Blossom-end rot is hands-down my least favorite gardening problem. It’s immensely frustrating when the tip of a developing squash or tomato suddenly turns soft and dark. What’s more, nothing will stop the rot once it begins.
However, it is possible to prevent blossom-end rot. Blossom-end rot is caused by internal calcium deficiency, not a pathogen. Remedy the deficiency to prevent the rot. This article discusses the symptoms of blossom-end rot, its underlying causes, and methods for preventing this problem.
Blossom-end rot affects most fruiting plants in the edible garden: summer squash, zucchini, winter squash, melons, tomatoes, eggplants, and peppers. The rot begins as a dark, depressed, water-soaked spot on the end of the fruit opposite the stem (see picture). In peppers, the rot may appear on the side of the fruit, resembling sunscald (see picture). The dark spot enlarges as the fruit grows and could end up being half the size of the fruit (see picture).
Inside the fruit, the affected area will be under-developed (see picture). Seeds are small or missing, and the solid tissue is mushy, but the rest of the fruit appears unaffected. That said, it’s best to cut affected fruit off the plant at the first sign of blossom-end rot. Nothing will stop the rot, and the developing fruit will sap calcium from the rest of the plant.
Causes and Prevention
Blossom-end rot occurs when plants have an internal calcium deficiency. The key word there is “internal.” Adding calcium to the soil is not necessarily the solution, and in most cases it is not. Typically, the underlying problem is that plants cannot move calcium from the soil to the fruit. Here are a few ways that can happen and how to remedy them:
Low soil moisture Plants need water to deliver calcium to developing fruit. Even if plants are growing in calcium-rich soil at the optimal pH, blossom-end rot is likely to occur when there isn’t enough water to move calcium throughout the plant.
The solution is simple: keep the soil moist. There are several ways to accomplish this:
- Irrigation. Most garden plants need about 1” of water per week. This works out to roughly 2.5 quarts per square foot, or 5 gallons in a 3-foot diameter circle. Do not over-water, though; this can lead to root rots.
- Mulch. A few inches of straw, grass clippings, or shredded leaves will reduce evaporation and the need to irrigate.
- Organic matter absorbs water and helps keep soil from drying out. A soil test will tell you how much organic matter your soil contains.
As much as possible, keep soil moisture levels consistent. In other words, don’t let the soil dry out. In hot, dry weather, this could mean irrigating every day or two.
Low soil pH Calcium availability in soil is strongly dependent on soil pH (acidity). Availability begins to decrease around pH 6.0, and is very low at pH 5.0 or below. The acidic soils in the Virginia Piedmont can therefore be a big problem in this regard, even though they usually are rich in calcium! High pH, above 8.5, also reduces calcium availability, but this situation is rare.
The solution is to raise the soil pH. Start by having your soil tested. The results will include soil pH and instructions for modifying it. You might want to contact the Piedmont Master Gardener horticultural help desk (email@example.com and 434-872-4583) for help interpreting the soil test analysis.
Note that it takes a long time – several weeks at minimum – to change soil pH. Autumn is a great time to do a soil test and make any needed amendments for better results the following year.
Excess nitrogen fertilizer Leaves compete with fruit for water and, therefore, calcium. Since nitrogen promotes leaf growth, an excess of nitrogen fertilizer can shift the balance in a growing plant from flower and fruit production to foliage production.
Plants need nitrogen, but how much nitrogen is too much? Unfortunately, soil tests usually do not measure nitrogen levels because these can fluctuate rapidly, depending on temperature and moisture. The general guideline for nitrogen is to supplement with 0.2 pounds (3.2 ounces) nitrogen per 100 square feet. For heavy feeders like tomatoes, supplement with 0.3 pounds (4.8 ounces) per 100 square feet. Note that these numbers are the weight of nitrogen, not fertilizer. Use the nitrogen content of the fertilizer – the first number on the label is the percent nitrogen by weight – to calculate how much fertilizer to add.
When should you add nitrogen? Here are some general guidelines. For a synthetic nitrogen fertilizer like nitrate of soda, work half of the total into the soil a few days before planting. Then add the rest to the top of the soil 4-6 weeks later. Organic nitrogen fertilizers like blood meal break down in the soil and release their nitrogen slowly, so it’s fine to add the whole amount to the soil before planting.
For details about how to fertilize particular crops consult the “Crop Specific Guides” section of the Virginia Cooperative Extension Home Vegetable Gardening webpage.
Cold soil Low soil temperature can be a problem in the spring. Cold soil reduces the ability of roots to take up calcium. What’s more, new transplants have small root systems with little capacity for nutrient uptake. Together, these effects make early “rounds” of fruit prone to blossom-end rot. These early fruits remain susceptible to blossom-end rot after the soil warms up. Fruit tissue that forms without adequate calcium remains weak and rot-prone even if the plant’s internal calcium levels eventually increase.
The solution is patience – and a soil thermometer. For cool-season crops like kale and Brussels sprouts, wait to transplant until the soil temperature reaches 50℉. For warm-season plants like tomatoes, peppers, cucumbers, squash, and melons, hold out until the soil hits 60℉ – though 65-70℉ is better. Virginia Tech’s “Home garden vegetable planting guide” has more information about the timing of planting for specific crops.
If you’re eager to plant in the spring, clear or black plastic “mulch” can raise soil temperatures by 10 or more degrees. Buy plastic mulch in rolls at garden stores, or use contractor-grade trash bags to cover smaller areas. Remove the mulch before planting to avoid overheating your transplants.
Calcium-deficient soil Low soil calcium is rare in the Virginia Piedmont but can be a problem elsewhere. Test your soil to determine if it’s calcium-deficient. If soil calcium is low, the test results will include instructions for amendments.
Don’t add calcium before having your soil tested. Excess soil calcium can block uptake of other nutrients, especially magnesium.
Foliar calcium spray: an ineffective solution
Spraying a calcium solution on plants’ fruit or leaves doesn’t prevent blossom-end rot. Fruit doesn’t take up calcium through its skin. In botanical terms, fruit lacks stomata, the pores where fluids can enter the plant. Moreover, while leaves do have stomata and can absorb external calcium solutions, the calcium in leaves is immobile and won’t move to the fruit.
The best overall practice for preventing blossom-end rot is to pay attention. Note how you treat your plants and soil, and how they respond to your interventions and to natural conditions. A garden journal is a fantastic tool for tracking what’s happening in your garden. And of course, soil testing!
It may take a couple of years to fine-tune your practices and minimize blossom-end rot. Everyone’s soil and local climate are different, and changing soil pH is slow. But with attention, care, and patience, you can mitigate this annoying issue and move on to worrying about blights, wilts, and insect pests.
References and further reading
Blossom end rot, University of Wisconsin Division of Horticulture.
Blossom end rot (PDF), University of Kentucky Cooperative Extension Service.
Blossom end rot and calcium nutrition of peppers and tomato, University of Georgia Extension.
Blossom end rot causes and cures in garden vegetables, Michigan State University Extension.
Blossom end rot in tomatoes and other vegetables, University of Minnesota Extension.
Blossom end rot on vegetables, University of Maryland Extension.
Fertilizing vegetable gardens, University of New Hampshire.
Fertilizing vegetables, University of Maryland Extension.
Gardening myths: fix blossom end rot with calcium sprays, Clemson Cooperative Extension.
Home vegetable gardening, Virginia Cooperative Extension.
Let soil temperature guide you when planting vegetables, Oregon State Extension Service.
Mulches for the home vegetable garden, Virginia Cooperative Extension.
Virginia home garden vegetable planting guide, Virginia Cooperative Extension.