Greenhouse Space Heaters Need Air Mixing

Q. I have a small greenhouse I would like to heat at night to prevent the freezing of some seedlings. I am guessing I will only need heat for the month of February. I was thinking of a small space heater.

A. It is much easier and more effective to provide bottom heat to warm seedlings than a space heater that only warms the air. Provide bottom heat by using waterproof heating mats beneath the seedling containers or trays. They are available locally from nurseries and garden centers.

            Many vegetables that grow during the summer months require warm soils for good germination. Seeds like tomatoes and peppers will fail or germinate very slowly if the soil is not warm.

Years ago we would place seedling trays on top of the television or refrigerator where it is warm enough to complete or speed germination. As soon as they germinated, they would be placed in some light or directly under very bright lights.

Drape plastic or a light blanket over the seedlings and heating mat at night and remove it during the day. Seedlings require 6 to 8 hours of full sunlight, or the same intensity from electrical lights, as soon as they germinate. Sunlight intensities or durations shorter than this will produce spindly transplants that will grow and transplant poorly.

You don’t need expensive grow lights for growing transplants. Fluorescent lights or LEDs are fine but they must be placed within an inch of the leaves to produce enough intensity to support good plant growth.

Be very careful when using incandescent bulbs, or lightbulbs, for growing seedlings. They produce a lot of heat and damage plants at distances close enough to produce enough light intensity. I leave electrical lights on about 16 hours each day when growing transplants.