Gadenin日本語Get the appWhat you are actually growing
Pentas lanceolata is a woody-based tender perennial from tropical east Africa and the Arabian peninsula. Where winters stay frost-free it lives for years and slowly builds into a loose shrub two to four feet (60 to 120 cm) tall. Everywhere else it is grown as a summer annual, planted out once the nights are warm and pulled or brought indoors before the first frost. Knowing which of those two plants you have changes almost every decision you make about it, so settle it first: if your winter low ever reaches freezing, treat it as an annual with an option to overwinter.
The thing that makes it worth the space is heat tolerance. Petunias go leggy and stop, impatiens wilt by noon, lobelia is finished by midsummer. Pentas keeps opening flat, five-pointed star clusters at the top of every shoot right through the part of summer when nothing else in the bed is doing anything, and it does it in humidity that ruins most bedding plants. If your summers are genuinely hot, this is one of the few reliable performers.
The flower is built as a dome of many small tubular stars, two to four inches (5 to 10 cm) across. Each cluster opens over one to two weeks, then browns from the outside in. What happens to that spent cluster is the single biggest control you have over how much the plant flowers for the rest of the season, and it is covered in the pruning guide.
A pentas that is still flowering hard in the hottest stretch of the year is behaving normally. If yours has stopped in that window, look at deadheading and water before you look at anything else.
Sun, warmth and where to put it
Give it a minimum of six hours of direct sun, and eight is better. In full sun the plant stays compact, the stems stay short between leaves and the clusters come thick and close together. In half a day of sun it still flowers but stretches, and in dappled shade you get a tall soft plant with a few small heads at the tips. In the very hottest, driest climates a little afternoon shade keeps the foliage from scorching without costing you many flowers, but that is a concession to a specific climate, not a general rule.
It is genuinely warm-season. Growth is slow and sulky below about 60F (16C) at night, and it will sit and do nothing for weeks if you plant it out too early into cold ground. Wait until the last frost is past and the soil has warmed to at least 65F (18C), which in most gardens is two or three weeks after you would plant tomatoes. A pentas planted into warm soil catches up and overtakes one planted early into cold soil every time.
Wind is worth a thought. The stems are brittle where they branch, and a plant in a windy corner loses whole flowering shoots in a summer storm. A spot with a wall or a taller planting behind it keeps the shape intact. In containers, choose something heavy enough not to blow over once the plant is top-heavy with flower.
Soil, water and feeding
Pentas wants moisture-retentive soil that still drains. It is not a drought plant in the way lantana is: let it dry to the point of wilting repeatedly and the flower buds abort and the lower leaves yellow and drop. Equally, it will rot at the crown in ground that stays soggy. Work two to three inches (5 to 8 cm) of compost into the bed before planting, and if your ground is heavy clay, raise the planting position by four inches (10 cm) so water moves away from the base.
In the ground, water deeply once or twice a week rather than a little every day. Push a finger two inches (5 cm) into the soil beside the plant; if it is dry at that depth, water until the ground is wet six inches (15 cm) down. A two-inch (5 cm) mulch of bark or shredded leaves, pulled back an inch from the stems, halves how often you need to do this and keeps the roots cool in hot weather.
Containers are a different plant altogether. A pentas in a twelve-inch (30 cm) pot in a hot summer will need water daily, sometimes twice on a windy day above 95F (35C). The mix should be a standard peat- or bark-based potting mix with a handful of perlite added, never garden soil. Water until it runs freely from the drainage holes, and empty the saucer afterwards.
Feed lightly and often rather than heavily. A balanced slow-release fertilizer worked into the bed or the pot at planting, plus a half-strength liquid feed every two to three weeks once flowering starts, is about right. Heavy nitrogen gives you a big dark green plant with few clusters, which is the most common feeding mistake with this species.
| What you see | Most likely cause | What to do |
|---|---|---|
| Large leafy plant, very few flower clusters | Too much nitrogen, or under six hours of sun | Switch to a low-nitrogen or high-potash feed; check actual sun hours |
| Flowering slowed in midsummer, lots of brown heads | Spent clusters left on the plant | Deadhead every cluster back to the next bud pair |
| Lower leaves yellow and dropping, plant limp | Repeated drying out, or waterlogging | Check soil two inches (5 cm) down before each watering; fix drainage |
| Pale yellow leaves with green veins on new growth | Iron chlorosis in alkaline soil | Apply chelated iron and mulch with compost to acidify slowly |
| Stems long and floppy, flowers only at the tips | Too little light, no pinching when young | Move to more sun; pinch tips on the next flush of growth |
Spacing, staking and the butterflies
Space plants twelve to eighteen inches (30 to 45 cm) apart in a bed, depending on how vigorous the plant is. Crowded pentas trap humid still air around the lower leaves, which is where powdery mildew and spider mites both start. You should be able to see daylight through the base of a mature planting. In containers, one plant to a ten-inch (25 cm) pot, or three in an eighteen-inch (45 cm) pot if you want a full look quickly.
Taller plants in rich soil sometimes flop after heavy rain. Rather than staking individual stems, cut the whole plant back by a third early in the season so it branches low and holds itself up. A plant that has been pinched twice while young almost never needs support.
This is one of the best nectar plants you can put in a summer border. The long tubular flowers are the right shape for butterfly proboscises and for hummingbird bills, and a mature clump will hold swallowtails, skippers and, in the Americas, hummingbirds all day. That has a practical consequence: be extremely reluctant to spray anything on a flowering pentas. If you have a pest problem, handle it with water, with hand removal, or with a targeted treatment applied late in the evening when nothing is feeding.