Overwintering Pentas: Frost Tolerance, Bringing Plants In and Cuttings

How much cold it actually takes

Pentas is a tropical perennial with essentially no frost tolerance. It is reliably hardy only where winter lows stay above about 30F (minus 1C), which in practice means frost-free subtropical and tropical gardens. There it behaves as an evergreen shrub that flowers on and off all year and is cut back once in late winter to keep it from getting woody and bare at the base.

Everywhere else it is an annual unless you intervene. A light frost blackens the growing tips and the flower clusters. A proper freeze kills the whole plant to the ground, and unlike a hardy perennial it does not come back from the roots. There is no mulching trick that changes this in a garden that actually freezes.

Cold without frost is a separate problem and it starts much earlier than most people expect. Growth slows markedly once nights sit below 60F (16C), and sustained temperatures around 40 to 50F (4 to 10C) cause leaf drop and a general sulk that the plant is slow to recover from even after it warms up. That is why the decision about overwintering has to be made while the weather is still mild, two to three weeks before the first expected frost, rather than the night before it.

Cuttings: the low-effort way to keep it

For most gardens this is the right answer. Two to three weeks before your first expected frost, take four-inch (10 cm) tip cuttings from healthy shoots, remove any flower heads, strip the lower leaves and root them in a gritty mix or in water on a bright windowsill. They root in two to three weeks and spend the winter as small pots that need a bright cool windowsill, water only when the surface has dried, and no feeding at all until late winter.

Take more than you think you need, six or eight for every plant you want next year. They cost nothing, they take up a tray on a windowsill rather than a whole pot in a room, and a small young plant survives an indoor winter far better than a big one because it needs much less light to keep itself going.

Inspect what you take. Cuttings from a mite-infested plant carry mites into a warm dry room with no predators, and by late winter you will have lost them. Take material only from clean shoot tips, and rinse the cuttings under the tap before you insert them.

Late winter is when overwintered cuttings wake up. Cut each one back by half to force branching, repot into fresh mix, move to your brightest position, and start feeding at quarter strength once new growth is clearly moving.

Bringing a whole plant indoors

If you want to keep a large established plant, do the work while it is still warm outside. Cut the plant back by a third to a half first, which reduces the leaf area it has to support in poor indoor light and makes it far easier to inspect. Then check every leaf underside with a hand lens for spider mite stippling and webbing, and hose the whole plant down thoroughly twice, a week apart, before it comes anywhere near the house.

For a plant growing in the ground, lift it with as much root as you can, pot it into a container barely larger than the root ball with fresh potting mix, water it in and leave it in a shaded sheltered spot outdoors for a week to recover from the root disturbance before moving it inside. Expect some leaf drop; it is normal and the plant recovers.

Indoors, the target is bright and cool rather than warm: 55 to 65F (13 to 18C) in the brightest window you have, ideally south-facing, or under a grow light for twelve hours a day. Keep it away from radiators and heating vents, which combine dry air and warmth to produce exactly the conditions spider mites want. Water only when the top two inches (5 cm) of mix have dried, which through winter might mean every ten to fourteen days. Do not feed until late winter.

The plant will look poor by midwinter: thin, pale, dropping the odd leaf, possibly not flowering at all. That is survival mode and it is fine. Judge it in early spring, not in the middle of winter.

1. Cut back first Take the plant back by a third to a half, cutting above healthy leaf pairs. Less leaf means less light needed and much less to inspect.
2. Inspect and wash Check leaf undersides with a hand lens for mite stippling and webbing. Hose the whole plant hard, twice, a week apart, before it goes indoors.
3. Pot and settle If lifting from the ground, pot into a container just larger than the root ball with fresh mix, water in, and leave it in shade outdoors for a week.
4. Move in before it gets cold Bring it inside while nights are still above 50F (10C), rather than waiting for the frost warning. A chilled plant does badly indoors.
5. Hold it cool and bright Keep at 55 to 65F (13 to 18C) in the brightest window, away from heating vents. Water only when the top two inches (5 cm) are dry, and do not feed until late winter.

Frost-free gardens, and going back out in spring

Where it never freezes, pentas is simply a shrub and the winter job is one cut. In late winter, before new growth starts, take the whole plant back by about half, remove any dead or crossing wood from the middle, and clear the old brown flower heads. Mulch afterwards with an inch or two (3 to 5 cm) of compost. Plants left uncut year after year become tall with bare woody legs and all the flowers out of reach at the top.

For plants and cuttings that have wintered indoors, moving back outside follows the same rules as any tender plant. Wait until frost is past and nights are reliably above 55F (13C), then harden off over seven to ten days, starting with two hours in dappled shade and building up to full sun. Indoor-grown foliage scorches badly if it goes straight into full sun, and the damage sets the plant back for weeks.

Once it is out and the soil has reached 65F (18C), plant it, water it in, and pinch the tips to start the branching for the season. From that point it is on the normal summer routine of deadheading, deep watering and a light feed every two to three weeks.