How far apart should I plant perennials?

The commonest border mistake is planting one of everything at even spacing, which gives a collection rather than a planting and leaves bare soil for two years. The two decisions that change the result are spacing based on eventual spread rather than the size of the pot, and grouping several of the same plant together.

Step by step

1. Find the eventual spread, not the height.The label gives both. Spread is the number that governs spacing, and it is the one most often ignored because the plant in your hand is 150mm across.
2. Space at roughly the eventual spread, or slightly less.Two plants each spreading 500mm, planted 450mm apart, will just meet and knit together. That is usually what you want in a border. For most medium perennials this works out at three to five plants per square metre.
3. Plant in odd-numbered groups of one variety.Threes, fives and sevens of the same plant read as a drift; one of each of seven different plants reads as a collection. This single change does more for how a border looks than any other decision.
4. Repeat the same plant along a border.Using a group of one variety two or three times along a bed gives rhythm and holds the whole planting together. It is what makes professionally designed borders look coherent.
5. Plant closer for quick cover and weed suppression.Dense planting shades the soil, which suppresses weeds far more effectively than mulch alone and reduces watering. Many designed borders are planted substantially closer than the labels suggest for exactly this reason.
6. Give more room to the plants that need it.Anything that resents competition, needs air movement around it, or is prone to mildew and rot in crowded conditions — bearded irises in particular need their rhizomes in the sun and hate being smothered.
7. Use vertical layering to fit more in.Bulbs under perennials, and early plants under later ones, occupy the same square metre at different times. Snowdrops and daffodils under hostas or hardy geraniums is the standard example.
8. Allow for the plants that spread aggressively.Mint, some hardy geraniums, physalis, houttuynia and running bamboos will take over a group planting. Site them where they can spread, contain them, or leave them out.
9. Fill the first two years with annuals rather than wider spacing.If you want correct final spacing but cannot bear the gaps, sow hardy annuals in the spaces for the first couple of seasons. It is better than planting too wide and having to remove plants later.
10. Expect to move things.Perennials transplant readily in autumn or early spring. A border is adjusted for several years rather than being right first time, and moving a plant that is in the wrong place is a normal part of gardening rather than an admission of error.

Tips

• Standing the pots out on the bed before planting anything, then walking away and coming back to look, catches most spacing errors while they are still free to fix.
• Buying three small plants usually costs less than one large one and produces a better group within two seasons.
• In a narrow border, plan for the plants leaning forward over the edge — the usable depth is less than the measured depth.
• Grasses and airy plants like verbena bonariensis can be planted near the front despite their height, because you see through them.

Common mistakes

• Spacing by the pot rather than the eventual spread — Everything is far too close within two years and the border becomes a fight. Read the spread on the label.
• Planting one of each of many different plants — It reads as a collection rather than a planting scheme. Groups of one variety are what make a border look designed.
• Planting at very wide spacing to be safe — Bare soil between plants grows weeds, dries out and takes years to fill. Closer planting with occasional thinning is the better error to make.
• Ignoring the vigorous spreaders — One aggressive plant can absorb a whole group planting within three seasons.

Questions people ask