With Hot Weather Comes Garden Woes

Triple-digit temperatures may be great for cactus, but not garden plants. Heat, wind and sun combine to cause a number of environmental problems on vegetables in particular. Most sensitive to these environmental stresses are the delicate flowers and developing fruit.

Pollination often suffers when temperatures are hot. Flowers may not be pollinated at all or they may have only partial pollination. Most commonly effected are bell peppers and squash. On bell peppers, fruit often develops misshapen and small. Cutting the pepper open reveals a definate lack of seed. In bell peppers, normal pollination and fruit set does not occur if temperatures are below 58 degrees at night or above 85 degrees during the day. In the desert, our wide swings between day and nighttime temperatures, make it very difficult to grow bell peppers. That's why local gardeners choose to grow hot peppers. They love the heat!

Squash are also effected by poor pollination. In the case of squash, and other cucurbits including cucumber, pollination is provided by bees. Hot weather reduces bee activity, and in-turn flower pollination. Plants poorly pollinated have fruit that turns brown, becomes shriveled, or remains small or flattened in shape. You can do the job of the bees by hand pollinating squash. It's just a matter of identifying the male and female flowers.

Female flowers of squash set atop an embryo, which resembles a baby squash. Male flowers do not have this attached embryo. What they do have are anthers covered with powdery yellow pollen. The anther can simply be cut off and rubbed against the stigma, at the center of the female flower, to achieve fertilization.

Once flowers are pollinated and fruit has formed it's important to maintain an even supply of water. Interruptions in watering can stress plants, but more importantly will interfere with the fruit's development.

Blossom end rot is a fruit disorder caused by interruptions in watering. It commonly effects tomato, pepper, cucumber and squash causing them to develop a round leathery brown or black patch on the bottom of the fruit. Inside, the tissue is hard and brown. The rot occurs when there is a lack of calcium supplied to the developing fruit. More than enough calcium is present in the soil. The problem develops when missed waterings disrupt the flow of calcium from soil to developing fruit. Therefore, to avoid blossom end rot, the soil must be kept evenly moist. Watering frequently will help. Using a straw mulch around vegetable plants is also beneficial as it helps to keep the soil from drying out between waterings. Straw bales can be purchased at the many livestock feed stores around town.

Over-exposure to our strong sun can also cause fruit injury. Tomatoes develop splits or cracks when over-exposed. Sunburned peppers turn brown and leathery. Corn kernels may not completely develop, or may form and later shrivel. Corn leaves exposed to the sun's most direct rays may develop a bleached appearance.

Growing healthy plants with a good canopy of shading leaves is the best way to avoid sunburned fruit. If plants are not producing sufficient growth, apply small amounts of nitrogen to encourage more leaves to develop. Don't overdo it! Too much nitrogen will overproduce leaves at the expense of fruit development. A water soluble fertilizer, such as Miracle Grow or Peters, works well for a "quick fix" of nitrogen.

Artificial shading will also help prevent sunburn. A simple structure made with wood framing, covered by shade cloth can be located to provide filtering of mid-day and afternoon sun. This can also help extend the production of vegetables into early Summer.

Finally, although we have no control over it, heat can cause cucumbers, squash, and melons to become bitter. Dry conditions also contribute to bitter fruit so keeping up on the watering may help. And with all environmental problems related to our desert heat and sun, it's best to select and grow varieties proven reliable in extreme-heat conditions.

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on May 14, 2000
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