Zucchini Rotting at The Ends: Treating Blossom-End Rot in Zucchini

Blossom-end rot affects a surprising amount of crops! Tomatoes, peppers, eggplants, squash, and zucchini are susceptible. Zucchini is particularly troublesome because the rot can spread from the butt up, ruining entire harvests.

The good news is this condition is easy to prevent and treat. We’ll ensure your plants avoid future rotting by looking at their cultural conditions. Fix the issues, and the rot goes away.

These guidelines also work well for other crops that experience blossom-end rot. It affects them similarly—look for ripe fruits with rotting ends. They’ll have mushy brown or black spots when you grasp them.

What Is Blossom-End Rot?

It’s not a disease, but a cultural condition. It looks like a bacteria or virus spreading from fruit to fruit but it’s a symptom of crops struggling to get their proper nutrients. Water, pH levels, fertilizer, temperature, and airflow are all factors that can cause it. 

This rot typically shows up halfway through your growing season, as zucchini vines start to produce excessively. The vines need lots of steady moisture and nutrients, and when they don’t get this they can’t ripen their fruits. 

How to Identify Blossom-End Rot

It’s easy to mistake rot on the blossom’s end for zucchini rot from inadequate pollination. Zucchini is monoecious, meaning it has separate male and female flowers on the same plant. If your zucchini rots before forming, and has a slight bulbous swelling near its stem, it’s most likely a pollination problem. 

Blossom-end rot differs from pollination rot in the way it spreads. Whereas pollination rot happens quickly after the flowers fade, rot on the blossom’s end takes longer to show. Look for large fruits that partially ripen and bulge before rotting at the end. These zucchinis look ripe, but their bottoms are uneatable, mushy, and black or yellow.

Look out for these other common symptoms when identifying this condition in zucchinis:

How Does Blossom-End Rot Occur? 

This condition isn’t always because of a calcium deficiency—it often happens when plants can’t take in calcium. Where soils are too acidic, dry, or cold, zucchini roots struggle to consume it. Fix the conditions, and the roots can access available calcium. 

When your crops get all they need and still have this condition, it’s most likely because of nutrient deficiencies. First, solve the cultural issues—if rot doesn’t go away, consider adding calcium-rich amendments that will boost the nutrient content in the soil over time.

These physiological settings cause problems when they’re out of whack—bring these factors back to baseline and your zucchini crops will ripen their fruits properly.

Watering Infrequency

Calcium and water availability have deep connections—when there’s too much or too little water, zucchini roots can’t suck up nutrients. Watering infrequency causes this, as the calcium in the soil can’t move from the particles to plant roots.

Shallow, infrequent watering also causes blossom-end rot. When plants suck up water, calcium comes with it. The roots can’t take in any calcium without enough moisture. Watch for heat waves or droughts, watering your crops plenty so they’re resilient. 

One easy method to ensure your vines have consistent moisture is by turning on a watering system with a timer. Do you not own a watering system? Simply set a timer on your phone to check on your zucchini three times a week. Dig your finger into the bed to see if it’s wet, and water well if it’s dry on the top two inches. Hold off on watering already moist ground.

pH Imbalance

Zucchini, like tomatoes and peppers, appreciates a neutral pH between 6.0 and 7.5. Acidic or extremely alkaline pH lowers Ca availability, making it impossible for your vines to form fully ripe fruits. They’ll swell and turn color, but rot on their butts in the last stages of ripening. You’ll want to amend your beds so its pH is around 7.0. 

Extreme Temperatures

Cold soils cause calcium deficiencies, which lead to zucchini end rots. This commonly occurs early or late during the growing season. Lingering spring frosts and early fall freeze shock zucchini plant roots. 

This also happens during excessive heat waves in dry beds. The hot temperatures affect water availability, which affects nutrient transfer and absorption. Keep plants cool during heat waves with shade cloth—maintain warmth during frosts with greenhouse plastic or row covers.

Low Calcium

If your plants have consistent moisture, good airflow, proper pH, and mild temperatures and still experience blossom-end rot, then it’s most likely a calcium deficiency. Calcium (Ca) helps this vegetable form ripe, dark green fruits. The mineral helps cells stay stuck together, creating tight, thick skin. When deficient, plants can’t ripen the skin and flesh properly. They rot it instead, turning fruits black, brown, or yellow.

A surefire way to know if your zucchini has Ca deficiencies is by getting a soil test. If the test comes back with low Ca levels, apply a fertilizer or amendment that has plenty of it. While there aren’t immediate sources of calcium for crops, prepping the ground ahead of the growing season helps.

Excessive Nitrogen

Minerals affect each other underneath the dirt. Too much nitrogen makes it nearly impossible for plants to take in micronutrients, which leads to zucchini rot. Avoid using nitrogen-rich fertilizer, and go for a well-balanced organic type meant for vegetables.

Prevention

Zucchini with this rot won’t fix itself right away—you’ll have to wait for more ripening fruits to see the condition disappear. Keep future rot away with these prevention techniques. 

