Calculating plant spacing: a practical guide

Why spacing matters

Getting the spacing right makes the difference between a border that fills in beautifully after two years and one that stays bare for ages or becomes an overcrowded tangle. Too close leads to competition for light, water and nutrients. Too far apart leaves bare patches and more weeds. Garden centres print recommended spacing on labels, but those numbers don't always suit your specific situation.

With GardenWorld you can preview how plants will look in your actual garden. Plan your planting to scale and avoid surprises after you've put everything in the ground.

The basic formula

The formula is straightforward. Divide 10,000 (square centimetres per square metre) by the square of the spacing in centimetres. At 30 centimetre spacing: 10,000 divided by 900 equals 11 plants per square metre. At 25 centimetres: 10,000 divided by 625 equals 16 plants per square metre.

This table gives a quick indication. In practice, round up and buy 10 per cent extra as a reserve.

Spacing by category

Ground cover

Ground cover plants go closest together: 15 to 25 centimetres. The tighter the spacing, the faster you get a closed carpet. Pachysandra at 20 centimetres provides full coverage after two seasons. Geranium macrorrhizum at 30 centimetres needs an extra season but ends up just as dense.

Low perennials (20-40 cm tall)

Space at 25 to 35 centimetres. Examples: lavender, lamb's ears, geranium, catmint. Groups of three to five give the best effect.

Medium perennials (40-80 cm tall)

Space at 35 to 50 centimetres. Examples: salvia, echinacea, rudbeckia, astilbe. Three per group creates a natural look.

Tall perennials (80+ cm)

Space at 50 to 70 centimetres. Examples: delphiniums, foxgloves, miscanthus. As specimens or in threes.

Hedging plants

Spacing depends on species and desired density:

• Beech hedge: 5 per running metre (20 cm apart)
• Privet: 5-6 per metre (17-20 cm)
• Yew: 4 per metre (25 cm)
• Cherry laurel: 3 per metre (33 cm)

Staggered vs. straight rows

In straight rows plants line up neatly. Simple but not the most efficient for coverage. In a staggered (zigzag) pattern, you offset each row by half the spacing. That fits 15 per cent more plants per area and gives faster, more natural coverage.

Staggered planting works best for ground cover and large-scale planting. For mixed borders a looser arrangement looks more appealing.

Practical calculation tips

First calculate the area of your border in square metres. It needn't be exact to the centimetre. Measure length and width and multiply. If the border isn't rectangular, break it into simpler shapes and add the areas together.

Sketch on paper or use a digital tool. Draw the border outline and place circles representing each plant's mature size. That gives a realistic picture of the quantities you need.

Allowing for growth

Spacing is based on the plant's mature size, not its size at purchase. A 9 centimetre pot grows into a 40 centimetre clump. The first year will always look bare. Fill gaps with annuals rather than planting too closely.

Common mistakes

The number one mistake is impatience. Gardeners want instant results and plant too closely. After two years the plants crowd each other, flower less and become disease-prone.

The second mistake is ignoring shade and soil type. A plant that spreads 50 centimetres in full sun may stay at 30 in partial shade. Adjust spacing to the conditions.

Prepare your planting plan at gardenworld.app and calculate exactly how many plants you need.