The splitting of tomatoes from the top of the fruit can be a bothersome problem in any garden.  First, the splitting occurs.  Then, fungus and mold move in and before you know it, you have a pretty ugly tomato. 

What causes this and how can it be avoided?

The splitting is actually caused by an imbalance in water supply.  Usually, a large amount of irrigation that follows a dry spell will cause the fruit to split.  The plant actually takes up more water than it needs during wet periods.  The tomato plant loses some water through transpiration.  However, if the weather is cooler following a rain or excessive watering, the plant cannot release enough moisture in this way.  The tomato fruit take some of the excess water (actually it is forced into them) from the plant.  When a critical level is reached, the fruit begins to split radially along the top.

One way to avoid this is to mulch your tomatoes.  Mulching provides an environment where the soil neither takes up water too quickly nor loses it too quickly.  This allows for a more consistent soil moisture level.

Another way to avoid splitting tomatoes is with the use of drip irrigation.  This actually goes hand in hand with the mulching method.  Drip irrigation can be soaker hoses or other drip material that supplies water slowly to the tomato plant.  When used in conjunction with a timer to provide systematic frequent watering, drip irrigation will greatly decrease your chances of having split tomatoes.

No one likes tomatoes with cracks and splits in them.  They can be ugly and, in some cases, unusable.  It is difficult to avoid this problem altogether.  However, using mulch and drip irrigation can be helpful.  Some things like heavy rainstorms are out of your control and may cause splitting no matter how hard you try to prevent it.

For most people, a split tomato will not cause too much heartache.  The split (and mold) can simply be cut out with a knife and the rest of the tomato will be fine.

Happy gardening to you!

-Thomas