Why Do Heirloom Tomatoes Crack on the Vine?

Why Do Heirloom Tomatoes Crack on the Vine?

Heirloom tomatoes crack on the vine because of rapid changes in water availability. When a tomato plant takes up water faster than the fruit can expand to accommodate it, the skin ruptures. This is most common after a heavy rain or a deep watering following a dry period — the plant suddenly has more water than it knows what to do with, and the fruit splits under the pressure. Heirloom varieties are more prone to cracking than modern commercial hybrids because they have not been bred for thick, elastic skin.

The cracks are usually concentric (rings around the stem end) or radial (lines running from stem to blossom end). Both are caused by the same mechanism: the flesh inside the tomato swells faster than the skin can stretch, and the skin gives way. The tomato is still edible — in fact, a cracked tomato is often a sign of a very ripe, juicy fruit — but the cracks are entry points for mould and bacteria, so cracked tomatoes do not store well and should be eaten quickly.

The Water Pressure Problem

A tomato is about 95% water. As the fruit ripens, it continues to take up water from the plant through the stem. The skin has some elasticity and can stretch to accommodate moderate growth, but it has limits. When the water uptake suddenly increases — as it does after rain or heavy watering after a dry spell — the internal pressure rises faster than the skin can adjust, and it splits.

This is the same mechanism that causes watermelons to split and cherries to crack after rain. Any thin-skinned fruit that takes up water rapidly can split. The problem is worse for heirloom tomatoes because heirlooms have generally thinner, less elastic skin than commercial hybrids. Commercial breeders have selected for thicker, more crack-resistant skin precisely because cracking is a major cause of post-harvest loss. Heirlooms, which have not gone through this selection, keep their thinner skin — and their better flavour.

Concentric vs Radial Cracks

There are two main types of cracking in tomatoes, and they tell you slightly different things about what happened.

Concentric cracks are rings that form around the stem end of the tomato. They are the most common type and usually happen during the final stages of ripening. The skin near the stem is the thickest and least elastic, so it cracks first when pressure builds. Concentric cracks are often shallow and the tomato is still good to eat if you cut around the crack.

Radial cracks run from the stem end down toward the blossom end. They are more serious because they go deeper into the flesh and create larger openings for mould and bacteria. Radial cracks usually happen when the water uptake is very rapid — a sudden downpour on a very ripe tomato, for example. A tomato with radial cracks should be eaten the same day or within a day, because the deep cracks will lead to quick spoilage.

Why Heirlooms Crack More Than Hybrids

Heirloom tomatoes are open-pollinated varieties that have been passed down for generations without the intensive breeding that commercial hybrids go through. They are grown for flavour, not for transport or storage, and they have not been selected for traits like crack resistance, thick skin, or uniform ripening. This is why they taste so much better than supermarket tomatoes — see our supermarket tomato guide — but it is also why they are more fragile.

The same genes that give heirlooms their thin, tender skin and delicate texture also make them vulnerable to cracking. Commercial hybrids have been bred to have thicker, more elastic skin that can stretch further before splitting. The tradeoff is flavour — the thicker skin of commercial varieties is tougher and less pleasant to eat, and the flesh inside is often mealier. Heirlooms sacrifice durability for taste, and cracking is one of the costs. See our tomato season guide for when heirlooms are available in Australia.

How to Prevent Cracking in Home-Grown Tomatoes

If you grow heirloom tomatoes at home, you can reduce cracking with consistent watering. The key is to keep the soil moisture as steady as possible — not bone dry one week and soaking wet the next. Mulch around the plants to retain moisture and prevent the soil from drying out quickly between waterings. Water deeply and regularly rather than in sporadic heavy sessions. During rainy periods, there is not much you can do — nature does the watering for you — but good drainage helps prevent the worst waterlogging.

Picking tomatoes at the "breaker" stage — when they have started to change colour but are not fully ripe — also reduces cracking. A breaker-stage tomato will finish ripening on the counter over a few days and will taste nearly as good as a fully vine-ripened one, with far less risk of cracking. This is a common technique among heirloom growers who want the flavour without the loss. See our regrowing guide for more on growing from scraps.

Is a Cracked Tomato Safe to Eat?

A freshly cracked tomato is safe to eat. The crack itself is just a mechanical rupture in the skin — it is not a sign of disease or spoilage. Cut away the cracked area and eat the rest. The flesh underneath is usually fine and often the sweetest part of the tomato because cracked tomatoes are typically very ripe.

A cracked tomato that has been sitting for several days is a different story. The crack is an entry point for mould and bacteria, and a cracked tomato will spoil faster than an intact one. If the crack has mould, is weeping liquid, or smells fermented, discard the tomato. If the crack is fresh and dry and the flesh underneath looks and smells fine, it is safe. See our mould safety guide for the general rules on salvaging cracked and damaged produce.

How to Buy and Store Heirloom Tomatoes

Heirloom tomatoes are most commonly found at farmers' markets, specialty greengrocers, and the premium produce section of some Coles and Woolworths stores. They are usually more expensive than standard tomatoes because they are harder to grow, more fragile, and have a shorter shelf life. Look for tomatoes that are firm but yielding, with smooth skin and no cracks or soft spots. Cracked tomatoes at the shop should be heavily discounted — if you buy them, eat them the same day.

Store heirloom tomatoes at room temperature, stem-side down, away from direct sunlight. Never refrigerate — cold damages the flavour irreversibly, as we explain in our supermarket tomato guide. Use heirlooms within 2–3 days of purchase for the best flavour and texture. If you have too many, roast them or make a sauce — see our Fridge Scan for recipe ideas based on what you have. And if you want to check freshness before buying, scan the tomato with PickFresh.

The Case for Growing Your Own

Because heirlooms are fragile and expensive to buy, growing your own is often the best option. A single plant can produce several kilograms of tomatoes over a season, and you can pick them at the breaker stage to avoid cracking losses. Heirloom seeds are widely available in Australia from seed companies like Diggers and Eden Seeds. Popular Australian heirloom varieties include Grosse Lisse, Mortgage Lifter, Cherokee Purple, and Tommy Toe. Plant in spring after the last frost, and expect fruit from January to March in most of Australia. See our tomato season guide for regional timing.