Q: Our tomato fruits start filling out very nicely but almost overnight some fruits develop open bottoms, almost like it was sliced off. What causes this and what can be done? – Mike H.

A: Your photos show one of the most common tomato disorders, called blossom end rot, which results in sunken, leathery, soft or hard lesions on the bottom of the fruit. The disorder isn’t caused by disease organisms, which means disease control sprays aren’t effective for prevention.

The disorder is caused by the tomato plant’s inability to utilize soil calcium, and the plant sucks the nutrient from the developing fruit, resulting in the classic blossom end rot symptoms. Because most area soils are already plentiful in calcium, adding more can be dangerous.

To allow tomato plants to utilize existing soil calcium, consistent soil moisture is important. Calcium uptake is prevented when moisture fluctuates between wet and dry, or if close cultivation damages plant roots. Early fruits are sometimes affected until the plant overcomes the situation.

Mulching around tomato plants is one of the best ways to keep soil moisture uniform and lessen blossom end rot frequency. Use shredded bark, straw, or dried grass clippings from lawns that haven’t been sprayed with herbicides. Calcium sprays can be tried, following label closely, but results seem inconclusive.

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Q: My snow-on-the-mountain groundcover has made its way into my creeping juniper. What can I do to get rid of it, where it’s spreading among the juniper without harming the creeping juniper? – Barb O.

A: I wish I had an easy solution, but any herbicides that will kill snow-on-the-mountain will also harm the juniper. Even carefully painting glyphosate (original Roundup) onto the groundcover’s leaves runs the risk of collateral damage to the creeping juniper or to snow-on-the-mountain growing in its desired location as glyphosate can travel through the groundcover’s network.

Although it’s not a fun task, carefully digging out the groundcover where it has invaded the juniper is probably the safest route. Unfortunately, this will be an ongoing task, unless a deep vertical barrier is installed to keep the groundcover’s vigorous root system from invading neighboring territory.

Q: I have a Limelight hydrangea that I’d like to fertilize, but I’ve heard you’ll change the flower color if you use the wrong fertilizer type on hydrangeas. What’s the best way to fertilize them? - Sandy M.

A: Limelight hydrangea is a beautiful paniculata-type hydrangea, which is a hydrangea species well-suited to most soil types in the Upper Midwest. The pyramidal-shaped flower clusters open to a fresh chartreuse green in summer, turning pink by fall, making a beautiful landscape shrub.

Paniculata hydrangeas, like Limelight, do not change color in response to using various fertilizer acidities. For maximum growth, you can use granular 10-10-10 fertilizer, water-soluble types like Miracle Gro, or fertilizers formulated for shrubs. The best time to fertilize hydrangeas is in spring, just before periods of active grow. An application in early May, and again in June will provide good nutrition.

Fertilizing in the heat of midsummer isn’t recommended. Late summer fertilizing also isn’t wise because it can stimulate late-season growth that doesn’t have enough time to toughen up before winter, increasing the potential for winter dieback. At this point of summer, it might be best to delay fertilizing until next spring.

If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at donald.kinzler@ndsu.edu. Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.