This page has the numbers for cucumbers: 12 inches between plants when trellised, 48 inches between rows when left to sprawl, direct sown a week after your last frost or started indoors three weeks before it, an inch and a half of water a week, and 5 to 10 pounds of fruit per plant over a 50 to 70 day season.
Cucumbers fail in two predictable ways, and the page deals with both: they run short of water, which makes the fruit bitter, and they get powdery mildew, which ends the crop. Trellising helps with the second and the spacing figures below assume it.
Sowing and planting
Cucumbers hate cold soil. Direct sow about a week after your last frost date, or start indoors three weeks before it and plant out two weeks after. They resent root disturbance, so if you start them indoors use a large cell or a pot and get them out before they are pot-bound — which happens fast.
Two sowings a fortnight apart is a better plan than one. The first plants crop heavily for a few weeks and then tire or succumb to mildew; the second batch takes over.
Space them 12 inches apart if trellised, with 48 inches between rows if left to sprawl. That row figure is the honest measure of how much ground an untrellised cucumber takes.
Trellis them
This is the single best decision available to a cucumber grower in a small garden. A vining variety on six feet of netting occupies a foot of ground and produces straighter fruit that is easier to find and pick. Air moves through the foliage, which delays mildew by weeks. The fruit hangs clear of the soil, so less of it rots.
Bush and compact varieties exist for containers and small beds, and they do not need support, but they also produce less.
Water and food
Cucumbers want an inch and a half a week, and they show a shortfall faster than almost anything else in the garden — wilting at midday, then bitter fruit. Mulch two or three inches deep once the soil warms, and water the soil rather than the leaves.
Feed generously by cucumber standards: compost worked in before planting, then a balanced feed once the vines start running. They are hungry, fast-growing plants and will use what you give them, unlike tomatoes and peppers where restraint pays.
Picking
Pick often and pick young. A cucumber left on the vine to go fat and yellow tells the plant its work is done, and production slows sharply. Going over the plants every couple of days in high season is what keeps them cropping for two months instead of three weeks.
Cut rather than pull; the stems are more brittle than they look.
Common trouble
Cucumber beetles chew leaves and, more seriously, spread bacterial wilt. A plant that wilts suddenly and does not recover overnight has probably got it, and there is no cure — remove it. Covers over young plants until they flower are the main defence.
Powdery mildew is a question of when, not if. Spacing, trellising and dry foliage buy time. Choosing a resistant variety buys more.
Hollow or misshapen fruit points at water and pollination rather than disease.
In containers
Cucumbers grow well in a large pot with something to climb. Use a container holding at least five gallons of mix — the container calculator will tell you what your pot actually holds — and expect to water daily in summer. A cucumber in a small pot is a plant that wilts every afternoon and produces bitter fruit.