Q: I have a problem with my heirloom tomatoes cracking after heavy rain (a rarity here in central Texas, but it does occur).
---Brooks in Austin
Although I don't have any big problems growing tomatoes, they do tend to split. What am I doing wrong and how do I fix it?
--- Kim in Norwich, CT
After reading your tomato growing book last yearI am happily harvesting organic Brandywines and Romas. However, most of the Brandywines developed deep crevices up near the stem. Other than that, they are beautiful. I looked in your 'A to Z archives' but didn't find any answers. Do you know why tomatoes would crack?
--- Ed in Milmont Park, PA
A. 'Cracking' and 'splitting' are such common problems that I was kind of shocked we hadn't addressed them yet—especially since the answers are all relatively good news: Cracking and splitting aren't caused by some dread disease or awful insect; they're more of a cultural problem—like opera and my radio show.
Anyway, tomatoes split open when the skins of the ripe fruit can't keep pace with the growth of the insides—especially when that growth is sudden and rapid, like right after a rain that falls heavily in a short period of time; and especially especially if the rain was preceded by a long dry stretch. Once a tomato is ripe, the outside is pretty much done growing, but the inside is still going to take in some of that water—and sometimes the skin gives way.
This isn't typically a problem with green tomatoes; their parts are still growing at a relatively equal rate. And the fact that only ripe tomatoes are affected means you shouldn't lose any fruits. If a heavy rain is predicted—especially after a dry spell—go out and pick all your ripe-to-mostly-ripe tomatoes (they're only going to lose flavor from all that excess water if you don't). Otherwise, pick fruits that start to split promptly and use them to make sauce or salsa. Heck, if the splits are small and you act fast, they'll still make good slicing tomatoes. Don't leave split tomatoes on the vine—the openings will attract opportunistic insects like ants.
Although it may seem similar, 'cracking' around the tops of heirloom tomatoes is a different issue—more of a 'price you pay for a great tasting big old-fashioned tomato' kind of thing. Almost all of the old original big 'beefsteak' sized tomatoes are prone to at least a little bit of cracking around the tops, and most of the time it isn't a problem. Just pick them, slice off that top 5% and enjoy the rest. And besides—some of the really huge heirlooms never seem to want to ripen all the way to the top, so it makes sense to pick them when they're still a little green-shouldered anyway; otherwise you risk them getting overripe on the vine and losing flavor.
But again, this is at least partially a 'cultural' problem, and so there are some things you can do up front to lessen the possibility of splitting and cracking. Growing in raised beds, for instance. Tomatoes grown in raised beds are always going to have the problem less, because heavy rains will drain away faster in the light, loose, un-stepped-on soil of a properly raised bed. Flat-earth gardeners with compacted soil will always have more cracking and splitting.
A one to two inch mulch of compost, shredded fall leaves or straw will also help by keeping the soil moisture more constant. Again, you get the worst splitting when a lot of rain follows a very dry spell, and mulch can help keep the moisture levels higher during dry times, which helps the skins stay more flexible.
And then there's the importance of calcium. Having access to adequate soil calcium allows tomatoes to better regulate their water uptake, which is why we always advocate adding calcium at planting time to prevent the heartache of blossom end rot—when ripe tomatoes turn black and rot out on the bottom ("the blossom end").
A dozen finely crushed eggshells in the planting hole pretty much totally prevents blossom end rot, and should help prevent splitting. In fact, blossom end rot is a kind of 'worst case splitting', as both are caused by too much moisture building up inside ripening fruits. And this is a great time to start saving dried eggshells for next season.
Don't eat eggs? Natural plant foods labeled for use on tomatoes will contain a lot of added calcium, specifically to prevent these kinds of problems. If it's a granulated fertilizer, add some directly to the hole at planting time, and then add the recommended amount to the surface of your soil (or mulch of compost), then cover the fertilizer with a little more soil or compost. (Granulated plant foods always work best when they're incorporated into your soil as opposed to just sitting on top of it.)
And finally, you can plant varieties that are known to resist the problem. Look for tomatoes whose catalog descriptions say things like 'crack-free'; these varieties are bred to have skins that continue to expand when the fruits are ripe.