Why Are My Grapes Splitting and Cracking?

Splitting grapes are one of the most demoralising sights in late summer — a bunch that was close to perfect suddenly develops cracks in the skin, the juice oozes out, and botrytis follows within days. In the vast majority of cases, cracking is caused by the internal pressure of the berry increasing faster than the skin can expand. Once you understand what causes that pressure surge, preventing future cracking becomes straightforward.

Irregular watering — the primary cause

When soil moisture fluctuates dramatically — a prolonged dry period followed by heavy rain or a large irrigation — the vine rapidly absorbs water and pumps it into the fruit. The berry flesh expands faster than the skin can stretch, and the skin splits. This is almost identical to the mechanism behind tomato splitting. The fix is consistent, moderate irrigation throughout the growing season rather than large infrequent doses. Deep mulching helps buffer sudden rainfall events by slowing water infiltration into the root zone.

Tight bunches with berries pressing together

In compact-clustered varieties, individual berries are under physical pressure from their neighbours as they expand. If the cluster is very tight, berries crack even with consistent watering because there is simply no room for normal expansion. Thinning the bunch at fruit set — removing the tips of the side laterals of the cluster with small scissors — creates space between berries and dramatically reduces splitting in tight-clustered varieties. This technique is called bunch thinning or berry thinning.

Calcium deficiency weakening skin

Calcium is essential for cell wall strength. Calcium-deficient berries have fragile skins that crack under pressure more readily. Calcium deficiency in grapes is often a transport issue rather than an absence in the soil — in hot, dry conditions, transpiration slows and calcium, which moves with water in the xylem, reaches the fruit in insufficient amounts. Foliar calcium sprays applied from fruit set onwards can supplement supply when soil conditions limit uptake.

Heavy rain at harvest time

A sudden period of heavy rain when the grapes are nearly ripe causes rapid uptake and cracking across the whole vineyard in a short window. In wet climates, timing harvest is critical — once grapes reach the desired sugar level, harvest promptly rather than waiting for a slightly higher Brix reading if rain is forecast. Growing under glass or a temporary poly cover eliminates this risk entirely.

Botrytis entering cracks

Any skin crack — however small — provides an immediate entry point for botrytis spores. In humid weather, cracked berries can develop grey mould within 24 to 48 hours. Harvest cracked bunches immediately if they are otherwise ripe, or remove them to prevent the infection spreading to intact bunches nearby.