Some years, I’ve had a serious problem with splitting, which means I’ve had to throw out more tomatoes than I like because they split and were later infested with worms or rot. Annoying.
Most books say that splitting is caused by watering issues. Periods of dry, followed by lots of water like a heavy rain, can cause splitting. It happens when the tomatoes are still green, and their skin/exterior is hard and inflexible. As the excess water rushes into the fruit, it causes a growth spurt that the young fruit cannot handle, so it splits.
The splitting is especially bad in years with very heavy rains. However, I’ve noticed something interesting in the test garden: 90% of the splitting is confined to the tomatoes growing in the coconut coir grow bags. The tomato pictured here is a Better Boy hybrid, so it should be tough as nails—and I promise my watering is absolutely consistent. I water the grow-bag tomatoes every morning, just the same as the container-grown tomatoes in sphagnum. And the heirlooms and beefsteaks grown in sphagnum peat hardly split at all.
So … I’ve done tons of research on this and I can’t find any proof that tomatoes grown in coconut coir are more liable to split. So have mine own eyes thus deceived me? I don’t know. I do know that coconut coir has a different water-holding capacity than sphagnum peat; it’s possible that daily watering is simply too much because the coconut coir holds water for so much longer. Then again … professional growers using coconut coir water with daily drip irrigation, although I assume they deliver much less water than I do with a standard hose. It’s possible I have yet to really understand how to use coconut coir.
In any event, at the most basic level, tomato splitting is caused by inconsistent watering, with periods of dry followed by lots of water. I suspect in my case, there’s a learning curve for using coir as a growing media, and perhaps I should try every-other-day-watering. If anyone out there has any insight, I’d love to hear it.