Four o’clocks (Mirabilis jalapa) respond well to cutting back at several points during the growing season, and the timing depends on what you’re trying to accomplish. A midsummer trim controls their often aggressive sprawl, deadheading through the bloom period reduces rampant self-seeding, and a hard cutback in fall or after the first frost tidies the garden before winter. These are vigorous, forgiving plants, so even aggressive pruning rarely causes lasting harm. The details, though, vary depending on your climate zone and whether you’re growing them as annuals or returning perennials.
Why Four O’Clocks Need Cutting Back
Four o’clocks are not delicate plants that require careful, measured pruning. They’re fast growers that can reach three to four feet tall and equally wide in a single season, forming dense, bushy mounds that crowd out neighboring plants if left unchecked. A single plant can easily occupy a four-foot circle by midsummer. The stems branch freely, and because each branch tip produces flowers that eventually set seed, an unpruned plant can scatter hundreds of seeds across your garden by the end of the season.
The main reasons to cut them back are size control, tidiness, encouraging fresh growth, and managing self-seeding. Four o’clocks that have been growing unchecked since spring often develop woody, leggy lower stems with most of the foliage and flowers concentrated at the top. Cutting them back forces new branching from lower nodes, which produces a fuller, more compact plant with more bloom sites closer to eye level. If you’ve ever let a four o’clock go all season without any trimming, you know the result: a sprawling mass that flops open in the middle after a hard rain.
Midsummer Shaping
The first opportunity to cut back four o’clocks comes in midsummer, roughly six to eight weeks after they’ve leafed out and begun blooming. By this point, the plants have put on most of their vegetative growth and may already be flopping or encroaching on walkways and neighboring plants. You can cut the entire plant back by about one-third to one-half of its height using hedge shears or hand pruners. This sounds drastic, but four o’clocks recover quickly. New growth typically appears within a week, and the plant will resume blooming within two to three weeks.
The best time for this midsummer trim is during a stretch of warm weather when the plant is actively growing. Avoid cutting back during a heat wave if you can, since the exposed stems lose moisture faster when temperatures are extreme. Early morning is ideal, giving the cut surfaces time to dry before the afternoon sun hits them. Water the plant well after cutting back, and consider a light application of balanced fertilizer to fuel the regrowth.
If your four o’clocks are growing against a wall or fence and you want them tall rather than bushy, skip the all-over shearing and instead selectively remove the side branches that are reaching where you don’t want them. Leave the central stems intact to maintain height.
Deadheading and Seed Management
Four o’clocks produce small, dark, round seeds that ripen inside papery husks at the base of each spent flower. The seeds drop freely when ripe, and because each plant produces flowers nightly for months, the seed output over a full season is enormous. If you’ve ever had four o’clocks pop up in every crack and bed across your yard the following spring, you already understand the problem.
True deadheading, meaning removing individual spent flowers, is impractical with four o’clocks because the flowers are small and numerous. A more realistic approach is to periodically shear off the outer few inches of growth, which removes the developing seed heads along with spent blooms. Doing this every two to three weeks during peak bloom season significantly reduces the number of seeds that make it to the ground. You won’t eliminate self-seeding entirely, but you’ll reduce it from a nuisance to a manageable number of volunteer seedlings the following year.
If you actually want seeds for replanting or sharing, let a few flower clusters mature fully before cutting them. The seeds are ripe when the husk turns papery and brown and the seed inside is hard and black. Collect them, dry them for a few days indoors, and store them in a cool, dry place. Then shear the rest of the plant to prevent unwanted spread.
Cutting Back After Frost
In most climates, the first hard frost of autumn kills the above-ground stems and leaves of four o’clocks overnight. The foliage turns black and mushy, and the stems collapse. At this point, you should cut the dead growth down to the ground, leaving just an inch or two of stubble above the soil surface. There’s no reason to leave the dead stems standing through winter unless you simply haven’t gotten around to the cleanup. They don’t provide meaningful structure or wildlife habitat the way ornamental grasses or seedheads of coneflowers might.
Use loppers or heavy-duty pruners for the thickest stems near the base, which can become surprisingly woody over the course of a season. Gather the cut material and either compost it or bag it. If your plant was heavily diseased or infested with spider mites during the growing season, it’s better to bag and discard the stems rather than composting them, since some pathogens can survive in compost that doesn’t heat up sufficiently.
