What Causes Blossom End Rot In Squash

Blossom end rot in squash is caused by a calcium deficiency in the developing fruit.
 
It shows up as a dark, sunken, leathery spot on the blossom end of the squash, which is the part that flowers first.
 
If you’ve been wondering what causes blossom end rot in squash, the short answer is that the fruit isn’t getting enough calcium when it needs it most.
 
This isn’t always because your soil lacks calcium; it’s often because the plant can’t move calcium to the fruit fast enough.
 
In this post, we’ll break down why blossom end rot in squash happens, what triggers it, and how to fix it so your harvest stays healthy.
 
Let’s dig in.
 

What Causes Blossom End Rot in Squash

Blossom end rot in squash happens when calcium can’t reach the rapidly growing fruit.
 
Here’s what’s really going on:
 

1. Calcium Needs to Move Through Water

Calcium travels through the plant’s water-conducting tissues, and it can only get to the fruit if the plant is moving enough water.
 
When that flow gets interrupted, the blossom end of the squash is the first place to suffer because it’s the furthest from the roots.
 
That’s why blossom end rot in squash shows up at the tip and not on the stem end.
 

2. Inconsistent Watering Is the Biggest Culprit

If your squash plants go through cycles of drought and then heavy watering, calcium delivery gets thrown off.
 
Dry spells slow the water flow, so calcium can’t reach the fruit, and then a sudden soak doesn’t fix the damage that’s already done.
 
Blossom end rot in squash is far more common in gardens with uneven moisture than in gardens with steady watering.
 

3. Soil pH That’s Too Low or Too High

Calcium is present in most soils, but it becomes unavailable if the pH is out of range.
 
Squash grows best in soil with a pH between 6.0 and 6.8, and when the pH drops below that, calcium locks up.
 
If the pH climbs too high, calcium can also become less accessible to the roots.
 
That’s why a simple soil test can reveal a lot about what causes blossom end rot in squash in your own garden.
 

4. Too Much Nitrogen or Potassium

Over-fertilizing with nitrogen pushes lots of leafy green growth, but it can crowd out calcium uptake.
 
Potassium competes with calcium too, so a fertilizer heavy in potassium can make blossom end rot in squash worse.
 
The fix here isn’t more calcium; it’s balancing your fertilizer so the plant isn’t overwhelmed.
 

5. Root Damage or Compacted Soil

If roots are damaged by cultivation, pests, or disease, they can’t take up water efficiently.
 
Compacted soil makes it hard for roots to spread, which limits their ability to pull in calcium.
 
Blossom end rot in squash often appears in gardens where the soil is heavy clay or hasn’t been loosened in years.
 

How to Fix Blossom End Rot in Squash

Fixing blossom end rot in squash starts with the soil and the watering can, not with a calcium spray.
 
Here’s what to do:
 

1. Water Deeply and Consistently

Give your squash plants about an inch of water per week, and aim for one or two deep soakings instead of light daily sprinkles.
 
A soaker hose or drip irrigation keeps moisture even and helps calcium move steadily to the fruit.
 

2. Test and Adjust Your Soil pH

A soil test tells you exactly where your pH stands and whether you need lime to raise it or sulfur to lower it.
 
Getting pH into the 6.0 to 6.8 range makes calcium available again, which is one of the most reliable ways to stop blossom end rot in squash.
 

3. Mulch to Hold Moisture

A layer of straw, shredded leaves, or grass clippings keeps soil moisture from swinging up and down.
 
Mulch also keeps roots cooler, which helps them function better during hot spells.
 

4. Go Easy on the Fertilizer

Switch to a balanced fertilizer and avoid heavy nitrogen or potassium blends once fruit starts forming.
 
If you’ve been feeding your squash a lot, cut back and let the plant focus on fruit instead of leaves.
 

5. Remove Affected Fruit

Pick off squash with blossom end rot so the plant doesn’t waste energy on fruit that won’t recover.
 
New fruit that sets after you fix the watering and pH should come in clean.
 

So, What Causes Blossom End Rot in Squash?

Blossom end rot in squash is caused by a calcium deficiency in the fruit, usually triggered by inconsistent watering, off-balance soil pH, or too much nitrogen and potassium.
 
It’s rarely a simple lack of calcium in the soil, so tossing in more calcium without fixing water and pH won’t solve it.
 
Keep your squash evenly watered, test your soil, mulch well, and feed lightly, and you’ll see far less blossom end rot in squash going forward.
 
Hope this post has helped you understand what causes blossom end rot in squash and how to keep it out of your garden.