Planting seedlings indoors Part 3: light, soil, humidity

This is the last in a series of articles about how to start flower and vegetable seedlings indoors under artificial lights. We have already seen that we can get a jump on spring by providing the plants with the right conditions; the right kind (color) of light – a combination of red and blue wavelength light which is best found in a combination of Cool White and Warm White fluorescent bulbs used together, or Grow Lux bulbs used alone; the right intensity (brightness) of light, which could be accomplished with two four-foot 40-watt bulbs placed eight or nine inches above the seedlings; and the right duration of light-at least 12 hours but not more than 16 hours per day.

Now we come to the relationship between the amount of light present, the temperature of the soil and the surrounding air, and the humidity around the plants. The best daytime temperature for most flower and vegetable seedlings is 70-80 degrees. Placing your setup near a stove or furnace or providing bottom heat to the flats with a heating cable will accomplish this. At any rate, the soil temperature should not be allowed to fall below 65 degrees or uneven germination or seedling diseases may be a problem.

The best medium for seed germination and subsequent plant growth is a sterile commercial soilless mix containing some combination of finely ground peat moss, perlite, and vermiculite with a pH of between 6.0 and 6.5. These mixes also contain a very small amount of fertilizer, or starter nutrient charge, as they are called. Common trade names are Jiffy-Mix, Redi-Earth, Terra-Lite, and many others.

Once the seeds have germinated, the seedlings should be given a temperature of 70-80 degrees during the day and 60-65 degrees at night, depending on the crop. This daytime temperature allows the plants to maintain a high rate of photosynthesis for food production. During the night, however, the plants’ food production ceases and its other physiological processes speed up, and food produced during the day is consumed. Lowering the night temperature ensures steady, even plant growth, which makes for strong, compact plants for the garden.

Humidity is important too. Seedlings need a minimum of 35 percent relative humidity, and the optimum is around 60 percent. To achieve the latter, you may have to place your trays and pots on pebbles in standing water.

Air circulation is another vital component to good plant growth. You may consider running a room fan for this, since it makes more carbon dioxide available and decreases the incidence of disease.

Most seedlings are ready to be transplanted when they have grown their first set of true leaves. A true leaf is one, which looks like a mature plant leaf. They will usually appear two to four weeks after germination, depending on the kind of plant being grown.

Good luck with your spring seedlings. Let me know if I can help.

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