If you're new to growing winter cabbages and kale, deciding when to harvest isn't always obvious.

At a gardeners' question time recently, someone asked what a kale plant would look like when ready to harvest. It can also be hard to know when to pick broccoli since different varieties become ready at various points over winter and early spring, as do cabbages. So how do first-time veg growers judge when to harvest their crops?

Seed packets advise on the best sowing and harvesting times, but this can only be a guide. As a general rule, the dates on the packet apply to Bournemouth or Hertfordshire, not Glasgow or Sutherland. So, if you're told to harvest between February and April, expect to do so in April or May.

Vegetable varieties are programmed to mature at different times. So don't buy a plant described simply as "cabbage". Check whether it's Derby Day, ready in early summer, or the late winter Tundra.

Some types of brassica need to be harvested as soon as they're ready, while others can be ignored for several weeks. Broccoli waits for no man. As soon as it produces spears, cut and consume, otherwise it will burst into flower and you will have missed the boat. When cutting spears, limit yourself to the heads and short stems. The plant will sprout new flower heads from nodes in the stem, giving you a longer harvest. If growing calabrese broccoli, don't expect more than a large central head.

Even when the plant looks fully grown, cauliflower waits a few tantalising weeks before hearting up. Like calabrese, it protects its developing head with tightly curling leaves. The fully formed head should be ready to cut several days after first pushing through these leaves. As with broccoli, you must act quickly; don't let the curd get too large. Harvest while it's still tightly packed.

Be careful to choose caulis that are ready to cut before winter sets in. Although some broccolis, like Rudolph, start to produce spears in February, cauliflower curds won't tolerate frost. So, if you're tempted to try Autumn Giant, you'll need to start it in March to be sure of an autumn harvest. A safer alternative is to sow a variety like All Year Round in June to let it establish by autumn. It will then heart up the following spring. If you have planted a late autumn cauli, hope for a mild November or cover with fleece if a cold spell is forecast, keeping the fabric well clear of the plant.

Cabbages are much more forgiving and mature plants will stand for several weeks. Kilaton, one of the tastiest drumheads I've come across, is ready now, but will stay in the ground in good condition until Christmas. Meanwhile, January King and Savoy King can safely be left until the New Year, regardless of what the weather throws at them.

Kale is another safe bet. It keeps slowly putting on new growth in autumn, but the leaves will stay fresh until you need them. Although the likes of Nero di Toscana only provides an autumn harvest, toughies like Dwarf Green Curled or Ragged Jack produce a fine autumn crop, go dormant over the winter and start into life again in the spring.

In the autumn, allow plants to grow to around 45cm before picking leaves. Always choose fresh green leaves near the top of the plant, limiting yourself to two or three leaves per plant and carefully avoiding the leading shoot. In spring, the plants start producing tiny clumps of leaves on the side of the main stems. Keep picking these delicious little tufts, as this delays the production of flowering shoots. But, desperate to go to seed, the plant will finally defeat you. Then dig out or leave to flower for your own seed.