Needs to be harvested on time otherwise the curds (White heads) will turn brown and rot. What a shame considering all the work you've put in! The image above show an over ripe head alongside one that is perfect.
If you planted a number of cauliflower it's best to start harvesting some of the small ones while you're waiting for the rest to mature. This is because they will all be ready at the same time and don't store well. It the florets begin to separate then you've waited too long.
Cut the cauliflower head at the base of the plant. It's a good idea to keep the leaf wrapping intact to prevent the curd from damage. The best time to pick is in the early morning when there's still dew on the plant, if it's frosty you're better off waiting till midday.
If you wish to store the cauliflower for 2 or 3 weeks lift the whole plant including the roots, shake off any excess soil and hang upside down in a cool shed. Spray the head occasionally with water to prevent drying out.