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Extending Vegetable Garden Season

Posted September 05, 2017

These techniques are relevant to anyone who grows plants in a temperate climate. They are primarily used for growing vegetables, but they can also be used for annual flowers or tender and semi-hardy herbs or any plant that you want to have live outdoors in any other than its usual season or climate.

Planting and Harvest Windows

One of the simplest ways to keep a steady stream of vegetables to your table is to pay close attention to the length of time required from seeding to harvest. Then, depending on the crop, you may be able to plant early and reseed often, and something will always be ready. This works particularly well with fast growing crops, such as many kinds of leafy greens or radishes. Alternatively, plant several varieties with different maturity dates, which can work well with crops such as cucumbers, determinate tomatoes, summer squash, and sweet corn, which have short periods of yield and then they're done.

Low Energy Efforts

We have already mentioned mulch and row cover in other contexts. Mulch can keep in soil moisture, prevent weeds, and add organic matter to the soil in the case of organic mulch. Used in the fall, it can also extend the harvest season by keeping the soil from freezing longer, thus making it easier to harvest root vegetables. In the spring, organic mulches keep the soil cool longer, which may slow down the growth of heat-loving crops, but may be a desired result for holding root crops in the ground a bit longer. Plastic mulch, on the other hand, has the opposite effect on soil temperature. Laid in early spring, it can warm the soil early, allowing the germination of seeds or success of warmth-loving plants much earlier than in bare soil.

Row cover can be used spring or fall, to hold in warmth closer to the plants. It can be used as an emergency frost-protection, draped over tender plants on spring or fall nights when temperatures might dip below freezing. Depending on how thick the row-cover is, it can provide up to 8‚ °F protection against frost. The same row cover can also act as a mini-greenhouse during the day. Since it blocks some wind and holds heat, temperatures under row cover tend to be warmer than outside of it. Heat-loving plants can be started earlier in the spring, or will put on more growth in cool weather, when covered by row cover.

Many gardeners devise home-made season extension tools, such as these 'hot caps' made of empty milk jugs.

Cold Frames and Hotbeds

Cold frames and hotbeds (left) are miniature greenhouses that utilize the sun's energy for heat. They are useful as season extenders, for propagating cuttings, for starting vegetable and flower transplants, etc. In the case of hotbeds, the sun's energy is supplemented by heating cables or fresh manure. These structures have movable sashes or lids, which can be hinged to allow for easy opening and closing. The lid will allow access to the plants inside and be the source of ventilation and temperature control.

During the day the sun's energy is transmitted through the lid and stored in the soil floor of the frame. At night, heat loss from the soil is slowed by the cold frame. Insulating the inside walls and covering the lid at night with old blankets can further reduce heat loss. A hotbed, in addition to slowing heat loss, would be able to replenish some of this lost heat due to its additional heat source.

Depending on your needs cold frames and hotbeds can be very simple or quite elaborate. Sizes can vary also. The back should always be higher than the front to allow as much sunlight as possible to enter the frame. You can purchase easy to assemble cold frames or build your own.  Although the frame can be built to any size, a 3-foot wide by 6-foot long structure is convenient for home gardeners. Wood such as cedar, black locust, catalpa, or cypress will last a long time but can be expensive. Other woods such as pine, spruce or oak are less expensive but will need replacing more frequently. You can make the front, back and sides for a 3 foot by 6 foot frame from a standard 4 foot by 8 foot sheet of plywood also.

Using stainless steel or galvanized screws instead of nails will allow you to take the frame apart so it can be stored for the winter, thus increasing its useful life. Conversely, winter might be a good time to use the cold frame, and have a full-grown crop of cold-hardy greens in there for occasional harvesting over the winter. Automatic vent controllers are reasonably priced and these will open the lid on the cold frame to maintain a preset temperature inside. These can be very advantageous since temperature control during the day can be one of the more difficult problems to overcome when growing plants in a cold frame or hotbed. Old wooden storm windows with glass panes are often used as lids for cold frames, but be aware of the possibility of lead paint, which could contaminate your soil and crop. You can also use polyethylene film or acrylic plastic sheets as the glazing material, which would be incorporated onto a wood frame. Acrylic is a more permanent material having a life span of 10-15 years. It is usually available at home centers or hardware stores. A 1/8-inch thick material should be sufficient.

Location of the frame is very important. It should be constructed in an area free of any shade and built facing directly south. A site with a slight slope will allow rainwater to run away from the frame lessening the problem of water accumulating inside. It should also be conveniently located. Consider having a source of water nearby so that plants inside can be easily watered. Many times plants may have to be brought indoors when very cold temperatures are expected so locating it near your home can be advantageous. If you are using heat cables, a nearby source of electricity will be needed. Some people choose to locate the frame against the home so that the back is actually "made" of the foundation wall of the home.

A cold frame structure is simply placed over a cleared area of soil. Soil can be mounded against the base of the structure to prevent heat loss and wind from entering. A hotbed, on the other hand, will be located over a bed of compost or composted manure and soil. Usually a hole slightly wider and longer than the dimensions of the frame is dug to a depth of approximately 3 feet. Fill the hole with 24-30 inches of compost or composted manure and top this with 6-12" of topsoil. The frame is then placed over this manure bed.

Low Tunnels

Low tunnels are constructed out of small metal or plastic hoops (right), and a long piece of clear plastic sheeting. They also act as mini-greenhouses, like row cover does, only low tunnels tend to have better light transmission through their clear plastic. On the down side, they require irrigation underneath, as rain does not penetrate, and need a careful eye to ventilation, as they can become quite hot underneath. They also do not keep insect pests off the crop, as low tunnels have openings to allow ventilation.


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