All is not lost with blackened tomatoes

Wednesday, August 21, 2002

Sometimes the cause and cure for a gardening problem are immediately obvious, such as when tomato fruits have, on their ends, blackened areas that are sunken or flattened -- blossom end rot.

The cause of blossom end rot is calcium starvation of tomato fruits. Calcium is a component of cell walls, which just do not form when calcium is deficient.

But hold off running outside to dose the soil with eggshells, lime, gypsum or some other calcium source. Insufficient calcium in the soil might or might not be at the root of the problem. If the soil pH is near neutral or higher, soil calcium usually is adequate.

More commonly, the problem is that calcium is not getting to the tomato fruits. Roots damaged by hoeing or drowning in waterlogged soil could be the cause.

Even if the soil has enough calcium in it and the roots are functioning perfectly, tomato fruits still might be starved for calcium. Fruits are poor competitors, within the plant, for calcium. When a tomato plant is growing rampantly, most of the calcium goes to feed new leaves and shoots. (Calcium works its way through a plant in the transpiration stream. Leaves, having more pores than fruits, transpire more water and, hence, suck up more calcium than do the fruits.)

So much for theory. What is one to do? To avoid the problem next year, of course, check the soil pH. If growth was rampant this year, cut back on fertilizer next year. Because pruning decreases the size of the root system, and increased air circulation about a plant makes it dry out faster, staked tomatoes are more susceptible to blossom end rot than are sprawling tomatoes. (Nonetheless, this disadvantage might be outweighed by the earlier, cleaner, and larger fruits from staked tomatoes.)

For the remainder of this season, keep the soil evenly moist. Wide swings in soil moisture, especially if the season begins wet, then becomes dry, exacerbates blossom end rot. And finally, if you're hoeing to control weeds, put that hoe away. A thick, organic mulch keeps the soil evenly moist and smothers weeds without damaging the tomato roots.

There is some consolation. First, blossom end rot usually afflicts only the first tomatoes of the season to ripen. And even those fruits that are afflicted are fine to eat once you cut away that ugly black.

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