What Causes Blossom End Rot On Summer Squash

Blossom end rot on summer squash is caused by a calcium deficiency in the developing fruit, which is almost always triggered by inconsistent watering rather than a lack of calcium in your soil.
 
That’s the short answer, and it surprises a lot of gardeners who rush out to buy calcium sprays.
 
The truth is, most soils already contain plenty of calcium, but the plant can’t move it to the fruit fast enough when moisture levels swing up and down.
 
So if you’ve been wondering what causes blossom end rot on summer squash, the real culprit is usually your watering can, not your soil.
 
In this post, we’ll dig into what causes blossom end rot on summer squash, why calcium matters so much, and how to stop it before it ruins your harvest.
 
Let’s get growing.
 

What Causes Blossom End Rot on Summer Squash

Blossom end rot on summer squash shows up as a dark, sunken, leathery spot on the blossom end of the fruit, and it happens because calcium isn’t reaching the growing tips.
 
Here’s why that calcium shortage occurs:
 

1. Inconsistent Watering

The number one cause of blossom end rot on summer squash is uneven moisture in the soil.
 
Calcium moves through the plant dissolved in water, so it can only travel where water flows.
 
When your soil dries out completely and then gets drenched, that stop-and-go flow disrupts calcium delivery to the fast-growing fruit.
 
This is why blossom end rot on summer squash often appears right after a dry spell followed by heavy rain.
 

2. Too Much Nitrogen Fertilizer

Overfeeding your summer squash with nitrogen pushes tons of leafy green growth.
 
The plant then pours its energy into those big leaves instead of the fruit, and calcium gets stretched thin.
 
So excess nitrogen is another common cause of blossom end rot on summer squash because it throws off the plant’s nutrient balance.
 

3. Root Damage or Shallow Roots

Summer squash has a shallow root system that’s sensitive to disturbance.
 
If you hoe too close to the plant, grow it in a cramped container, or let the soil bake dry, the roots struggle to take up water and calcium.
 
Damaged roots mean less calcium reaches the fruit, and blossom end rot on summer squash follows.
 

4. Soil pH That’s Out of Range

Calcium is only available to your summer squash when the soil pH sits between 6.0 and 6.8.
 
If your soil is too acidic or too alkaline, calcium gets locked up and the plant can’t absorb it, even if it’s sitting right there in the ground.
 
That’s why a simple soil test can reveal one more hidden cause of blossom end rot on summer squash.
 

5. Cold Soil Temperatures

Early in the season, cold soil slows root activity, which limits how much water and calcium the plant can pull in.
 
This is why the first few squash of the season often show blossom end rot on summer squash.
 
Once the soil warms up and watering becomes regular, later fruit usually comes in clean.
 

How to Fix Blossom End Rot on Summer Squash

The good news is that blossom end rot on summer squash is easy to manage once you know what’s causing it.
 
Here’s how to fix it:
 

1. Water Deeply and Consistently

Give your summer squash about one to two inches of water per week, delivered slowly so it soaks down deep.
 
Water at the base of the plant in the morning, and aim for moist soil rather than soggy or bone-dry ground.
 
A layer of mulch helps hold that moisture steady, which is the single best defense against blossom end rot on summer squash.
 

2. Remove Affected Fruit Right Away

Pick off any squash with blossom end rot on summer squash as soon as you spot the spot.
 
Those fruits won’t recover, and leaving them on the vine just wastes the plant’s energy.
 
Toss them in the trash, not the compost, and keep the healthy fruit growing.
 

3. Ease Up on Nitrogen

Switch to a balanced fertilizer or one that’s lower in nitrogen and higher in phosphorus and potassium.
 
Too much nitrogen is a frequent cause of blossom end rot on summer squash, so cutting back helps the plant direct calcium to the fruit.
 

4. Test and Adjust Your Soil

A soil test tells you your pH and calcium levels in one shot.
 
If pH is off, add lime to raise it or sulfur to lower it, and work it in before planting next season.
 
Getting pH in range prevents blossom end rot on summer squash by keeping calcium available to the roots.
 

5. Don’t Bother with Calcium Sprays

Here’s a myth worth busting: spraying calcium on the leaves rarely fixes blossom end rot on summer squash.
 
Calcium doesn’t move well from leaves to fruit, so the spray never reaches the spot that needs it.
 
Fixing your watering and soil is what actually stops blossom end rot on summer squash.
 

So, What Causes Blossom End Rot on Summer Squash?

Blossom end rot on summer squash is caused by a calcium deficiency in the fruit, usually brought on by inconsistent watering, excess nitrogen, root damage, off-balance soil pH, or cold soil.
 
It’s rarely about missing calcium in your ground and almost always about calcium failing to reach the fruit.
 
Water deeply and evenly, mulch your beds, skip the high-nitrogen feeds, and test your soil, and you’ll see far less blossom end rot on summer squash.
 
Now you know exactly what causes blossom end rot on summer squash, so you can protect your harvest and enjoy smooth, healthy squash all season long.