Gadenin日本語Get the appWhy an unsheared mat hollows out in the middle
Moss phlox grows by running outward. Each trailing stem lengthens a few inches a year, roots where it touches soil, and pushes on. The flowers are carried at the tips of that year's growth, so the flowering zone moves steadily away from the center of the plant while the oldest wood is left behind in the middle, shaded by everything that grew over it.
Left alone, that center thins, goes woody and eventually dies out. By the fourth or fifth year you have a ring: a band of green and flower around the outside, and a patch of bare stems and soil in the middle that no amount of watering or feeding brings back. This is the single most common complaint about the plant, and it is not a disease. It is what happens when the plant is never cut.
Shearing reverses the direction of growth. Cutting the stems back forces buds along the older wood behind the tip to break, and those new shoots fill the space inside the mat rather than outside it. Do it every year and the plant thickens instead of hollowing. Skip it for three or four years and you are into repair work rather than maintenance.
| Treatment | What the mat looks like at five years | Flowering |
|---|---|---|
| Sheared every year after flowering | Dense, even carpet two to three inches (5-8 cm) deep all through | Full sheet of bloom to the center |
| Sheared occasionally | Patchy, thin over the older parts, some woody gaps | Uneven, heavier around the edges |
| Never sheared | Open ring of growth around a bare woody middle | A band of flower at the outer edge only |
Shear the whole mat by a third, as soon as the flowers fade
Timing matters more than technique. Cut as soon as the last flowers brown, which is usually two or three weeks after the display opens in mid spring. At that point the plant still has the whole warm season ahead of it to regrow, and the new shoots harden off long before cold weather. Shearing in late summer or autumn leaves soft growth going into winter and loses you flower buds, because the buds for next spring are set on this year's growth during late summer.
Take roughly a third off the length of the stems. That sounds drastic on a plant only four or five inches (10-13 cm) deep and it looks shocking for a fortnight, but moss phlox breaks readily from old wood and the mat is green again within three or four weeks. Cutting by only an inch does almost nothing; the flowering tips are removed but no dormant buds further back are triggered.
Cut the entire mat, not just the flowered edge. The interior is exactly the part you are trying to make branch, and it is the part most people leave alone because it is not carrying dead flowers. Run the shears across the whole plant at an even height, working from the center outwards so you can see where you have been.
A hedge shear or a pair of grass shears is the right tool for anything larger than a dinner plate. Deadheading moss phlox flower by flower is a waste of an afternoon and does not achieve the branching that a hard shear does.
- Wait until the last flowers have browned The display lasts two to three weeks. Cut when perhaps nine flowers in ten have gone over. Cutting a few days early costs you almost nothing; cutting a month late costs you a season of thickening.
- Sweep out the dead leaves first Run your fingers or a small hand rake through the mat and pull out the papery brown leaves and any debris that has blown in over winter. It is far easier before you cut than after, and it shows you where the bare patches are.
- Shear the whole mat down by about a third Hold the shears flat and take an even layer off the top, leaving the mat roughly two to three inches (5-8 cm) deep. Cut across the middle as well as the flowering edge. Do not cut into bare woody stems with no green at all.
- Clear every clipping off the plant Rake or brush the cuttings away completely. A layer of damp clippings sitting on the crowns holds moisture against the stems in warm weather and is a common cause of rot in the weeks after shearing.
- Water once if the soil is dry, then leave it One thorough soak helps the regrowth start if the ground is dry. After that, treat it as normal. Do not feed, and do not water on a schedule, or the soft regrowth outruns its roots.
Do not mulch after shearing, however bare the plant looks. Bark or compost laid over a moss phlox holds moisture against the crowns and the stems rot underneath it. Bare soil, grit or fine gravel between the plants is what this species wants.
Feeding, watering and the mistakes that follow a shear
The instinct after cutting a plant hard is to feed it. Resist it. Moss phlox flowers best on poor, lean ground, and nitrogen applied after shearing produces long, soft, dark green growth with very few flower buds on it. The plant regrows perfectly well on whatever the soil already holds. If the mat is genuinely starved on pure sand or gravel, a very light scattering of a balanced granular feed in early spring, before flowering, is enough for the whole year.
Watering is the same story. Water the fresh cut mat once, then only during a genuine drought. This is a plant from thin, dry, sunny ground; its roots are shallow and it copes with dry soil far better than it copes with wet. Regular summer watering on a plant that has just been cut is the fastest route to stem rot, because the cut surfaces are open and the mat is dense enough to stay damp underneath.
The other common mistake is cutting into hard bare wood in the hope of forcing regrowth from it. Old moss phlox stems that carry no green shoots at all rarely break again. Cut into the green, and deal with the bare wood by layering new growth over it instead.
Repairing a mat that has already gone bare in the middle
If the center is already dead there is no shearing regime that fills it directly, because there is nothing living left there to break into growth. What works instead is bringing living stems back across the gap and rooting them into it.
Right after shearing, pull several of the long healthy stems from the outer ring inward across the bare patch, press each one down onto the soil, and pin it with a bent wire staple or lay a small stone on it. Scratch a little grit and soil over the pinned section so the stem sits in contact with damp ground rather than on top of dead wood. Those stems root within a season and become the new center of the plant.
For a mat that has gone past repair, the honest answer is to start again from its own material. Lift the whole plant after flowering, cut away and discard the woody dead middle, and replant only the vigorous rooted outer pieces, spaced out, at the same depth they were growing. Each piece re-covers the ground in a season or two and you have lost nothing but the old wood.
Whichever route you take, write down what you did and when. The date of the shear, and how deep you cut, are worth having next year, because the honest test of whether you cut hard enough is what the mat looks like the following spring.
Keep a note of the shearing date each year and a line on how dense the mat looked the following spring. Two or three seasons of that record tells you exactly how hard your plants like to be cut in your conditions, which no general figure can.