Five ways to fail a tomato
Almost everything that goes wrong with a tomato comes down to the same handful of small impatiences, and most of them can be undone.
Tomatoes have a reputation for being easy, which is unfair to the gardeners who fail them every year — myself among them, more than once. The truth is that they are forgiving of almost everything except inconsistency, and the mistakes that ruin a crop are nearly all variations on a single theme: we are in a hurry, and the plant is not.
The first and most common is uneven watering. A tomato wants steady, even moisture, and when it gets a drought followed by a flood the fruit swells faster than its skin can grow and splits down the side. The same erratic supply makes it harder for the plant to take up calcium, which is how you end up with blossom-end rot — that sunken, leathery black patch on the underside of an otherwise perfect fruit. The fix is dull and reliable: an inch or two of water a week, and a thick mulch of straw or wood chips to hold the moisture and slow the evaporation. Mulch is the closest thing the vegetable garden has to a steady hand.
The second mistake is watering from above. Splashing the soil up onto the leaves is the surest way to introduce the fungal and bacterial blights that overwinter in the ground, and foliage left wet overnight is an invitation. Water at the base, in the morning, and let the leaves stay dry. The third mistake compounds the second — planting too close together. Crowded tomatoes hold humid, stagnant air between them, and disease moves through that air like gossip. Give them two to three feet, and strip the lowest foot or so of leaves once the plant is established, so air can move underneath.
The fourth is kindness misapplied: too much nitrogen. Tomatoes are hungry, but feed them heavily on nitrogen early and you will grow a magnificent green bush with almost nothing to eat. Start with compost-rich soil and, once the flowers set, switch to a feed higher in potassium than nitrogen — the plant's energy follows the food you give it. The last mistake is the most impatient of all: planting into cold ground. A tomato is a heat-lover, and set out before the soil has warmed it sulks, stalls, and never quite recovers its nerve. Wait until the frosts are done and the nights hold above fifty degrees. The plant put in two weeks later, into warm earth, will overtake the brave one every time.
There is one watch worth keeping through high summer, and it is not a mistake so much as a visitor: the tomato hornworm. They are enormous, finger-thick, the exact green of the stem, and you will usually find the stripped branches and the dark pellets of frass before you find the caterpillar itself. Pick them off by hand and that is the end of it. But before you do, look closely — if a hornworm is studded with what look like grains of white rice standing on end, leave it exactly where it is. Those are the cocoons of a braconid wasp whose larvae have been feeding inside the caterpillar and are now pupating on its back. That hornworm is already doomed and will eat very little more, and the wasps emerging from it will go on to parasitise the next generation for you. The gardener's instinct is to remove the dreadful-looking thing; the better move, just this once, is to do nothing and let the garden settle its own account.