How to prevent blossom end rot in your tomatoes

Every summer, I get phone calls about problems with tomatoes in people’s gardens. The most common malady is a leathery rotten spot on the bottom of the fruit, and by the time people ask me about it, the growing season is coming to a close.

Blossom end rot can happen to both green and red tomatoes and starts by having a wet spot on the bottom of the tomato, which then turns into a black leathery patch and can enlarge to cover the entire bottom half of the tomato. It is important to note that this is not caused by a fungus or bacteria, but rather a physiological imbalance of water and nutrients in the plant. The imbalances that cause this are water stress and calcium deficiency.

The calcium deficiency can be caused by (1) insufficient calcium levels in the soil, (2) excessive or low amounts of water in the soil that interferes with calcium being taken up into the plant, or (3) excessive fertilization.

Most cases of blossom end rot in our area are not caused by insufficient calcium, but rather too much water or too little water. Large fluctuations in soil moisture can also trigger a plant to grow rapidly or slow down in growth, and start this disorder in the fruit. Consistent moisture that does not overwater or underwater, will give the home gardener a stable trend of growth, and help to prevent this issue. Avoid planting tomatoes into waterlogged soils that don’t drain well. One of the best ways to accomplish this is to grow your tomatoes in raised bed gardens where you can effectively control the moisture retention qualities of the soil by selecting a planting mix that drains well. Using mulch around the base of the plants can help to reduce soil moisture fluctuations, and conserve water. There are many organic mulches such as grass clippings, leaves, or compost you can use, or black plastic can be used to cover the soil under a tomato plant. But, do not use grass clippings such as mulch from a lawn that has had herbicides applied to it. Those herbicides can leach from the grass clippings and damage your tomatoes.

Selecting a variety that is less susceptible to blossom end rot is another good tool for preventing this issue. Good varieties that have a lower chance of blossom end rot are “Better Boy”, “Super Star”, and the cherry type varieties of tomatoes. Do not over-fertilize your tomatoes. An over-fertilized tomato plant has a hard time putting on fruit, can imbalance the growth of the plant, and ultimately the overall water uptake. Use a moderate amount of fertilizer, especially nitrogen, so that the plant is green and healthy, but not excessive in growth. The fertilizer requirements for each garden are different, so each fertilizer application will vary. Having an organic-rich soil to plant your tomatoes into will give them a good start to a successful growing season. Using composted manure is a way to increase the organic matter content, and give your plant a natural slow-release fertilizer. Organic matter also helps to loosen the soil and balance drainage issues.

Similar in looks, but different in cause from the blossom end rot is sunburn on peppers. It appears very equal in appearance but is caused by the fruit being exposed to direct hot sun. This issue on peppers is easily remedied by trimming back the top end of the branches while the plant is young, which causes it to become bushier and shade the fruit rather than it becoming sunburned.

For further gardening questions, contact Lance at (208) 624-3102.