Garden Phlox Year Round: What to Do Each Season, from Thinning to Autumn Cleanup

The shape of a phlox year

Border phlox is a herbaceous perennial that dies back to nothing each winter, rebuilds a full-sized plant between spring and midsummer, flowers in the second half of summer for four to six weeks, then dies down again. Every stage has one task that matters and several that do not.

What makes the plant unusual is how much of the outcome is fixed early. The number of stems you leave in spring sets the head size and the mildew risk for the whole year, and neither can be corrected in summer. If you only do one thing to a phlox, do that.

Use the table as a checklist and the sections beneath it for the reasoning. Seasons are given rather than dates so the sequence reads the same in either hemisphere.

Spring: the season that decides the year

Start before the shoots appear by clearing away anything left around the crown, because mildew spores overwinter in old leaf litter and reinfect the new growth from below. Top-dress with an inch (2.5 cm) of compost at the same time.

As the shoots break the surface, decide whether the clump needs dividing. Many more stems than last year, all thinner, with a hollow or woody center, means lift and split now while you can see the buds. Take vigorous outer pieces of three to five shoots each and discard the exhausted middle.

When the shoots are four to six inches (10-15 cm) tall, thin every established clump to five or six of the stoutest, snapping the rest out at the base. Then walk the bed and pull anything growing outside the original crown, because self-sown seedlings appear at the edges and will swamp the parent within a few years.

Finish spring by feeding once with a balanced fertilizer, laying two inches (5 cm) of mulch over the root area with the material pulled back from the stems, and putting supports over any clump you know flops. Supports go in while stems are short; they are useless once a plant has collapsed.

1. Clear the crown before growth starts Rake away every scrap of last year's stems and leaves and bin it. This is the cheapest mildew control there is, because it removes the spore reservoir sitting directly beneath the new shoots.
2. Divide if the clump has run to a thicket Lift clumps showing many thin stems and a woody center, split the outer ring into pieces of three to five shoots with good roots, discard the middle and replant 18-24 inches (45-60 cm) apart.
3. Thin to five or six stems Once shoots are four to six inches (10-15 cm) tall, snap out the weakest at the base until five or six evenly spaced stems remain and you can see daylight through the clump.
4. Pull the seedlings Remove every shoot growing outside the original crown, roots and all. Seedlings revert to plain magenta, grow more strongly than the parent, and take the clump over if they are left.
5. Feed, mulch and support One dressing of balanced fertilizer, two inches (5 cm) of mulch kept off the stems, and a ring support or twiggy sticks over any clump that flopped last year, put in while the stems are short.

Summer: water, watch and deadhead

From early summer the plant is building stem and bud in the hottest, driest part of the year on a shallow root system, and drought at this stage means small heads or buds that dry up before opening. Give a deep soak once or twice a week rather than a daily sprinkle, wetting the top eight inches (20 cm), and always at the base of the plant in the morning rather than over the foliage in the evening.

Once a week, part the stems and look at the lowest leaves in the center of the clump. That is where powdery mildew starts, and you will see it there a fortnight before anything shows on the outside. Pick off and bag any leaf with a floury patch. Early stripping often stops an infection outright, and a fungicide is only worth applying while the plant is still largely clean.

When the heads open, deadhead as each one fades, cutting back to the first pair of side buds below the spent panicle. That usually brings a lighter second flush and, more importantly, prevents seed being shed and starting next year's seedling problem.

Do not cut whole stems to the ground after flowering. The foliage is still feeding the crown for next spring, and stripping it early costs you stems next year.

Autumn, winter and the notes that make next year easier

Autumn is a good planting and dividing season provided you leave six weeks before the ground freezes. Later in the dormant season you can take root cuttings, which is the one method that produces clean plants from a parent carrying stem eelworm.

Once the foliage has yellowed, cut the stems down to two or three inches (5-8 cm) and clear every leaf and stem fragment away from the crown. Bin or burn the material; do not compost it and do not leave it as a mulch. This is the second half of the mildew cycle and skipping it undoes the spring thinning.

In winter the plant needs nothing except not to be waterlogged. On heavy soil that lies wet, a crown sitting in standing water rots, and if that has happened before, lift and replant on a slight rise in spring. Cold itself is rarely the problem for border phlox.

Use the quiet weeks to review your notes clump by clump: when the first head opened, how wide the biggest head was, how many stems you left, when mildew first appeared and on which plants, and the flower color each clump showed. Those five records answer next spring's questions before you have to guess. They tell you which clump is due for division, which position in the garden is genuinely too still, which plant is susceptible enough to be worth replacing, and whether the color of any clump has quietly drifted towards magenta since you planted it.

Mark the position of every crown before the top growth disappears in autumn. The plant is invisible from then until well into spring, and a fork through a dormant crown is the most common accidental loss in a herbaceous border.