Water Consistently

Ensure zucchini roots have a happy home with consistent moisture. Use a timer, or check your garden daily to see when the dirt is dry. Water as often as you need so the soil is moist but not soggy. I like to let the top inch or two dry before a deep soaking. 

Water deeply and infrequently rather than often and shallowly. Shallow waterings encourage short roots that are less resilient to drought. Deep waterings encourage them to reach further into the ground, creating strong plants with fewer blossom-end rot cases. Roots have better access to nutrients at a deeper level too.

Finally, avoid overhead watering as it causes powdery and downy mildew. Water plants near their base, and you’ll get their roots wet without touching the leaves. 

Balance Your Soil’s pH

Acidic or extremely alkaline soils make it impossible for roots to intake nutrients. Most vegetables prefer a somewhat neutral pH, and zucchini is no different. Use a soil test to determine pH levels, then add amendments to keep them between 6.0 and 7.5. 

If your soil’s pH is too high, add garden sulfur to lower it. Raise acidic levels quickly by adding calcium carbonate or crushed eggshells. Crushed eggshells are perfect for treating this condition because they raise pH and are rich in calcium. However, they aren’t available to plants until they’ve broken down, and are best used ahead of the season as a preparation. 

All amendments take weeks to months to take effect—adjust your beds this year for a better crop next year. Add compost or organic matter to your garden this season—it helps your vines despite the pH level.

Protect Weak Plants

Your plants benefit from extra protection during intense heat waves or fall freezes. Use shade cloth, a greenhouse product, to give ripening zucchini shade during the hottest days of summer. Temperatures above 100°F (38°C) are excessive, and your plants need extra shade and water. Other shade structures, like sun umbrellas, work well too—just be sure to give your plants some space from any materials with posts or canopies.

Early spring and autumn seasons surprise ripening fruits with nighttime freezes. Where this type of frost is common, use a row cover or UV-resistant greenhouse plastic to cover the plants. Drape the materials over posts to keep them off the plants.

Add Compost or Organic Mulch

A good dressing of compost injects organic goodness into your garden. It not only adds beneficial nutrients that bolster growing vines—but it also makes the soil more acidic with time. It’s really useful for alkaline dirt that needs amending.

Even when the pH is out of whack, compost helps plants survive. Microorganisms work to decay nutrients and make them more available to plant roots. Extra air pockets in this amendment allow roots to breathe despite clay or sandy texture. Compost has high porosity, meaning it can absorb moisture and hold it for plant roots.

When in doubt, add some compost. It’ll create humus-rich pockets of nutrition that vining plants reach into when they need it most. Use it or a similar amendment like leaf mold, straw, or grass clippings with dried leaves.

Treatment

Blossom-end rot doesn’t disappear from zucchini that already have it. Once they struggle to form cell walls and start rotting, this condition spreads from the bottom up. You’ll want to address the physiological issues first, then watch for new vegetables to form without rot. If they continue ripening and rotting, they need something they’re not getting, whether it’s water, calcium, or a proper pH level.

You don’t have to waste those rotting vegetables though! This condition is easy to mitigate in its early stages. Cut off the mushy ends and cook the rest of the zucchini how you like best. 

Some treatments help this condition stop spreading. This means the rot remains on zucchini butts, but it won’t grow any bigger. Use these treatment techniques to stop this condition in its tracks. Combine them with prevention methods and you won’t see this issue again.

Apply Amendments

Calcium feeds deliver nutrients to plants in dire straights. These ensure plants stop rotting while slower-acting ones maintain nutritional balance in the long run. A good rule of thumb is adding slow-acting organic amendments at planting. Bring out the faster-acting sprays, liquids, and foliar feeds if this rot is already occurring. 

These are excellent slow-acting calcium amendments to add to planting holes during early spring:

For quick fixes, apply a foliar feed. You’ll still need to address the root causes of this condition. Apply foliar feeds as a part of multiple techniques to mitigate rotting. 

Leach Nitrogen Out Of Your Soil

If a soil test tells you your soil has too much nitrogen, you’ll want to leach it out as quickly as possible. Excess nitrogen holds calcium hostage, making less of it available for your vines. Rather than starting over with new soil, water your beds for a long time to get nitrogen flowing. 

Do this by turning on a hose with low water pressure so it trickles slowly. Set the nozzle on your garden beds, and leave it until the moisture saturates the area. You’ll need to wait an hour or more for the water to seep enough nitrogen out of the dirt. Move the nozzle as you see fit to leach it out throughout your beds. 

One leaching session should be enough. Know for sure by taking another soil test a week later. If nitrogen levels are still higher than normal, set your hose to low and soak it out of your beds as many times as necessary. Note that other nutrients will leach out too, and your soil may need additional amending after this process. 

Wait It Out

Oftentimes, the first round of fruit displays this rot while the next round ripens correctly. Waiting it out may be all that’s necessary for this condition to vanish. If your garden meets all proper guidelines, wait for the second round of zucchini. Harvest your first round of rotting vegetables, cutting off any mushy parts before eating them.