Last Updated on October 23, 2026
Blossom End Rot: The Real Reason Tomatoes Rot on the Bottom
Blossom end rot is the sunken black patch on the bottom of a tomato, and it usually comes from uneven watering, not from a lack of calcium in your soil. Calcium only reaches the fruit with water, so when the soil swings from bone dry to soaking wet, the fast-growing bottom of the fruit runs short and collapses. The fix is steady water: water deeply on a schedule, mulch a few inches deep, and build the soil with compost. University of Wisconsin Extension puts it plainly: more often there is plenty of calcium in the soil, but its uptake and transport to the fruit is impaired. Eggshells won’t fix that.
Blossom end rot at a glance
| Question | Short answer |
|---|---|
| What it looks like | A water-soaked spot on the bottom of the fruit that turns sunken, leathery and black |
| Real cause | Uneven soil moisture keeps calcium from reaching the fruit |
| Crops that get it | Tomato, pepper, eggplant, squash, pumpkin, watermelon, cucumber |
| Fix 1 | Water deeply on a schedule |
| Fix 2 | A few inches of organic mulch |
| Fix 3 | Compost in the bed |
| Makes it worse | Too much nitrogen, root damage, hot weather |
| Fruit that has it | Can’t be reversed; pick it off |
What blossom end rot looks like
It starts small. A water-soaked spot shows up on the bottom of the fruit, the end opposite the stem. Then it turns sunken, leathery and black. Wisconsin Extension says it can spread across half the fruit. University of Minnesota Extension says it is most common when the fruit is one-third to one-half grown.
It is not a disease. Oregon State University Extension says it is not caused by fungi or bacteria and does not spread from plant to plant, though mold can move into the dead patch later.
Don’t confuse it with sunscald. UMass Extension says sunscald only shows up on the side facing the sun, while blossom end rot can show up where no sun hits.
Why it usually isn’t your soil’s calcium
Here’s what surprises most people. Your soil very likely has calcium in it already. The problem is getting it into the fruit.
Calcium rides up the stem with water, like a passenger in a stream. Clemson Extension explains it the same way: calcium is taken up by the roots and carried up with the water, so if the water supply is interrupted, the calcium supply is interrupted too. When the soil dries out, the stream slows and the calcium stops reaching the fruit.
The bottom tip of each young tomato grows fastest, so it needs calcium the most. When your watering swings from dry to soaked, that tip gets starved and collapses. Nebraska Extension adds that this is rarely a lack of calcium in the soil, but plants that can’t pull calcium up fast enough for the growing fruit.
What sets it off
- Moisture swings. Wisconsin lists drought stress, alternating wet and dry soil, and waterlogged or cold soil.
- Root damage. Wisconsin and Minnesota both list it.
- Too much nitrogen. It makes extra leaves, and Wisconsin says calcium goes to leaves before fruit.
- Hot weather. Nebraska says high temperatures speed up fruit growth, so the fruit needs more calcium.
Why eggshells don’t help
If you’ve been crushing eggshells into the soil, I’ve got bad news. University of Illinois Extension says eggshells can take one to five years to break down and may not be available to plants, crushed or not. And Illinois says its soils already hold enough calcium for garden plants, so blossom end rot is often a water problem.
The fix isn’t something you buy, either. Clemson points out that calcium sitting in the leaves can’t move into the fruit. The fix is watering, not a product. If you like saving eggshells, Illinois suggests the compost pile.
Which crops get it
University of Maryland Extension lists pepper, eggplant, pumpkin, squash and watermelon along with tomato, and Wisconsin adds cucumber. The fixes are the same for all of them.
- Tomato types. Maryland says paste and plum tomatoes get it the most and cherry tomatoes rarely do.
- The first fruits of the season are hit hardest. UMass explains why: as young plants get established and start growing fast, they need more water and calcium.
- Peppers. Maryland says long peppers get it the most. Wisconsin says the spots can look like sunscald and form on the sides of the fruit near the blossom end. UMass says shorter, rounder peppers tend to tolerate it better.
- Zucchini and summer squash. Iowa State Extension says the blossom end begins to rot and soon the whole fruit has rotted. Poor pollination looks similar: the fruit starts to grow, then shrivels. Rainy weather means fewer pollinators, so rule that out first.
- Tomatoes in pots. Illinois Extension says containers dry out quickly, so potted plants are more likely to get it without consistent watering. More in how to grow tomatoes in containers.
Fix 1: water deeply, on a schedule
Water deeply and on a regular schedule, instead of a little splash whenever you remember. Push your finger two inches into the soil. If it’s dry, give it a good soak. If it’s still damp, wait.
Too much is as bad as too little. UC IPM says to watch soil moisture so the root zone neither dries out nor stays saturated. Maryland says regular, deep watering will relieve the problem if the soil has enough calcium.
