Give every plant the room it becomes.

Twenty-four to thirty-two inches in the row for most varieties, with rows three to four feet apart. Determinate bush types can go at the tighter end; indeterminate vining types want the wider one and a sturdy support. Crowding tomatoes is the fastest route to early blight, because the foliage never dries.

Substantially. Up to a point, more plants means more total yield, but past it the plants shade each other, compete for water and nutrients, and each produces far less. Total yield per bed peaks and then declines, and disease pressure rises the whole way. Correct spacing usually beats extra plants.

Take the sowing density for the crop — about 60 seeds per square metre for lettuce, 200 for carrots, 10 for tomatoes — multiply by your area, and add 10 to 15%. If the seed is more than two years old, add considerably more, or test its viability first.

It varies enormously by species. Onion, parsnip and leek are effectively one-season seeds and drop off sharply after that. Tomato, cucumber and lettuce hold up for four or five years in cool, dry storage. A paper-towel test with ten seeds tells you the real rate in a week.

Most vegetables germinate fastest between 68 and 77 °F at the seed, and stall badly below 59 °F. Cool-season crops such as lettuce and spinach are the exception and germinate happily in the fifties, while peppers and eggplant effectively refuse to move below 65 °F.

Almost always soil temperature. Peppers can take three to four weeks at 59 °F and under a fortnight at 77 °F, and in cold, wet compost they frequently rot before they sprout. A heat mat under the tray is the standard fix and it is the difference between a crop and an empty tray.

No, and the gap is what catches people out in spring. Soil warms slowly and lags air temperature by days to weeks, more so in heavy clay and in shade. A cheap soil thermometer pushed into the bed to seed depth is the only reliable way to know.

Multiply the root-ball diameter by about 1.5 for a leafy plant, 2 for something that will fruit, and only 1.2 for a cactus or succulent. Step up gradually — jumping straight to a very large pot leaves a mass of wet, rootless soil that stays saturated and rots the roots you have.

No. Nursery pot sizes are trade names rather than measurements, and a nominal 1-gallon pot typically holds around 0.7 US gallons. If you are calculating potting mix for a batch of pots, measure one rather than trusting the label.

Generally no, and this is one of the most persistent pieces of bad gardening advice. A coarse layer beneath fine potting mix creates a perched water table: water will not cross the texture boundary until the mix above is saturated, so the soggy zone ends up higher in the pot, not lower. Drainage holes and an open mix are what actually work.

When drainage holes are large enough that mix washes out, or when a tall pot needs ballast so it does not blow over. Both are real reasons — neither of them is drainage. And nothing at all helps a pot with no drainage hole; use it as a cachepot with a liner inside instead.