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Garden Q&A: Blossom end rot on tomatoes and whether ants help

Tomatoes can develop blossom end rot like this for a variety of reasons including soil issues, inconsistent watering, too much fertilization and intense light.
Tomatoes can develop blossom end rot like this for a variety of reasons including soil issues, inconsistent watering, too much fertilization and intense light. (Ellen Nibali/For The Baltimore Sun)

My tomatoes are getting a large dark spot on the bottom as they ripen. What causes this, and what can I do? It makes them worthless.

Your tomatoes are suffering from blossom end rot, a metabolic disruption from stresses resulting in fruit cell breakdown and calcium shortage in enlarging fruits. At first, dark blemishes appear on the blossom-end but can spread until the entire bottom of the fruit becomes dark, shrunken and leathery. Factors that encourage blossom-end rot include low soil pH and low levels of calcium, inconsistent water levels, high temperatures, intense light, and excessive nitrogen fertilizers. It also can be an issue when cold spring temperatures slow growth.

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The key in summer months is to maintain consistent soil moisture while the fruit is developing. Easier said than done, but soil moisture levels need to be monitored as best you can. When you see all the tomatoes on a cluster with blossom end rot, you know the soil moisture fluctuated greatly.

Foliar calcium sprays can help, but can’t overcome soil moisture extremes. Leaves can serve as shade to moderate temperature and sunlight. Shade cloth is an option. Dispose of affected fruit and stay on stop of soil moisture management. It is more common in pear-shaped tomatoes and does not occur in cherry tomatoes. Pepper, eggplant, pumpkin, squash and watermelon also can get blossom end rot.

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Gardeners often stir a handful of lime into each tomato transplant hole to ward off blossom end rot, but when you’re seeing blossom end rot on a yearly basis, do a soil test. It will determine if you need to add lime to raise the pH of the entire garden. Test results will recommend the correct amount of lime to add. (Search ‘soil testing’ on the Home and Garden Information Center website.)

I have a constant battle with ants along the foundation of my house. They mostly congregate around decaying objects. They are now uninvited guests at our hummingbird feeders. I’ve been using sugar and boric acid on a paper towel on the ground, hoping they’ll take it back to their nest. Aside from the hummingbird feeder, are they beneficial or pests?

We can’t get along without ‘em. Ants are beneficial, absolutely. Their tunneling opens up hard compacted soil, so roots can grow through. Tunnels allow both water and oxygen — which roots must have — to penetrate deeper into soil. This is crucial for healthy plants, since our foot traffic and mowers are always compacting soil.

As ants tunnel, they churn up soil, too, thereby bringing upwards deep nutrients to where shallow roots can use them. Ants hate termites and keep they away, which is always good around foundations! As you observed, ants are scavengers.

By consuming the constant debris dropped by plants and animals, they clean up our environment. You might want to declare a truce with your ants, which sound like they are healthy and doing their job. Incidentally, special moat-like attachments are sold for hummingbird feeders that intercept ants.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Maryland’s Gardening Experts” to send questions and photos.

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