One year your zucchini crop is so overly productive that you’re leaving them on every neighborhood doorstep, ringing the doorbell and running. The next year your zucchini, or other summer squash, barely reach finger-length before the ends turn soft and rot. And you certainly don’t want to admit you’ve failed at zucchini-growing when everyone else is peddling wheelbarrows full of the baseball-bat-sized vegetable.
What causes zucchini and other summer squash to rot instead of developing normally? The problem is actually quite widespread.
Sometimes it’s caused by poor pollination if bees are scarce, or cloudy damp weather reduces pollinator activity.
But the usual cause is blossom end rot. If this sounds familiar, it’s because it’s the same malady commonly troubling tomatoes in which the bottom of the fruit, opposite the stem end, turns brown/black and deformed.
Blossom end rot is a common problem with tomatoes, peppers, zucchini, yellow summer squash, cucumbers and watermelon. It’s not caused by disease organisms, so it doesn’t spread like a contagious disease from plant to plant, and it doesn’t carry over as organisms in the soil.
It’s called a physiological disorder, meaning instead of disease-causing organisms, it’s caused by something in the environment (soil, air, water, sun) that adversely affects plant growth. Blossom end rot is caused by the inability of plants to utilize calcium that is present in soil. Although our soils have plenty of calcium, certain conditions interfere with the root system’s ability to access the mineral.
The causes of this calcium-uptake interruption can be root disturbance from too-close cultivation, applying too much nitrogen fertilizer, or most commonly, fluctuations in moisture. Soil that is too dry, too wet, or when it swings between wet and dry, disrupt roots’ calcium uptake. The plant compensates by sucking calcium from the end of the fruit (the blossom end) opposite the stem attachment. The end of the fruit becomes discolored, shriveled, soft or rotten.
How can blossom end rot be prevented on any of the susceptible vegetables? Avoid disturbing roots when weeding or cultivating. Avoid high nitrogen fertilizer. Prevent overly wet or overly dry soil and strive to prevent wide moisture swings. Watering once or twice weekly is usually enough. If the plants are in containers or raised gardens, more frequent watering is usually needed. Mulching soil with straw, compost or dried grass clippings helps greatly. Because calcium is plentiful in regional soils, adding more calcium isn’t recommended. Foliar sprays of calcium have also been found ineffective. The internet advice of Epsom salt applications have also been debunked by university research.