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Larry Sagers
The colder and darker it becomes in the winter, the harder it is to grow plants in a greenhouse.

Greenhouses are not magical. They attempt to re-create a suitable outdoor environment so plants can grow out of season. Providing that environment is sometimes more difficult than inexperienced gardeners realize.

Plants grown in a greenhouse need the same things outdoor plants need — warmth, light, water, mineral nutrition, air and space to grow.

Some of these are easy to provide in the greenhouse and cost you nothing. Others are expensive to provide, and the colder and darker it becomes in the winter, the harder it is to grow plants out of season.

First, consider warmth or temperature. With subfreezing temperatures, it is clear that temperature is a limiting factor right now. Want evidence? Look out your window and see what is growing in your garden. Everything is dormant, and other than hardy evergreens, nothing looks alive.

How warm does it need to be in a greenhouse for plants to grow? The temperature needs of different plants vary, but a convenient way is to divide plants into cool-season and warm-season crops. Warm-season vegetables include tomatoes, peppers, squash and melons, while cool-season vegetables include peas, lettuce, cabbages and radishes.

Flowers can be divided into similar groups. Some cool-season types are buried under the snow right now. These include pansies, wallflowers, foxgloves and many others that will survive nicely and bloom this spring. Warm-season flowers are those killed by the frost last fall, including petunias, fuchsias, impatiens and nicotiana.

The warmer you keep the greenhouse, the more fuel it uses and the more it costs. Keeping a greenhouse above freezing and growing cool-season vegetables and flowers is much less expensive than keeping it near 80 degrees and trying to grow tomatoes.

Light is the other expense. High-intensity lighting is expensive to buy and install, and it is even more expensive to operate. Since our days are so short and overcast right now, light is often the most limiting factor.

The cheapest and most effective way to deal with the limited light and low temperatures is to shut down the greenhouse or scale back what you are growing. The expense of trying to force the plants to grow is often not worth it when even Mother Nature isn't providing the basic needs of the plants.

In my own greenhouse, I'm not providing supplemental heat or artificial light, and it has been easy to see which plants are doing well and which are not.

The caladiums, which are tropical bulbs, did OK until the really cold weather arrived. Now they're dead. Other flowers have managed to stay alive, although they won't grow much until February when the more intense sunshine adds both light and heat to my greenhouse.

I knew when I built my greenhouse I would be depending on solar heat and, therefore, would not be successful in growing warm-season crops year round.

The other growth needs — water, mineral nutrition, air and space for your plants are much easier to provide. Most gardeners tend to over-water plants in the winter. Culinary water is suitable for most greenhouse needs. If your water comes from a different source, get it tested to make certain it is suitable.

Occasionally, people make the mistake of hooking into a soft-water system to irrigate their greenhouse plants. Never do that because softened water is high in sodium, which will damage your plants.

When growing plants indoors, most gardeners use a premixed soil-free growing mix. These do not contain nutrients, so get a good fertilizer and add it as needed on a regular basis. One frequent mistake gardeners make is that they often try to use fertilizers to overcome a lack in other growth factors.

Fertilizers cannot make a plant grow when light or heat are limited, so use them sparingly during the winter months. Fertilizers are salts and will burn the plants if they are not needed for growth.

Air is the one free thing plants need to grow. You do not need to worry about adding more air or changing the air. The plants in your greenhouse will extract what they need.

The last need is space. You are on your own there. If you are really into growing plants, you will probably complain that your greenhouse is too small. When you pay your heating bill the complaint will probably be that it is too big. Whatever you decide, I hope your plants, excluding the weeds, grow better in this new year.

Larry A. Sagers is the horticulture specialist, Utah State University Extension at Thanksgiving Point.