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Extending the Growing Season

By Skip Richter,
Contributing Editor

Here in Texas we gardeners enjoy a very long growing season. In the southern parts of the state gardening continues year round, while in the northern areas the winter cold is but a brief interruption, especially compared to some of our northern neighbors, who may sit for months waiting for the bitter cold to end.

However, in most parts of Texas a few really cold snaps can put an end to an otherwise nice cool-season garden. It is a shame to lose a warm-season crop to an unusually early frost when there are still a few more frost-free weeks to follow, or to have an unseasonably hard freeze shut down an otherwise winter-hardy plant.

You can grow vegetables 12 months out of the year in your Texas garden. In most of the state, the heat of summer makes that a more formidable “dormant season” than the cold of winter. You can take measures to protect against cold, but I have yet to see a Texas gardener set up a plastic tunnel over a row and place a window AC unit at one end… although the thought has occurred to me.

So with summer being such a challenge, it would be good to get more gardening in during the cool season. In fact, there are a number of ways to extend the season, whether it is to get a warm-season crop past the first or last couple of frosts or to protect a cold-hardy crop on a night when temperatures drop into the low 20s or teens. Here are a few tips and techniques to get more out of your garden this coming fall and winter.

Rowcover Fabric
Spunbound polyester fabrics come in several weights. The lighter weight products (approximately .6 ounce per square yard) are suitable for protecting plants from insects but not very helpful when frosts threaten. Heavier weight fabrics at around 1.2 ounces or more per square yard can protect plants against frosts and light freezes. They will provide a few degrees of protection by holding the heat of the earth beneath them and around the underlying plants.

When practical, the fabrics should be removed during the day to allow the sun to heat the soil and plants. Then in late afternoon or early evening, re-cover the plants for overnight protection. These fabrics are most helpful in protecting low-growing plants such as strawberries and vegetable gardens. Fruit trees and vines may be too large to cover easily and effectively.

Water sprinklers
Water can help protect plants against a frost or light freeze. Water provides some heat, being well above freezing as it lands on the plant. Additionally, as it reaches the freezing point, it gives off heat energy in the process of turning from a liquid to a solid (ice). As long as more water is applied to the surface of the ice, the heat released from the constant freezing process on the surface will prevent the interior temperature from dropping below about 32 degrees. While this will not be sufficient to help a warm-season vegetable, it can protect a fruit blossom or perhaps a semi-tender cool-season vegetable on a cold night.

Having said this, now here is the catch. On a long, cold night there is a double problem: the buildup of ice and, subsequently, the excessively saturated soil. Plants can be broken and crushed beneath the heavy load of ice. In a light freeze when the temperature only drops below freezing for a few hours, it is possible to use water to protect certain plants. But when the freeze is hard, say in the low 20s, and lasts for an extended period of time, water is just not a practical option.

Continue to apply water until the ice is melted from the surface of the plants. As the ice changes back to water, heat energy is absorbed, supercooling the plant. Thus, after air temperature rises above freezing, the plant can sustain freeze damage when covered in melting ice. This process is similar to the “evaporative cooling” that occurs when water goes from a liquid to a gas (vapor).

Another use of water is as a “heat sink” in the soil. Water the soil around plants prior to a freeze. This not only keeps the plants from being drought stressed and more prone to damage, but also enables soil to hold heat and release it more slowly on a cold night. Just do not overdo it. Soggy soil does not hold heat any better than adequately moist soil, and can be very stressful to plant roots that need oxygen to survive.

PVC hoop tunnels
A great way to extend the season with little expense is to build plastic hoop tunnels down garden rows. Sections of PVC pipe can be used to make a series of arched “hoops” spaced about 5 to 10 feet apart down a group of garden rows. Drive pieces of rebar or other thin metal rods into the ground leaving a few inches sticking above the soil line to slide the PVC pipe onto. Additional PVC sections can be tied to the hoops down the row on a centerline to provide added strength and support.

Next, cover the PVC hoops with clear plastic, available in most home supply stores. These products are in various lengths, widths and thicknesses, are quite inexpensive and will last through the winter season. Secure the plastic along the sides of the hoop tunnel with soil or bricks. The plastic at the end of the row should be brought together and held down with bricks also. This prefab greenhouse will provide considerable protection on a cold night and will increase soil and plant warming during the day. On a sunny day, it will be necessary to open the ends to prevent overheating.

