Growing vegetables in changeable weather

Tomato growing out of Wall O'Water. These
tomatoes are the same ones that were protected
from the hail discussed in the previous post.

By June 9 you might expect the weather to be consistently warm to support the growth of warm season vegetables. At Denver's mile high elevation it isn't so. Low temperatures last night reached 43 degrees F and tonight's low is predicted to be 49 degrees.

Plants such as tomatoes and peppers are set back by lows under 50 to 55 degrees F and take days to resume growth. Development and harvest of your tomato or pepper crop is delayed from the labeled days to harvest number for the variety. That number is based on favorable growing conditions.

Tomatoes growing in Wall O'Waters with a source of heat (warm water in side channels) fare better. Even though the tops may have grown out of the water walls the heat is enough to moderate a cold night and keep the plant actively growing for a close to on-time harvest.

Climate modification through devices such as Wall O'Waters is important for vegetable gardeners to practice in our changeable Front Range Colorado climate.

Squash seedlings growing in a hill.

On another subject I was recently asked why it is recommended that squash and other vine crops be planted as a group in hills. The answer is soil drainage. In addition to last nights cold, my rain gauge measured 0.8 inch of rainfall yesterday. That's enough to saturate and water-log soil without good drainage. This can lead to stunted growth and root rots particularly in our compact clay soils. Although gardeners may get tired of hearing people recommend the virtues of soil improvement for growing vegetables, it is good advice.

Photo credit: Tomato growing out of Wall O'Water, Squash planted in a hill - both Carl Wilson

Telling powdery mildew by its spots

Powdery mildew can be a chronic late summer and fall disease in the vegetable garden. I’ve found that some people mistake the normal color variation in leaves for the disease. Many squash have silver-white blotches like spots on a leopard. See photo of the normal blotches on the leaves of the All America Selections winner, ‘Papaya Pear’ squash right.

When squash becomes infected with powdery mildew, the dusty flour appearance of the disease looks more irregular and is on the surface instead of being part of the leaf. See photo left. Note that the disease appears on the upper leaf surface, not leaf undersides. Infected leaves can turn yellow, become distorted and fall prematurely (photo below right).

The severity of the disease depends on many things including plant variety, age of the plant, health status of the plant and weather conditions. Young, succulent growth is more susceptible than older plant tissue. Avoid late summer applications of nitrogen fertilizer.

Powdery mildews tend to be severe in warm, dry climates. High relative humidity is needed for fungal spores to germinate once they land on leaf surfaces. Shaded sites with poor air circulation favor disease. This is another reason to grow in open, full sun locations. These sites promote plant health by having ample sunlight for photosynthesis. They also ensure that humidity around leaves is quickly dispersed by adequate air movement.

Ground applied (drip) watering rather than overhead sprinkling also helps humidity control. Water in the morning rather than evening to take advantage of sunshine that quickly dries leaves.

If these cultural controls are not adequate, supplement with chemical applications of potassium bicarbonate (preventive) and neem oil (eradicant after infection). Read all label instructions and make sure the product you purchase is labeled for use on squash.

Photo credit: Healthy ‘Papaya Pear’ squash leaf, Powdery mildew infestation on squash leaves, Severe infestation and leaf yellowing – all three Carl Wilson