Tomatillo: growing, harvesting and turning it into salsa verde

Anyone cutting open a tomatillo for the first time is usually surprised: no sweet berry aroma like the orange nibble from the supermarket, but a tart, almost lemony bite that tastes of lime and fresh grass. That is exactly what makes it indispensable in Mexican cooking. And in the garden it is more straightforward than its exotic reputation suggests: it is grown like a tomato, but crops more willingly and needs less fiddly pinching out. One catch remains, and I will tell you right at the start, because it decides between a harvest and a disappointment.

A tomatillo is not a cape gooseberry, even if the husk suggests it

Both carry their fruit in a papery husk, both belong to the genus Physalis, and both get muddled up in the shops. Yet they are two different plants with two different aims. The tomatillo forms green to purple-flushed fruit that you harvest green and still firm, when it tastes tart. The cape gooseberry (Physalis peruviana) turns golden yellow, soft and sweet instead, and heads for the fruit salad or the tart.

Remember the rule of thumb by taste: what you nibble raw off the plant and what is sweet is the cape gooseberry. What you harvest green, cook or roast and what stays sour is the tomatillo. They also differ in growth. The tomatillo grows broader and more sprawling, often over a metre tall and wide, while the cape gooseberry stays more compact and shrubby. Set both side by side and you have salsa and dessert in one bed.

The one reason you must plant two

Here comes the catch. The tomatillo is largely self-sterile, which means a single plant barely pollinates itself and so sets almost no fruit. You then get lots of empty husks that stay disappointingly flat, because no fruit ever grew inside. The fault is not in your green thumb but in the biology of the plant.

The solution is simple: plant at least two specimens within reach so insects can carry the pollen from flower to flower. Two plants are the minimum; three or four make the harvest even more reliable and abundant. This holds for the tomatillo more strongly than for almost any other nightshade in the bed, because tomatoes and peppers can manage solo at a pinch. So if you can only spare a single spot, the tomatillo is the wrong choice.

Sowing and raising on the windowsill

The tomatillo needs a long, warm season, and you gain that by starting it indoors. Sow from mid-February to mid-March, that is CW 8 to 12. Sowing directly outdoors is not worth it in our latitudes; it turns warm enough too late.

1. Pre-soak the seeds

   Put the seeds in room-warm water for a day. This softens the seed coat and speeds up germination.

2. Sow in CW 8 to 12

   Sow shallowly in seed compost, about half a centimetre deep. Keep the soil evenly moist, but never wet.

3. Germinate warm

   At 20 to 27 degrees on the windowsill or in a heated propagator the seeds germinate within one to two weeks. A bright, warm spot is essential.

4. Prick out

   As soon as the seedlings have two true pairs of leaves, prick them out singly into larger pots. Set them a little deeper, as they root along the stem like tomatoes.

5. Harden off and plant out

   From early May, put the plants outside during the day to accustom them to wind and sun. Plant out only after the last frosts, around 15 May (CW 20).

Position, spacing and soil in the bed

The tomatillo wants sun, warmth and space. A full-sun spot with at least six hours of direct sun brings out the sweet-sour balance best. Because the plants grow broad and sprawling, they need more room than you first give them credit for.

As a moderate feeder the tomatillo is less demanding than the hungry tomato. In spring, before planting out, work ripe compost and a little horn meal in shallowly; that is enough of a starter. Go easy on nitrogen: too much of it drives lots of leaf and little fruit. A modest dose of diluted nettle feed in June and July is plenty.

And a word on crop rotation: the tomatillo belongs, like tomato, potato and pepper, to the nightshades. Leave the same bed at least four years before another nightshade stands there again. This prevents soil fatigue and late blight. As neighbours it likes basil, lettuce and nasturtium; right next to potatoes you should not set it, however.

Through the season: support, water, let it grow

Once established, the tomatillo makes little work. Pinching out like a tomato is not needed and even counterproductive, because the fruit sits on the side shoots. What it does need is support.

The plant grows sprawling and the branches bend to the ground under the weight of the fruit. A sturdy stake, a tomato cage or a low support keep the shoots up and the fruit clean. Water evenly, but without drowning the roots: the water need is moderate, and in hot weeks it may be more. Uneven watering can, as with tomatoes, lead to split or poorly formed fruit.

From June the small yellow flowers open with their dark throat markings, where bees and bumblebees like to work. Each pollinated flower first forms the empty husk, which then fills from within as the fruit grows. This is exactly the process you now watch week by week.

Harvesting: when the husk turns plump

The best ripeness test is the husk itself. As long as it is slack and clearly larger than the fruit, wait longer. The tomatillo is ripe when the papery husk is fully filled, feels taut and begins to split at the seam. The fruit beneath is then firm and mostly still green; exactly how you want it for salsa verde.

The harvest runs from August into October, often until just before the first frost. If a ripe fruit drops of its own accord, that too is a good sign; gather it off the ground while it is still firm. To store them longer, leave them in the husk: this way tomatillos keep for several weeks in a cool, airy room. Peeled and frozen they even survive the winter.

Salsa verde and what else reaches the kitchen

The classic goal is salsa verde, the green salsa, without which tacos and enchiladas lose their backbone. For it you peel the tomatillos out of their sticky husk, wash the fruit and roast them briefly under the grill or in the pan until the skin blisters and takes on dark spots. Roasting takes off the sharp acidity and brings a deep, smoky note.

For a simple salsa verde, blend the roasted tomatillos briefly with onion, garlic, fresh coriander, a green chilli and salt. If you prefer it fresher, leave the fruit raw for a brighter, zingier salsa. Both versions keep a few days in the fridge and freeze well in portions.

But salsa is only the start. Finely diced, tomatillos give a stew or a bean chilli a fine acidity that otherwise only lime would provide. They suit green curries, work as a chutney with grilled food and, fully ripe, even go into an unusual jam. In the kitchen they behave a little like a firm, sour tomato: braise, roast, preserve, anything goes.