilize a little more, increasing the dosage gradually so you don’t go to the other extreme and burn the small seedlings. A ‘burned’ look may indicate too much fertilizer in the planting medium. In this case, water the planting container from the top, using plain water, until it runs out of the bottom, thus removing excess fertilizer salts. Then reduce feeding rates.