Making sense of greenhouses and cold frames

Cold-Frame, poly-tunnel, high- tunnel, low-tunnel, cold-frame greenhouse, high-hoop greenhouse, gutter-connected greenhouse. … Confused yet? These are all descriptions of growing structures that I recently found in the FarmTek Growers supply catalog. Let me try to sort this out for you.

The term "greenhouse" in its strictest sense, unless otherwise specified, denotes a structure with transparent covering to maximize light transmission, with heating/cooling systems, to maintain optimal growing conditions year-round. This is the Cadillac of growing structures.

Having said that, they do come in all shapes and sizes, with a multitude of different coverings and environmental control systems.

Quonset, Gothic, sawtooth, A-frame and gutter-connected are just a few of the terms that describe the various shapes of greenhouses. Most often the shape is functional in nature, but sometimes it is aesthetic, as you might see, for example, in a botanical garden conservatory. Commercial operations often utilize gutter-connected greenhouses – many individual units that are connected at the gutters, forming one large, contiguous structure. This makes for better energy efficiency and workflow. Home greenhouse designs tend to be simple A-frame or Quonset.

Coverings can include polyethylene, glass, tempered glass, polycarbonate and acrylic. Because of cost considerations, commercial greenhouses are generally covered with polyethylene.

The top-shelf covering material is tempered, low-iron glass, but it's also the most expensive. It offers the maximum light transmittance and is reserved for high-value crops. Polycarbonate and acrylic are becoming increasingly more popular due to their high light transmittance and heat-retentive properties. Some greenhouses use a combination to maximize light and balance energy costs.

Heating/cooling systems are as varied and operate with different fuels. Heating systems predominately are forced-air, but in-ground or under-bench hot-water radiant systems are becoming popular. Solar systems, once unthinkable because of cost, also are gaining in popularity. Fuels used vary considerably and fluctuate with price.

Cold frames are basically overwintering structures that are similar in structure to greenhouses, and can range from miniature to full-size. Cold frames, technically, are not heated and offer winter protection, but not the growing-on conditions that heated greenhouses offer. Some nurseries will utilize these, covered with opaque plastic, to provide cold and wind protection over the winter. And sometimes heating mats or in-ground heating coils are utilized in cold frames to help maintain temperatures above freezing or aid propagation.

The cost of the structure/covering is just the beginning. The necessary add-ons – environmental systems, benches, water systems, supplemental lighting and such – will add expenses to a fully functioning greenhouse.

These structures, particularly greenhouses, can be a big investment in both money and time. Choose one wisely.

ask Ken

Q. I would like to get a jump on the growing season. I have a small greenhouse that I will bring indoors for winter. I tried to use it last year, but my plants became too leggy to stand. When is the best time to plant inside and which vegetable should be started when? – S. Deneau, Chesapeake

A. Your issue is light, or rather lack of light. Naturally, the intensity and duration of sunlight is much less in the winter than summer. It is difficult at best, even in a sunny window, to start seedlings or maintain plants indoors in the winter without supplemental lighting.

The lack of light is only compounded when your seedlings are inside another structure inside the house. Your plants are not receiving ample light through your window and an additional material.

To answer the second question, you first need to determine when you actually need your started plants outdoors. This depends on the particular species and, of course, the spring planting conditions in your area. The starting time indoors will also vary considerably, depending on species and the environment you provide.

Usually this is divided into germination and growing-on time. I’ve found this resource at Harris Seed Co. (http://bit.ly/2nvUoRm) invaluable as a guideline for starting and growing vegetables/herbs/flowers transplants from seed. Germination times, temperatures, crop times and the like are here for most of the common plants. Remember that conditions vary from year to year and adjustments must be made as needed on the fly. Good luck.

Send questions to [email protected] or to Home + Living c/o The Virginian-Pilot, 150 W. Brambleton Ave., Norfolk, VA 23510.

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