Greenhouse Growing Guide: Tomatoes

Tomatoes are prolific producers that don't require complicated growing procedures once basic conditions are met. Growing ripe, tasty tomatoes in a home greenhouse all starts with the seed. Be sure to purchase seeds that were developed particularly for use in greenhouses — reputable seed catalogs make this designation. Tomato varieties are divided into two different camps. Determinate, or bush, varieties are better suited for greenhouse conditions than their indeterminate counterparts. Indeterminate varieties form long, heavy vines that require substantial support and take up too much room for all but the largest greenhouses.

Starting the Seeds

Tomato seeds need warmth in order to germinate, so they're best started on heated mats under grow lights in your greenhouse. You can also use this method to get a good head start on the growing season if you're planning on growing your tomatoes out of doors. Start the seeds about six weeks before the last frost in your area if you're going this route. They can be started anytime if they're going to be grown strictly in the greenhouse, but most people choose to start them in autumn so they can enjoy fresh, ripe tomatoes in the middle of winter through spring. Don't be tempted to skimp and use recycled potting soil to plant your tomato seeds in, as doing so runs the risk of introducing pests and pathogens that can damage the plants into the greenhouse environment

Transplanting the Seedlings

The seedlings should be transplanted after 4 to 6 weeks in containers that hold at least 5 gallons. You should use 10-gallon containers for the larger tomato varieties. Wipe containers down with a solution containing nine parts of water to one part of bleach prior to transplanting, or use a product specifically designed to disinfect tools and containers used for cultivating plants in greenhouses. To ensure optimal stability of the seedlings, position the base of the stem about one half-inch under the surface of the potting mix. For best results, use a potting mix specifically designed for growing tomatoes. If this isn't possible, use a mixture of three parts standard potting soil, two parts mushroom compost, and one part each of perlite and coarse sand.

Light and Heat

Light and heat are essential to growing thriving tomato plants. Supplementary light won't be necessary if you're growing them in a greenhouse during the summer months, but you'll need to use high-pressure sodium lights for 16 to 18 hours each day if you're going to be growing tomatoes in the winter. Daytime temperatures in your greenhouse should be kept between 70 and 80 degrees Fahrenheit, while nighttime temperatures should never be allowed to dip below 65 degrees.

Water and Fertilizer

Your tomato plants should be watered when the top inch of the soil is dry. Use a 5-10-5 fertilizer at half-strength one month after transplanting and every two weeks after the plants set fruit. Always add the fertilizer first at each watering time and make certain to water thoroughly after it's applied.

You'll need to use tomato cages or stakes to keep your plants upright throughout the growing process. Keep a sharp eye out for any signs of common greenhouse pathogens such as leaf spot and blight — typical signs are leaf-wilt and small lesions on the surface of the leaves. You'll have to destroy any plants that come down with these conditions in order to prevent an infestation among the others. Tomatoes also attract whiteflies, but you can easily control them by applying insecticidal soap to the leaves of the plant every three days until you no longer see any trace of the insects.

When it comes time to harvest, the squeeze test is still the best way to determine when a tomato is ripe. Remember, a ripe tomato will be firm but give a little bit when gently squeezed. Fortunately, tomatoes have a fairly long pollination period, so once they begin to ripen, you'll be able to enjoy freshly picked tomatoes for weeks to come.


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