Beans (Phaseolus spp.) and tomatoes (Solanum lycopersicum) are frost-sensitive crops grown as annuals. Beans can be grown in United States Department of Agriculture hardiness zones 3-10, while tomatoes can be grown in USDA hardiness zones 2-10. These crops can be be grown as companions because both crops thrive in similar environments.
Beans are nitrogen fixing -- they convert nitrogen in the air to useable nitrogen in the soil -- so very little nitrogen fertilizer should be used when growing beans. They benefit from being mulched with 1 to 2 inches of compost or a low nitrogen fertilizer such as 5-10-10 and according to package instructions. Because of their ability to create nitrogen in the soil, they can provide living fertilizer for a companion crop of tomatoes. While pole beans can be grown with tomatoes, they do require extensive trellising, which could be crowded when used with tomato cages. Bush beans are a better choice. They grow up to 2 feet tall and 2 feet wide, so they make a nice low crop between tomato plants.
Tomatoes are heavy feeders, requiring a lot nutrients in the soil to thrive, but too much nitrogen can cause problems. Plants that have been over fertilized with nitrogen will be large and full, but they might not develop many blooms or set fruit. When companion planting with beans, use a low nitrogen fertilizer such as 5-10-10 or 6-24-24 to prevent too much nitrogen in the soil. Both indeterminate varieties, which grow until the first frost, and determinate varieties, which grow to a certain height, can be grown successfully with beans.
The key to healthy companion plants is proper spacing. Planting crops too close together creates competition for water and nutrients, while planting too far apart negates the benefits of companions. Place one bean plant in between your tomato plants. They will grow under the canopy of the tomato, while shading the ground. This will inhibit weed growth and reduce water evaporation from the hot sun, working almost like a living mulch. Spacing will depend on your variety, but most determinate tomatoes grow to about 3 feet wide, so place your tomatoes about 4 feet apart to allow space for a bush bean to grown under the canopy for each plant. Indeterminate varieties can grow 4 to 5 feet wide, so place these varieties 5 or 6 feet apart with the bush bean plant in between.
Fertilize your plants with a low-nitrogen fertilizer according to package instructions. It is better to water deeply and infrequently, so water well any time the soil is dry 1 inch below the surface. Harvest tomatoes and beans as soon as they are ripe. If fruit is left on the vine beyond maturity, fruit production may slow down or cease.
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