Researchers have never shown that gardening gets into the bloodstream, but I, for one, am firmly convinced it does. Few if any gardeners don't long for a chance to grow something this time of year. It has been just long enough to forget about pesky weeds, lawn mowing, insects and other irritants to our summer gardening. Now we wish for sunlight, warm temperatures and good growing conditions.

Conditions in our area require a greenhouse for successful winter plant production. Growing structures vary from small, inexpensive cold frames built from scrap lumber and polyethylene plastic to elaborate free-standing or attached models worth thousands of dollars. It seems that anyone who gardens would want a greenhouse. That simply isn't practical or even necessary. Greenhouses are not tropical paradises transported to northern Utah, and they can be expensive, time-consuming and troublesome.Do a brief analysis of growing needs and desires when considering off-season growing structures. First, what do you want to produce indoors? If you are avid about growing indoor plants, a greenhouse may be just what's needed. If you are only trying to produce a few dozen transplants for the garden, you may elect to spend your time and money in other ways. Second, don't fall into the trap of thinking that a greenhouse will reduce the grocery bill. Producing vegetables off season is expensive and rather difficult. Just as there is no good argument, as far as saving money, for hobbies like golf, fishing, and others, there is none for a hobby greenhouse. Enjoy it as a hobby. That way you won't be disappointed when it costs money.

Greenhouse styles are as varied as other buildings. Coverings also vary considerably. The least expensive is polyethylene. Polyethylene lets in sunlight and traps heat. It is the lowest-cost material available, but it lasts less than one season. Ultraviolet-resistant polyethylene is more expensive but saves labor each year.

Corrugated fiberglass is another popular greenhouse covering. It's widely used on commercial greenhouses and is a good choice for backyard structures. It is flammable, so don't use it on greenhouses attached to the home. Buy greenhouse quality fiberglass coated with Filon or Tedlar to ensure a long life and high light transmission.

The first greenhouses were glasshouses. Glasshouses waned in popularity but are popular once again. They are unsurpassed in quality, light transmission and durability. Their biggest drawback is breakage. Glasshouses require careful construction. Use tempered glass to prevent serious injuries in the event of breakage. Sliding glass door panels are widely used because they are strong and constructed from tempered glass. Double glazing reduces heat loss.

There are other products for covering greenhouses. Polygal and other double-walled plastic materials combine the light transmission of glass with the shatter-resistance of plastics. These products are available at local plastic suppliers, but they are expensive.

Of course, other things to consider for a greenhouse are heating, electricity, water and other necessities. Automation is a must; otherwise you will be spending hours each day trying to keep up with heating, cooling and watering needs.

If you own a greenhouse or have thought about building one, our hobby greenhouse class is a great educational experience. My co-worker, Jerry Goodspeed, and I teach the course to help gardeners decide the answer to the question, "Do I really want a hobby greenhouse?" Other questions covered are "How should I build my greenhouse?" "What should I cover it with?" and "How can I best grow plants in the greenhouse?"

Soil mixes, watering techniques and pest control are other topics. Classes are January 8, 15, 22, 29 (Tuesdays). Choose either the afternoon class from 2-4:30 p.m. or the evening class from 7-9 p.m. Cost is $10 to cover the materials and handouts for the course. Send registration to USU Extension Service, 2001 S. State St., Room S1200, Salt Lake City, UT 84190-3350. Call 468-3170 for more information.

College level courses offered through Utah State University in Salt Lake City will be held during winter quarter. Turf management (Plant Science 420) is on Wednesdays, 6-9 p.m. Floral design is on Thursdays, 6-9 p.m. For more information contact Vince Lafferty, Life Span Learning Class Division, at 562-9677.