Catch crops are useful, efficient space savers, which tuck into gaps when other crops have finished, or as they're still growing, thereby maximising yield.
Most crops can be grown close together, so if you spot any spaces here and there, sow some quick-growing, tasty produce for harvest in just a few weeks. You'll be picking and eating some of these a couple of times over before some of your other crops even get near maturing!
Once you've harvested your potatoes, for example, there'll be vast spaces ready and waiting for some quick croppers such as coriander, radish and lettuce, which will zoom up in the same soil. As potatoes are hungry plants and usually take all the available nutrients from the soil, just dig in a bit of compost before you sow to help the new crops on a bit. Because closely cropping plants compete slightly with each other, they need a good, rich soil to grow happily in.
Under late-sown runner bean plants, 'intercrop' some new lettuce leaves, which will grow way before the beans do.