- How much? Illinois says plants need about one inch of water a week, from rain or watering. Iowa State says 1 to 1.5 inches. Nebraska says more for sandy soil or very hot, dry weather. One inch over a 4×8 bed is 19.9 gallons; the math for other bed sizes is in how many gallons is 1 inch of water.
- Water ahead of heat. Illinois says if hot, dry weather is coming, water well before it arrives.
- Take the guessing out. Clemson suggests drip irrigation on a timer for consistent watering. Size one with the raised bed drip irrigation calculator or compare soaker hose vs drip irrigation.
Fix 2: a few inches of mulch
Put a few inches of mulch around the base of your plants. Straw, shredded leaves or grass clippings all work. Mulch keeps the soil moisture even, so it never swings hard. Wisconsin Extension agrees: water evenly and mulch.
Stick with organic mulch. Montana State University Extension, quoting its horticulture specialist, says organic mulches work better than black plastic for controlling blossom end rot. Saved fall leaves work well here; see what to do with fall leaves.
Fix 3: build the soil with compost
This is the piece most gardeners skip. Compost helps the ground hold water evenly, like a sponge. UGA Extension says compost improves water-holding capacity, and in sandy soil it raises moisture-holding capacity and cuts drought damage. UC IPM adds that blossom end rot is more common on sandier soils. UGA suggests working 1 to 2 inches of well-decomposed compost into the top 6 to 8 inches of soil.
I feed the soil rather than the plant, so I don’t fertilize as a rule. My tomato bed gets a layer of compost at the end of the season, and that is all the feeding it needs. More in empty raised beds in fall and composting for beginners.
Two things that make it worse
- Too much nitrogen. It pushes fast, leafy growth that outruns the calcium. Wisconsin says too much nitrogen makes extra leaves, which pull calcium away from the fruit. Go easy on high-nitrogen feed.
- Digging close to the stem. Damaged roots can’t pull up water or calcium. Minnesota says not to cultivate within 1 foot of the base of the plant, and Clemson warns against hoeing or tilling near established plants. Let the mulch handle the weeds. More in how to transplant tomatoes.
When the soil really is short
Some soils do run low, so don’t guess. Wisconsin recommends a soil test to see if there is enough calcium, and Clemson says to test well before planting. If the test shows low pH, my soil pH for vegetables chart lists 6.0 to 6.8 for tomatoes, and the garden lime calculator turns your test result into pounds of lime for your bed. Start with the water, though. In most gardens that’s the main problem.
What to do with the tomatoes that have it
Once a fruit has blossom end rot, you can’t undo it. Fix the watering for the fruit still to come. The very first tomatoes often get it, and Minnesota says fruit that develop later on the same plant can be unaffected. Minnesota also says it can be minimized but is hard to eliminate completely, so don’t be surprised by the odd bad fruit.
- Pick off the affected fruit. Minnesota says it won’t develop normally, and removing it lets the plant put its energy into healthy fruit.
- Eat the good part. Montana State says tomatoes with blossom end rot are still edible; cut off the damaged part and eat the rest.
- Skip the soft ones. UConn Extension says plain blossom end rot feels thin and papery, and if the spot is soft, a secondary infection has likely set in.
Plan next year’s tomato bed
The best time to fix your watering is before the tomatoes go in. Plan My Garden is free to start. Enter your zip code, draw your beds and pick tomatoes and everything else you want to grow, and it places each plant at square-foot spacing and builds a planting schedule from your frost dates.
Quick questions
What causes blossom end rot on tomatoes?
Uneven watering, usually, not a lack of calcium in the soil. Calcium moves up through the plant with water, so when the soil swings from dry to soaked, the fast-growing bottom of the fruit runs short. Wisconsin Extension says there is more often plenty of calcium in the soil, but its uptake and transport to the fruit is impaired.
Do eggshells prevent blossom end rot?
No. University of Illinois Extension says eggshells can take one to five years to break down and may not be available to plants, even crushed. The problem is usually water getting calcium to the fruit, so steady watering, mulch and compost are the fix.
Can you eat a tomato with blossom end rot?
Yes. Montana State University Extension says tomatoes with blossom end rot are still edible: cut off the damaged part and eat the rest. Skip any fruit where the spot has gone soft.
Does blossom end rot spread to other tomatoes?
No. Oregon State University Extension says it is not caused by fungi or bacteria and does not spread from plant to plant. Plants with the same watering swings can each get it on their own.
Why do only my first tomatoes get blossom end rot?
The first fruits form while the plant is growing fastest, so they are the most likely to run short of calcium. University of Minnesota Extension says fruit that develop later on the same plant can be unaffected. Pick off the bad ones and keep the water steady.
