You've watched your tomatoes grow for weeks. They're finally getting big and starting to ripen. Then you notice it: a dark, sunken, leathery patch on the bottom of the fruit.
That's blossom end rot (BER). It's one of the most common and frustrating tomato problems - and one of the most misunderstood.
What Blossom End Rot Looks Like
Typical Symptoms
A dark, sunken spot at the blossom end (bottom) of the tomato. Starts as a small water-soaked area. Becomes brown or black and leathery. Can cover up to half the fruit. Often affects the first tomatoes of the season most severely.
BER isn't a disease or pest - it's a physiological disorder. The affected fruit is safe to eat if you cut away the damaged portion, but it's disappointing after all your growing effort.
The Real Cause (It's Not What You Think)
Here's what's actually happening:
Calcium moves through plants via water. When soil moisture fluctuates - wet, then dry, then wet - calcium transport becomes inconsistent. The fast-growing fruit cells at the blossom end don't get enough calcium, and the tissue breaks down.
This is why BER often appears:
- Early in the season (when watering habits aren't established)
- During hot, dry spells
- After a period of drought followed by heavy watering
- In containers that dry out quickly
What Actually Causes It
1. Inconsistent Watering (Primary Cause)
The #1 cause. Cycles of too dry and too wet stress the plant and disrupt calcium transport. Container plants are especially vulnerable because they dry out faster than garden beds.
2. Root Damage
Damaged or restricted roots can't absorb water efficiently. This includes being root-bound, compacted soil, or cultivation damage.
3. Over-fertilising with Nitrogen
Too much nitrogen causes rapid leaf growth at the expense of fruit. The plant puts energy into foliage instead of developing fruit properly.
4. Extreme Heat
High temperatures increase water loss through leaves. If roots can't keep up, calcium transport suffers.
What Doesn't Work
Skip These "Fixes"
Eggshells: Take years to break down. Won't help this season's tomatoes.
Calcium sprays on leaves: Calcium doesn't move from leaves to fruit.
More calcium in soil: Usually unnecessary - most soils have adequate calcium. The problem is transport, not supply.
Epsom salts: Provides magnesium, not calcium. Can actually worsen BER by competing with calcium uptake.
What Actually Works
Preventing Blossom End Rot
- Consistent watering - The single most effective prevention. Water deeply and regularly, maintaining even soil moisture. This is why self-watering containers nearly eliminate BER.
- Mulch - 5-10cm of mulch helps maintain consistent soil moisture and temperature.
- Don't over-fertilise - Avoid high-nitrogen fertilisers once plants are flowering. Too much nitrogen = too much leaf growth = less water for fruit.
- Proper pot size - Small pots dry out too quickly. Use at least 20-litre containers for tomatoes.
- Avoid root disturbance - Don't cultivate near tomato roots. Be gentle when transplanting.
Can You Save Affected Fruit?
Unfortunately, once a tomato has BER, you can't reverse it. The affected fruit won't recover.
Your options:
- Small BER: Leave on plant to ripen, cut away affected area before eating
- Severe BER: Remove fruit to redirect plant's energy to healthy tomatoes
The good news: once you fix watering consistency, new fruit will develop normally. BER typically affects the first few tomatoes worst, then decreases.
Why Container Tomatoes Are Most Affected
Tomatoes in pots get BER more often than garden tomatoes because:
- Containers dry out faster (especially smaller pots)
- Temperature fluctuates more in containers
- Root space is limited
- Watering is often less consistent than garden irrigation
This is why self-watering containers have become so popular for tomatoes. They maintain consistent soil moisture automatically, which is exactly what prevents BER.
Grow Tomatoes Without Blossom End Rot
Garden Stack's self-watering system maintains the consistent moisture tomatoes need. No more guessing when to water, no more BER ruining your harvest.
See Garden Stack