Hoop tunnels make it possible to grow cool season vegetables all winter through most of the state. Lettuce seedlings go right through a bitter cold snap in their protective tunnel and growth is enhanced in the warm interior on winter days. Tunnels can be just large enough for a single row of broccoli or large enough to cover several rows. When you need to access the growing crops or perhaps pull a few weeds (God forbid!), just lift the plastic on one side and secure it back when you are done.

For additional protection, place milk jugs or clear soda pop bottles full of water next to cold tender plants beneath the cover, with their lids closed. The water will hold heat and release it slowly on cold nights, warming the plants they are set up against.

Light bulbs
Prize plants that lack cold hardiness, such as small citrus trees, can be covered for some protection, provided the cover extends to the ground so it will trap rising soil heat. To simply wrap a plant like a “landscape lollipop” will do little good. You can increase the cold protection by adding a source of a little extra heat beneath the cover. Light bulbs or a string of the large outdoor Christmas lights will provide enough extra heat to give a few degrees more protection. Check cords for bare areas and do not allow the hot bulbs to contact plant stems or foliage.

Cold frames
Cold frames are basically a mini-greenhouse with the soil as its heat source. Boxes of various types are constructed near the garden with a clear, hinged lid slanted toward the south. As with the plastic hoop tunnels, the sun warms plants and soil during the day and the cover slows heat loss during a cold night. Special gadgets, called solar openers, can be ordered from some garden supply companies that lift the top of a cold frame when temperatures rise on a sunny day, to prevent tender seedlings from being damaged.

Innovative gardeners have even made a temporary cold frame by placing four bales of hay against each other to form a “box.” Then they place a section of clear fiberglass or an old window over the top, or cover the entire structure with clear plastic secured to the soil on all sides.

Greenhouses
While most gardeners may dabble in the hobby of growing things, as time goes on, we eventually start to consider the idea of putting in a greenhouse. Greenhouses allow us to grow things all winter and provide an additional benefit of helping us get a head start on spring transplants and multiplying cold-tender plants for the following spring.

There is something therapeutic about puttering around a warm greenhouse full of plants on a cold winter day. They are really not that expensive when you consider all the money you will save on growing food for the table, growing your own transplants, starting your own cuttings, and this is not to mention savings on therapist fees.

(Author’s note to gardeners with non-gardening spouses: since your dear one just may not grasp the wonderfulness of having a greenhouse, you may want to re-read the previous two paragraphs and memorize the points for “conversation fodder” with your loved one, as you drop subtle hints for ideal Christmas-gift ideas.)

Greenhouses vary in cost from the elaborate glass houses that are truly architectural assets to your homescape, down to some very inexpensive kit houses that may be as little as $200, but may require a bit of do-it-yourself elbow grease. When choosing a greenhouse, keep in mind that, like a garage, they are never big enough! So plan for all the house you can afford, and if possible, choose a design that allows for expansion… just in case.

While a heater system is offered in more high-end greenhouses, for most small houses a kerosene or electric heater is adequate and only needed for the coldest of nights. Take extra precautions to prevent electrical shorts and fire hazards whenever using electrical and combustion heaters. My unheated but well sealed greenhouse has stayed above freezing even on nights when temperatures dropped into the mid 20s.

Cooling systems are not really worth the money. They can be expensive to operate and usually cannot do enough to keep the house cool on a warm summer day. One possible exception is the use of an evaporative cooler in the arid western portions of the state. Even so, for a hobby greenhouse, it is probably not worth the trouble.

It is important to have a vent fan, because even on a winter day when the sun is out, it can get too hot inside. Some well-designed units allow for the removal of lower side or end walls. This, along with some shade fabric will allow you to keep on using the greenhouse in spring and fall, when it would be just too hot otherwise. With our short winter season, it makes sense to have a house that turns into an open-shade house for the long warm season.

With so many options for extending the season, there is no reason a little cold snap here and there should deter us Texas gardeners from growing on right through the winter. So try a few of these gardening techniques and have a great, productive fall and winter season!