The six crops worth starting in March (and three you should leave alone)

Every year, without fail, someone on our allotment site sows their courgettes in February. By April, they have enormous, leggy plants taking over their living room. By June, they've had to harden them off in weather that was entirely unsuitable. By July, they're fine — but they're no further ahead than the person who waited until May.

Timing matters. Not as a rule to follow obediently, but because plants have preferences, and ignoring those preferences wastes both your time and your seeds. So here's a clear-eyed look at what's worth starting now and what can wait.

Worth starting in March: Tomatoes; yes, now, on a warm windowsill. They need around 6–8 weeks indoors before planting out, so mid-March gives you a good run. Peppers and chillies too, they're slow to get going and appreciate an early start. Leeks are another one: sow them in modules now and you'll have decent-sized transplants ready by late spring.

Lettuce and salad leaves can go in undercover in a cold frame, polytunnel, or even a cloche, and you'll have your first pickings within 6–8 weeks. Spinach is similarly obliging. Broad beans, if not already sown in autumn, should go in now without delay.

Leave until later: Courgettes, squashes, and cucumbers all hate cold soil. Sow them in late April at the earliest, or they'll just sit there doing nothing while you worry about them. French and runner beans are the same, they want warmth. Sowing them now outdoors is an exercise in disappointment.

Sweetcorn is another one that's often sown too early. It germinates poorly in cold compost. Late April under cover is plenty early enough.

A good rule of thumb: if it's a tender annual that originated somewhere warm and sunny, it does not want to be outside in March. Give it a heated propagator or a warm windowsill and a bit of patience.

Patience, as it turns out, is the most important thing you can grow on an allotment.