Raising Pentas: Sowing Fine Seed and Rooting Cuttings That Take Easily

Sowing: light, warmth and surface sowing

Start seed ten to twelve weeks before your last expected frost. That is a long lead time and it is not padding: pentas germinates unevenly and grows slowly in its first month, and a plant started late goes out small and never catches up in a single season.

The seed is fine and needs light to germinate, so it goes on the surface and is not covered. Fill a tray with a fine seed-starting mix, firm it lightly, water from below until the surface is evenly damp, then scatter the seed thinly across the top and press it down gently with a flat board or the base of a pot so it makes contact without being buried. Cover the tray with a clear lid or a sheet of plastic to hold humidity, and put it somewhere bright but out of direct midday sun.

Warmth is the other half. Aim for 75 to 80F (24 to 27C) at the surface of the mix, which in practice means a heat mat. Below about 70F (21C) germination becomes slow and patchy. Expect the first seedlings in ten to fourteen days, with stragglers arriving for another week after that, so do not throw the tray out early.

Once about half the seedlings are up, take the cover off in stages over two or three days rather than all at once, and move the tray under bright light or into a very bright window. Seedlings left under a lid too long stretch and damp off. Keep the mix damp but never wet; water from below, and let the surface just dry between waterings.

Transplanting seedlings and growing on

Move seedlings into individual cells or small pots once they have their first pair of true leaves beyond the seed leaves, usually two to three weeks after emergence. They are tiny at this stage and the stems bruise easily, so lift each one with a dibber or the tip of a pencil under the roots and hold it by a seed leaf, never by the stem. Set it in the new mix at the same depth it was growing, firm gently and water in from below.

Grow them on at 65 to 70F (18 to 21C) under bright light for fourteen to sixteen hours a day, with the light source three to four inches (8 to 10 cm) above the leaves if you are using a lamp. Feed at quarter strength once a week starting two weeks after transplanting seedlings. Pot up into three- or four-inch (8 to 10 cm) pots when roots show at the drainage holes.

Pinch out the growing tip when a plant reaches six inches (15 cm) with three or four leaf pairs. This is worth doing before the plant leaves the greenhouse or windowsill, because a young pentas pinched at that stage goes out into the garden already branching rather than as a single stem.

Damping off is the main loss at this stage. Sow thinly, use clean trays and fresh mix, water from below, give the seedlings moving air from a small fan once the cover is off, and never let the surface sit wet overnight.

Cuttings: faster, easier and identical to the parent

Pentas roots very readily from softwood cuttings, which is why it is the sensible way to keep a plant you like from one year to the next and to multiply it for nothing. Cuttings taken in late spring or early summer make flowering plants the same season; cuttings taken in late summer, two to three weeks before your first frost, are the standard way to carry the plant through winter as small pots on a windowsill.

Take four-inch (10 cm) tips from healthy non-flowering shoots, early in the morning when the plant is fully turgid. If every shoot has a flower head on it, take the cutting anyway and remove the head. Cut cleanly just below a leaf node, strip the leaves from the lower half, and reduce any very large remaining leaves by half to cut water loss.

Root them in a gritty free-draining mix, three or four to a four-inch (10 cm) pot around the edge, with the lower node buried. Rooting hormone speeds it slightly but is not necessary. Cover with a clear bag held off the leaves by sticks, keep at about 70F (21C) out of direct sun, and keep the mix just damp. Roots form in two to three weeks; you will feel resistance when you tug a cutting gently. Ventilate the bag progressively over a week, then pot each rooted cutting on individually.

Water rooting also works for a quick result: a tip cutting in a glass of water on a bright windowsill roots in about two weeks. Pot those up as soon as the roots are half an inch (1 cm) long, because water roots that are allowed to get long struggle to adapt to soil.

1. Choose the shoot Pick a healthy, soft but not floppy shoot tip with no sign of mite stippling on the leaf undersides. Avoid woody older growth low on the plant, which roots slowly.
2. Make the cut Cut about four inches (10 cm) from the tip, just below a leaf node, with clean sharp snips. Remove any flower cluster or bud so the cutting puts its energy into roots.
3. Prepare it Strip the leaves off the lower half of the stem, leaving two or three pairs at the top. Halve any large remaining leaves across the blade to cut water loss.
4. Insert and cover Push the stem into moist gritty mix so the lowest bare node is buried. Cover with a clear bag held clear of the leaves, and keep at 70F (21C) in bright indirect light.
5. Check and wean After two weeks, tug gently: resistance means roots. Open the bag a little more each day for a week, then pot each cutting into its own three-inch (8 cm) pot.

Seed or cuttings: how to choose

Take cuttings if you already have a plant that performed well for you, because they are quicker, need no heat mat, and give you exactly the plant you liked including its habit and flower color. Take seed if you are starting from nothing, want a large number of plants cheaply, or want a mix of colors in a bed.

The practical difference is time and equipment. Seed needs a heat mat, bright light and ten to twelve weeks of attention before planting-out weather. Cuttings need a windowsill, a plastic bag and three weeks. For most gardens, sowing seed once to establish a planting and then keeping it going by cuttings thereafter is the least work for the most plants.