How many plants does a border need

It is usually more than people expect. The number falls out of the area of the bed and the mature spread of each plant, and there is a sum for it that takes about two minutes.

Space plants at their mature spread

Spread is how wide the plant gets when it is grown, and the spread is the spacing. Two plants with a 60 cm spread go 60 cm apart, centre to centre, and in three years their edges meet and the soil between them disappears.

You can cheat it. Plant at two thirds of the spread, so 40 cm for that 60 cm plant, and the bed closes up a year earlier. The cost is real. You will be lifting and splitting or binning every third plant somewhere around year three, crowded plants flower less, and damp air trapped between them brings mildew on things like phlox. It is worth doing on a bed you will be looking at from the kitchen window all this summer. It is never worth doing with shrubs.

The sum for a block

For a block of planting deeper than a single row, divide the area by the spread squared. A 2 m by 1 m block is 2 square metres. Planted with something 50 cm across, the sum is 2 divided by 0.25, which is eight plants.

For a single row along the front or the back, that sum undercounts, because the band is shallower than the plant is wide. Divide the length of the run by the spread instead. A 3 m run of a 45 cm plant is 3 divided by 0.45, which rounds to seven.

Odd numbers, and drifts

Buy perennials in threes, fives and sevens, and plant each group as one patch. Two of something reads as a pair, and four in a square reads as a grid. Odd numbers in a loose shape read as one patch. Five phlox in one patch read as a block of colour. The same five spread around the bed do not.

Shrubs are the exception, and one is usually right, because a shrub is doing the work of five perennials on its own.

A 3 m by 1.5 m bed, counted

Three plants, with the spreads taken from the catalogue.

Layer	Plant	Spread	Band	The sum	Plants
Back row	Rose 'Munstead Wood'	90 cm	3 m run	3 / 0.9	3
Middle block	Ornamental Sage 'Caradonna'	45 cm	3 m by 0.5 m	1.5 / (0.45 x 0.45)	7
Front row	Pink 'Doris'	30 cm	3 m run	3 / 0.3	10

Twenty plants for four and a half square metres, at proper spacing, with the roses given their full 90 cm. Tighten everything to two thirds and the same bed takes five roses, sixteen sage and fifteen pinks, so thirty-six plants and close to double the money.

Watch what the heights are doing while the spreads do the counting. Rose 'Munstead Wood' (Rosa 'Munstead Wood') is 100 cm tall, Ornamental Sage 'Caradonna' (Salvia nemorosa 'Caradonna') is 60 cm and Pink 'Doris' (Dianthus 'Doris') is 30 cm, so the bed steps down evenly from back to front without anything hiding anything else.

Height and spread are separate numbers

People buy on height and then run out of floor. Tall Verbena (Verbena bonariensis) is 150 cm high and only 45 cm across, so it costs almost nothing in ground and can stand anywhere in the bed, including near the front, because you see straight through it. Hardy Geranium 'Rozanne' (Geranium 'Rozanne') is the other way round at 50 cm tall and 100 cm across, so a single plant covers a square metre and you need far fewer than the bed suggests.

When you are working out a count, the spread column is the one that matters.

Shrubs need their full spread

A shrub planted at two thirds of its spread becomes a problem you cannot undo without a saw. It will not thin out and it will not split, and in five years you are cutting a lopsided hole in one side of it to keep the path clear. Give shrubs the number on the label, accept the bare soil around them for two summers, or fill the space with annuals you pull out later.

Perennials forgive crowding because you can lift, split and replant them in autumn or spring and they are better for it. Crowding a perennial is recoverable, as long as you actually go back and split it.

Pot size decides how long you wait

The count stays the same whatever size you buy, since the plant reaches its spread eventually either way. Pot size only decides how long you wait for it. A 9 cm perennial takes two summers to reach its spread, a 2 litre one takes a single summer, and a 5 litre shrub is about two years ahead of a 3 litre.

On a large bed, the gap between 9 cm and 2 litre perennials across forty plants is serious money for one season of impatience. The exception is anywhere you have to look at all year, like the front of a bed beside a door, where buying the bigger pots for the front row and the small ones for everything behind is a reasonable compromise.

Border Builder does this for your own bed: enter its size, aspect and soil and it draws up the plan.