How to Plant Moss Phlox: Spacing, Sharp Drainage and Planting on a Bank

Choose the position before you buy the plants

Moss phlox is not a plant you can move around until it looks right. It wants a specific set of conditions and it fails slowly and unmistakably without them, so the decision about where it goes is the most important one in the whole job.

It needs six to eight hours of direct sun. It needs ground that drains sharply, which in practice means a slope, a bank, the top of a wall, a gravel bed, a rock garden or the front of a raised bed rather than flat heavy soil. And it needs that ground to be lean rather than enriched. Where those three things overlap, the plant will look after itself for years.

It is also worth being clear about what you are planting it for. The flowers last two or three weeks in mid spring and that is all. For the remaining eleven months you have a low evergreen carpet that colors bronze or purple in cold weather, which is a perfectly good thing to have, but plant it as a season-long flower display and you will be disappointed. Use it where a short, overwhelming sheet of color is worth having and where the foliage carries the rest of the year: spilling over a wall, along a path edge, holding a bank, threaded between rocks or planted among spring bulbs that come up through it.

When to plant, and what a good plant looks like

Early spring and early autumn are both good. Spring planting gives the whole growing season for the roots to establish, which matters most in cold-winter gardens where a poorly rooted autumn plant can be lifted out of the ground by frost. Autumn planting works well in mild climates and in hot summers, since the plant establishes in cool damp weather rather than fighting heat. Avoid planting into the middle of summer heat or into frozen or waterlogged ground.

Look for a plant that is dense right across the pot rather than one with long trailing stems and an open center, and tip it out to check the roots are white and working through the compost rather than circling in a solid mass. Needle foliage should be a fresh green, not gray or dusty, which can indicate mites. A slightly smaller plant with a tight habit will make better ground cover in two years than a large sprawling one.

Spacing for a carpet, and how long it takes to close

Space according to how quickly you want the ground covered and how much weeding you are prepared to do while it closes. Each plant spreads to about eighteen to twenty-four inches (45-60 cm) across in time, so twelve to eighteen inches (30-45 cm) apart is the standard planting distance for a solid carpet.

Stagger the plants in a diagonal grid rather than in straight rows. A grid closes visibly faster and does not leave obvious lines of bare soil while the mat is filling in, and it looks natural rather than planted once it has knitted together.

On a slope, set each plant slightly proud of the soil surface so that water runs away from the crown rather than pooling behind it, and work up the slope from the bottom so soil you dislodge falls onto ground you have already planted. On a wall top, plant far enough back from the edge that the roots have soil to work into, and let the stems find their own way over the face.

Expect one full season for the plants to touch at close spacing and two to three at wide spacing. Nothing you do speeds that up materially, and feeding to force it produces soft growth rather than faster cover.

Preparing ground that drains, without enriching it

The one thing you must not do is dig in a barrow of compost or manure to give the plants a good start. Rich soil produces long soft growth and poor flowering, and it holds moisture around stems that need to stay dry. Preparation for moss phlox means opening the soil up and getting the weeds out, not feeding it.

On heavy or slow-draining ground, work coarse grit or sharp sand into the top eight inches (20 cm) at something like a third by volume, and plant slightly high so the crown sits above the surrounding level. On sand, gravel or thin stony ground, do almost nothing beyond loosening it and removing weeds; that soil is already what the plant wants.

Clearing perennial weeds completely, before anything is planted, is the step that saves the most work over the life of the planting. Once the mat closes over, grass runners and creeping perennials cannot be got out without destroying the phlox.

1. Clear every perennial weed root Dig out grass runners, bindweed, docks and creeping perennials completely. This is the last chance you will get, because a closed moss phlox mat cannot be weeded properly once it has knitted together.
2. Loosen the soil and open up the drainage Fork over the top eight inches (20 cm). On clay or any ground that stays wet, mix in coarse grit or sharp sand at roughly a third by volume. Add no compost, no manure and no fertilizer.
3. Dig a hole twice the width of the rootball Width matters and depth does not, because the roots run sideways near the surface. Make the hole wide and only as deep as the rootball is tall so the plant cannot settle lower than it was growing.
4. Set the crown level with the soil, or slightly above The point where the stems meet the roots must sit at or just above the surface. A crown planted low collects water and is the most common cause of a plant lost in its first winter.
5. Firm gently and water in once Press the soil down with your hands rather than treading it, then water thoroughly to settle soil around the roots. That single soak is the only feeding and watering the plant needs at planting.
6. Top the surface with grit, never with bark A half inch (1 cm) of grit or fine gravel between the plants suppresses weeds, keeps the stems dry and gives the trailing shoots something to root into. Organic mulch over the crowns rots them.

Water new plants roughly once a week through their first summer whenever the weather is dry, then stop. From the second season onward an established moss phlox in open ground wants rain and nothing else except in a genuine drought.

The first two years, and what to write down

In the first season the plant puts its effort into roots and runners rather than flowers, and the display may be modest. That is normal. Water through dry spells, keep the weeds out of the gaps while they remain open, and shear the mat lightly after whatever flowering you get, so it starts branching from the first year rather than the third.

By the second spring the plants should be touching at close spacing and flowering properly, and from that point the routine is simply the annual shear after flowering, occasional weeding and nothing else.

Record the planting date, the spacing you used, and a sketch or note of where each plant went and what color it was. Colors are impossible to tell apart out of flower and moss phlox plantings almost always end up being extended or patched, at which point knowing what is where is the difference between a considered planting and a muddle. Note also which parts of the planting struggled in the first winter, because those are the spots where drainage needs work before you replant them.