How to Plant Garden Phlox: Spacing for Airflow, Soil Prep and Planting Depth

When to plant, and what to look for in the plant

Plant in spring as growth is beginning, or in early autumn while the soil is still warm enough to root into. Spring planting suits cold gardens and heavy soils, because the crown is not sitting wet through winter before it has rooted. Autumn planting suits mild gardens, and gives the plant a head start on the following summer.

Avoid planting in the middle of the flowering season. A phlox in full flower has a large canopy and heavy heads to support, and a freshly disturbed root system cannot supply them. If you must move one then, cut the flower heads off and accept a lost season.

Choose a plant with several thick shoots rather than a pot crowded with many thin ones, and look at the lower leaves before you buy. Any floury white patch means mildew is already present and you would be introducing it to the bed. Check the growing tips too: distorted, narrow, crinkled leaves suggest stem eelworm, which is not something you want anywhere near your soil.

Bare-root divisions arrive dormant in spring or autumn. Get them into the ground the day they arrive, and if that is impossible, heel them into damp soil or a pot of compost rather than letting the roots dry.

Spacing: leave more room than looks right

This is the decision people regret. A young phlox occupies a few inches and looks lost at proper spacing, so it gets planted close to its neighbors, and three years later the clumps are touching, the air has stopped moving between them, and mildew arrives every summer without fail.

Set crowns eighteen to twenty-four inches (45-60 cm) apart, and keep the same distance from any dense neighbor that will overtop the lower third of the stems. If you have a choice, run the gap along the direction of the prevailing breeze so air moves through the group rather than around it. Fill the gaps for the first two seasons with annuals you can simply remove, rather than with more perennials you will not want to dig up later.

Avoid the enclosed corner between a wall and a hedge, however sunny it looks at midday. Still air is what the fungus needs, and no amount of later care compensates for it. An open bed with sun for most of the day and a breeze through it is worth more than a warmer, sheltered spot.

Aim for at least six hours of direct sun, weighted towards the morning if you can, because early sun dries the dew off the foliage and shortens the window in which infection can take hold.

Preparing soil that stays damp in midsummer

Border phlox is a moisture-loving plant with a shallow root system, and midsummer is both its flowering season and the driest part of the year in most gardens. The soil has to bridge that gap. This is why phlox does well in heavy loam that gardeners complain about and struggles on a light sandy bed that grows everything else beautifully.

Prepare a wide area rather than a deep hole. The roots spread sideways, so fork over an area two to three feet (60-90 cm) across and work two to three inches (5-8 cm) of well-rotted compost or manure through the top eight to ten inches (20-25 cm). On sandy soil use more, and expect to top it up every year. On heavy clay that lies wet in winter, add compost and coarse grit and plant on a slight rise so the crown is not sitting in standing water.

Waterlogging in winter is the one wet condition phlox will not tolerate. Damp in summer, drained in winter, is the combination to aim for.

Do not add fertilizer to the planting hole. Compost through the whole area, then a balanced feed on the surface once growth is under way in spring, is a better pattern than concentrated feed against new roots.

1. Soak the plant before you start Stand the pot in water until the bubbles stop, or lay bare-root divisions in a bucket of water for an hour. A dry rootball planted into damp soil often stays dry for weeks, because water runs around it rather than into it.
2. Fork over a wide area and add compost Loosen an area two to three feet (60-90 cm) across and work two to three inches (5-8 cm) of compost through the top eight to ten inches (20-25 cm). Widen rather than deepen; the roots run sideways.
3. Dig a hole twice the width of the rootball Make the hole roughly twice as wide as the roots but no deeper than they are tall, so the plant sits on undisturbed soil and does not sink as loose soil settles beneath it.
4. Set the crown level with the soil surface Lay a cane across the hole to check. Buried crowns rot at the base and produce fewer shoots; crowns proud of the surface dry out and the shallow roots suffer first in a dry spell.
5. Backfill, firm and soak Fill in, firm with your hands rather than your heel to avoid compacting the soil around shallow roots, then water with two full cans to settle the soil into contact with the roots and drive out air pockets.
6. Mulch, keeping it off the stems Spread two inches (5 cm) of compost or bark over the root area, pulled back an inch or two from the crown itself so the base of the stems is not sitting in damp material.

Do not plant phlox where a previous phlox was dug out because of stem eelworm. The nematode persists in the soil for years and will infest the replacement. Choose a different bed and put something unrelated in the old position.

The first two seasons, and what to record

Water a new plant thoroughly once or twice a week through its first summer, even in cool weather, because the root system has not yet spread far enough to find its own moisture. A first-year plant that dries out is often set back for a second season as well.

Do not thin the stems in the first spring. A young plant only makes two or three shoots and needs all of them. Start thinning to five or six once it has been in the ground two full seasons and is producing more shoots than that.

Protect the emerging shoots from slugs in the first two springs. An established clump can lose a few shoots without anyone noticing; a new plant with three shoots cannot. Leave the plant undivided for at least three years.

Record the planting date, the exact position of each crown, the spacing you used, and the flower color when it first blooms. Position matters because the crown is invisible from autumn until well into spring and is easily forked through. Color matters because self-sown seedlings will appear around the plant in later years and revert to plain magenta, and a written note of the original color is the only way to be certain that the clump you are looking at in five years is still the one you planted.