This page has the numbers for zucchini: 24 inches between plants with 48 inches between rows — about nine square-foot squares per plant — sown a week after your last frost or started indoors three weeks before it, an inch and a half of water a week, and 6 to 12 pounds of fruit per plant over a 45 to 60 day season.
That space figure is the one people get wrong. A zucchini planted at one per square foot smothers its neighbours and mildews from the lack of air. The other two things covered below are the squash vine borer, which kills a plant in two days, and the blossom-end rot that comes from incomplete pollination rather than disease.
Space, honestly
A zucchini plant occupies far more room than its seed packet implies. Allow 24 inches between plants and 48 inches between rows, and in square-foot terms give it about nine squares per plant. Planted at one per square, as people often try, it smothers everything around it and gets mildew from the lack of air.
Plant it at the edge of a bed, facing outwards, so it can spill over the frame rather than over its neighbours.
Sowing and planting
Direct sow about a week after your last frost date, once soil is genuinely warm, or start indoors three weeks before and plant out two weeks after. Zucchini resents root disturbance, so use a big cell and plant out early rather than letting it get pot-bound.
Two seeds per position, thinned to the stronger one, covers the usual germination losses.
A second sowing in early summer is worth considering. The first plants often succumb to mildew or borer by midsummer, and a younger plant takes over just as the older one gives up.
Water and food
An inch and a half a week, delivered to the soil rather than the leaves. Those enormous leaves lose a lot of water on a hot day, and a zucchini that wilts every afternoon produces poorly. Mulch generously.
Feed it well. Compost worked in before planting, then a balanced feed when the plant starts running, and again mid-season. Unlike tomatoes, there is little risk of over-feeding into leaf at the cost of fruit — zucchini does both enthusiastically.
Pollination
Zucchini has separate male and female flowers, and both have to be open at once with something to carry pollen between them. Early in the season a plant often produces only males, which is normal and passes. Later, in wet or cool weather when bees are grounded, you may get female flowers that shrivel and rot at the end. A soft brush moved from a male flower to a female one takes ten seconds and solves it.
Female flowers are the ones with a miniature fruit behind them. Males sit on a plain stem.
Picking
Every day or two, without exception, once the plant is going. A fruit at six to eight inches is at its best; one left to become a marrow tastes of very little and signals the plant to stop. Regular picking is the difference between a plant that produces for two months and one that produces for three weeks.
Cut with a knife rather than twisting, and mind the small spines on the stems.
Common trouble
Squash vine borer is the serious one where it occurs. Row covers over young plants until flowering, and mounding soil over the lower stem so it can root along its length, both help.
Powdery mildew arrives late and is largely inevitable. Spacing and airflow delay it.
Squash bugs cluster at the base and on the undersides of leaves. Squashing the bronze egg clusters by hand, early, is more effective than anything applied later.