Tomato troubles, plastic covering and tasty veggies

What does it mean when the bottom leaves of my tomatoes turn yellow? It means that the bottom leaves are not healthy. If they turn yellow late in the growing season, it means only that the leaves are getting old. Yellow tomato leaves should not be occurring in early summer.

There are many causes for yellow leaves (and they usually start at the bottom of the plant) but most yellowing in most of the country comes from tomato diseases. Fortunately, in Montana we can forget about diseases like early and late blight. They are the curse of tomato growers in the eastern United States.

It is conceivable to have sickly tomatoes from poor air circulation. One summer I got the bright idea of protecting my tomatoes from wind by surrounding the plants with a vertical wall of shadecloth to serve as a windbreak. Within a few weeks the tomatoes looked sick-yellow leaves, spotted leaves, small leaves, dropping leaves. I quickly removed the shadecloth, and the plants recovered. That taught me the importance of moving air around tomato plants. It is one reason why I plant tomatoes at least two feet apart.

The most likely cause of early season yellow leaves on a Montana tomato plant has to do with water, either too much or too little. Leaves age and die from either problem; the symptoms are the same. Ideally, the top inch of soil around tomatoes should dry before they are irrigated, but not more than an inch should dry out. Then a generous amount of water should be offered, enough to last three or four days. That system of watering will encourage tomatoes to grow wide and deep roots, which will support healthy leaves.

Too much water is more likely to occur when tomatoes are growing in containers. In the open ground, water can drain away from the roots; in a container the soil may stay too wet near the bottom. Once the plants are large, though, they may need water every day, or even twice a day. Solve the problem by giving frequent thought to how much water is in the soil. In a container, do not water until the surface looks or feels dry. Keeping an inch of mulch on the surface will keep the dirt shaded, and its moisture will evaporate more slowly.

Why are my tomato leaves curling under? What do I do? The weather has been cool and that is how tomatoes react. You need do nothing at all. The leaf curl is harmless, and it will go away with warmer weather.

If I use heavy plastic to cover a cold frame or a greenhouse, will it last more years before it turns brittle and splits? Probably not. You also will have added a new problem: how much light makes it through the plastic to help the plants grow. What you want instead of thicker plastic is a special type of plastic called UV stabilized.

Plastic becomes brittle from the ultraviolet light in sunlight. When used at this altitude, the plastic is destroyed within a year or two.

UV stabilized plastic, available from greenhouse and garden suppliers, will last for several years. Also, it will block only as much light as glass does, permitting all the rest of the sunlight to pass through. Plants will grow faster and produce more.

Why do I imagine that vegetables from my garden taste better than ones I buy in a store? You are not imagining anything - the difference is real. Vegetable seed varieties are developed for specific qualities. Breeders working on seeds for home gardens aim for flavor above everything else. Seeds for commercial agriculture are bred to ripen all at once, to be picked mechanically, to withstand long distance shipping, to be uniform in shape and size, to remain in prime condition for several days, to look good on retailers' shelves. Flavor is not even on that list.

Hackett welcomes reader questions related to gardening, pest management, plants, soils and anything in between. Submit questions to mhackett@centric.net, call 406-961-4614 or mail questions to 1384 Meridian Road, Victor, MT 59875.