Blossom end rot on tomatoes, and why calcium is not the fix

September 18, 2025

What it looks like, and what it isn't

A tomato gets to about half size and the blossom end, meaning the bottom, goes pale, then sunken, then leathery and black. It spreads no further than that one fruit and the rest of the plant looks perfectly happy.

That's blossom end rot, and the first useful thing to know is that it isn't a disease. Nothing has infected the plant, so there's nothing to spray and no point pulling the plant out. It's a physiological fault, which means the plant grew the fruit faster than it could supply one of the things the fruit needed.

The bit everybody gets wrong

The thing the fruit was short of is calcium, and that sends people straight to the shelf for something with calcium in it. Most of the time that does nothing at all, and here's why.

Our soils are generally not short of calcium. The calcium is there. The problem is that calcium can only travel into a plant dissolved in water, moving up with the transpiration stream, and it isn't mobile once it arrives, so the plant can't move it around from an old leaf to a new fruit the way it can with nitrogen.

So the fruit at the end of the branch, growing fastest, in the driest part of the week, misses out. Adding more calcium to soil that already has plenty doesn't fix a delivery problem.

There's one honest exception. Very acidic or very sandy soil genuinely can be short, and a soil test will tell you that, and garden lime or gypsum will correct it over a season. That's a slow soil amendment though, not a rescue for the fruit you're looking at now.

What actually causes it

Uneven watering. The big one. Dry for four days then soaked interrupts the flow that carries calcium, and a hot dry spell followed by a drenching produces a whole truss of it.
Too much nitrogen. A heavily fed plant grows fast and leafy, and the fruit competes with all that new growth for whatever the roots deliver.
Damaged or shallow roots. Hoeing too close, or a plant in a pot too small for it, cuts the supply line.
Heat. On a 40 degree day the leaves transpire hard and take the water, and the fruit is last in the queue.

What to do about it

Pick off the affected fruit, because it won't recover and the plant is wasting resources on it. Then fix the supply rather than the mineral.

Water deeply and on a schedule you can actually hold, which usually means a soaker hose or a dripper and a timer rather than a hose when you remember. Mulch to about 5cm to even out what the soil does between waterings. Ease off the high nitrogen feed while fruit is setting and use something balanced instead. Keep the hoe away from the root zone and pull weeds by hand close to the plant.

Plants in pots get it worst because a pot dries out fastest, so if it keeps happening to the same container plant, the answer is often a bigger pot rather than anything you can pour on it.

Which tomatoes get it most

Long fruited types get it more than round ones, so Roma and San Marzano style paste tomatoes are the ones you'll see it on first. Cherry tomatoes are the least affected, which is worth knowing if you have had a bad run and want something forgiving while you sort the watering out.

Shop related products

D.T. Brown Tomato Red Cherry Seed. The most forgiving type while you get the watering steady.
D.T. Brown Tomato Grosse Lisse Seed. A reliable round tomato for a home patch.
Martins Organic Sugar Cane Bale 11kg. Mulch, which is the cheapest thing you can do about this.
Brunnings Premium Potting Mix 36L. For moving a struggling pot plant into something that holds moisture better.

Did you know we also keep garden lime, gypsum, soaker hose, drippers and tap timers in the shop, and a fair bit of that isn't on the website. Give us a ring on (08) 8536 4333 to check what's on the shelf, and the rest of the garden centre range is worth a look.