The asparagus harvesting season in England is May and June. The first few spears may come through a bit earlier, perhaps at the end of April. I always think the plants put up one or two "exploratory" spears at the start of the season, just to see what happens -- to test the weather conditions up above. The end of the season is traditionally Midsummer's Day. In any event, you should stop cropping the plants by the end of June, to give them enough time to build up their energy again for the following year. At this time, it is easy to forget that the plants still need some care to repay them for giving up to you all those lovely succulent shoots earlier on.
You need to promote the formation of as much "fern", or leaf, as you can, since this is the plant's mechanism for transforming sunlight into energy. (Anyone for a quick refresher course on Photosynthesis?). So what you do is give the plants a good feed (I use a top dressing of multi-purpose Growmore fertiliser) and keep on watering them, particularly during dry spells. The main thing of course is to resist harvesting the spears - which may be difficult. Let the spears grow on and they will produce huge ferny leaves, about 6 feet tall. These will benefit from some support -- I use bamboo canes and garden twine.
Here is a sequence of photos that shows some of this...
|Asparagus spear just coming through|
|The one at the back is past the "tempting" stage I think...|
|A fine head of fern|
|The fern can get top-heavy and will need some support|
In about late September the fern will go yellow and begin to wither. This is the time to cut it back to just above ground level. I like to leave about 5cm of stem, so that I can actually identify where the plants are, and can avoid damaging them if putting in some new canes.
I hope to be reporting on the cropping activities next year...