How to Divide and Propagate Garden Phlox, and Why You Must Pull the Seedlings

When a clump needs splitting

The clearest signal is a change in the shape of the plant rather than its health. A clump that has been in the ground four years or more starts pushing up a thicket of thin stems instead of a few thick ones, and every head is smaller than the same plant produced two seasons ago. Look into the center and you often find it going bare and woody while the outer ring keeps growing. That is a crown competing with itself, and no amount of feeding will change it.

Compare it against the alternative diagnosis before you dig. If the stems are thin and numerous but the center of the crown is still solid and vigorous, spring thinning may be all it needs this year. If the middle has gone hollow or woody, it needs lifting and splitting.

Expect to divide an established clump every three to four years on average, and more often on rich soil where the plant grows fast. Slow-growing clumps in leaner ground go longer. Judge by the stems and the head size, not by the calendar.

Division is also how you keep the plant you actually have. Every division is genetically identical to the parent, which seed emphatically is not.

Splitting the clump

Divide in early spring as the new shoots break the surface, which is the easiest moment because you can see where the buds are and there is no foliage to support. Early autumn also works if you leave at least six weeks before the ground freezes. Do not divide a plant in full flower.

The rule for every piece is the same: visible buds or shoots on top, a solid share of fibrous root underneath. A piece with roots and no bud will never grow, and a piece with a bud and almost no root sulks for two seasons. Three to five shoots per division is the practical size, giving a plant that looks like something in its first year and flowers properly in its second.

Throw the old center away. On an aged clump the middle is exhausted and woody and will not improve; the vigorous material is the outer ring. Keeping the woody heart out of misplaced thrift simply gives you back the plant you were trying to fix.

Work quickly and keep the divisions in the shade under damp sacking while you go. Fine phlox roots dry out fast, and dry roots are the usual reason a division fails.

1. Water the clump the day before Soak the ground a day ahead. A hydrated plant lifts more cleanly, the soil falls away from the fibrous roots more easily, and the divisions take much less of a check than pieces taken from dry ground.
2. Lift the whole clump with a fork Drive the fork in a spade's width outside the visible shoots and work right around before levering up. Phlox roots spread wider than the top growth suggests, and cutting in close throws away most of the root system.
3. Shake or wash the crown clean Clear enough soil to see where the shoots sit and how the roots are arranged. Dividing a soil-caked crown means cutting blind through buds, and you will lose pieces you meant to keep.
4. Cut the outer ring into pieces and discard the middle With a clean sharp knife or spade, split the vigorous outer growth into pieces of three to five shoots, each with plenty of root. Bin the woody exhausted center rather than replanting it.
5. Replant at the same depth, well spaced Set each piece with the crown level with the soil surface, eighteen to twenty-four inches (45-60 cm) apart. Firm gently with your hands, then soak with two full cans to settle soil around the roots.
6. Keep them watered for the first season Water thoroughly once or twice a week through the first summer. A reduced root system cannot reach far, and a division that dries out in its first midsummer takes two seasons to catch up.

Sterilize the blade with alcohol between clumps. Stem eelworm travels on tools and on divisions, and dividing an infested plant is the standard way it spreads through a whole border in a single afternoon.

Root cuttings and basal cuttings

Root cuttings are the traditional method and have one specific advantage: stem eelworm lives in the stems and leaves, not the roots, so plants raised from root cuttings can be clean even when taken from an infested parent. It is worth knowing if a plant you value has been condemned.

Take them in late autumn or winter while the plant is dormant. Lift the clump or scrape soil away from one side, cut several pencil-thick roots into pieces two to three inches (5-8 cm) long, and lay them horizontally on a tray of gritty potting mix. Cover with half an inch (1 cm) of compost, keep them just moist in a cold frame or unheated greenhouse, and shoots appear in spring. Pot them up individually once they have two or three leaves.

Basal cuttings are quicker if you only want a few plants. In spring, when the new shoots are about three inches (8 cm) tall, cut two or three of them off right at the crown so each carries a small heel of firmer tissue. Trim the lower leaves, insert them around the edge of a pot of gritty potting mix, keep them shaded and just damp, and they root in four to six weeks. Taking these from a clump you were going to thin anyway costs you nothing.

All three methods give you the plant you started with. That is the point, and it is the reason seed is not on the list.

Pull the seedlings, every single spring

Border phlox seeds itself freely, and the seedlings are almost always a plain, hard magenta with no resemblance to the parent. Named garden plants are selections and hybrids; their seedlings revert. The magenta ones are also markedly more vigorous than most selected plants, so left alone they grow faster and taller and swamp the clump they came from.

The takeover is slow enough to go unnoticed. Seedlings germinate at the edge of the parent, look like part of the clump, and by the third or fourth year make up most of the stems. Gardeners describe this as a plant that has changed color or reverted, but the original plant has simply been outgrown and shaded out by its own offspring.

The prevention is deadheading. Cut each head off as it fades, back to the first pair of side buds below it, before seed is shed. This both extends the flowering and removes the source. It takes a few minutes a week during the flowering season and saves the plant.

The cure, once seedlings are up, is to pull them in spring while the shoots are short and you can see which stems sit outside the original crown. Anything growing at the edge of the clump that you did not put there comes out, roots and all. If a clump has already been taken over, lift it, identify the true crown by its position and its finer growth, replant that alone, and discard the rest.

Write down the flower color and the exact position of each crown in the first season, and record every division: the date, the parent clump and where each piece went. Divisions and seedlings look nearly identical for their first two years, and the written note is the only way to tell later which is which.