In frost-free or very mild climates (roughly USDA zones 9 through 11), four o’clocks may not die back on their own. In these areas, the plants can become semi-permanent shrubby perennials that grow larger each year. A hard cutback in late winter, taking the plant down to about six inches above the ground, serves the same purpose as frost: it forces fresh growth and prevents the plant from becoming an overgrown, woody tangle. Late January through February is the usual window for this in subtropical regions.
Regional Differences in Perennial Behavior
Whether you need to worry about the underground tuber at all depends on where you garden. Four o’clocks develop a thick, tuberous root that stores energy for regrowth. In zones 7 through 11, this tuber typically survives winter in the ground and sends up new shoots in spring without any help from you. In zones 9 and warmer, the tuber can grow surprisingly large over multiple years, sometimes reaching the size of a large carrot or even a sweet potato.
In zones 5 and 6, winter survival is inconsistent. The tuber may make it through a mild winter, especially if the soil is well-drained and the spot is sheltered, but a prolonged cold snap with frozen, wet soil will kill it. If you’re in this borderline range and want to keep a particular plant, you can dig the tuber after the first frost, let it dry for a day or two, and store it in a cool, dry location much the way you’d store dahlia tubers. Replant it after the last spring frost. In zones 4 and colder, the tubers almost never survive outdoors, so most gardeners treat four o’clocks as annuals and start fresh from seed each year.
For gardeners who do dig tubers, the fall cutback and the tuber harvest happen at the same time. Cut the stems to a few inches, then carefully dig around the base of the plant with a garden fork, lifting the tuber without slicing into it. Brush off excess soil but don’t wash the tuber, since moisture promotes rot in storage. Pack the tubers in dry peat moss, vermiculite, or even shredded newspaper inside a cardboard box, and store them somewhere that stays between 40 and 50 degrees Fahrenheit.
Spring Cleanup and Timing New Growth
If your four o’clocks are perennial in your zone and the tubers overwinter in the ground, spring cleanup is minimal. New shoots emerge from the base once soil temperatures warm up, usually around the same time that tomatoes are ready to transplant outdoors. You can clear away any remaining stubble from last year’s stems at this point. Don’t worry if emergence seems late compared to other perennials. Four o’clocks like warm soil and won’t push growth until conditions are right, which can be as late as May or even early June in cooler parts of their range.
If you’re growing from seed, start them indoors about four to six weeks before your last frost date, or direct-sow outdoors after the last frost. The seeds have a hard coat, and soaking them overnight in warm water or nicking them with a file speeds up germination. Once the seedlings are established and growing actively, you can begin the cycle of midsummer trimming and seed management described above.
Tools and Technique
Four o’clocks don’t require specialized tools. For midsummer shearing, hedge shears or even a sharp pair of kitchen scissors work fine on the soft, green growth. For the fall cutback or late-winter hard prune, hand pruners handle most stems, with loppers reserved for the thickest basal stems. Clean your cutting tools with rubbing alcohol or a dilute bleach solution between plants if you’re working in a garden where disease has been a problem.
When making cuts during the growing season, cut just above a leaf node, which is the point where a leaf or pair of leaves attaches to the stem. This is where new branches will sprout. Cutting in the middle of an internode, between two leaf joints, leaves a stub that dies back to the next node anyway, which is harmless but looks untidy. For the all-over shearing approach, don’t worry about individual nodes; just take the whole top layer off evenly and the plant sorts itself out.
One common mistake is cutting back too late in the fall before frost, during a period when the plant is still actively growing. This stimulates a flush of tender new growth that’s immediately killed by the approaching frost, which wastes the plant’s stored energy. If you want to trim for tidiness in autumn, either do it early enough that the new growth has several weeks to mature before frost, or wait until frost does the job for you and then cut the dead stems.
Dealing With Excessive Self-Seeding
Even with diligent shearing, some seeds will escape, and four o’clock volunteers can become a genuine garden management problem over time. The seedlings are easy to identify: they emerge with thick, rounded cotyledons (seed leaves) and quickly develop the characteristic broad, pointed true leaves on sturdy stems. Pull them when they’re small and the taproot hasn’t anchored deeply. Once a volunteer is more than a few inches tall, the root has already thickened, and you’ll need to dig rather than pull to remove the whole thing.
If volunteers have taken over a large area, hoeing the seedlings when they’re still small is the most efficient approach. A sharp stirrup hoe slices them just below the soil surface. Doing this once or twice in late spring, before the survivors get established, dramatically reduces the population. Mulching beds with two to three inches of organic mulch also suppresses germination, since the heavy four o’clock seeds need direct contact with soil to sprout reliably.
Pre-emergent herbicides labeled for use in ornamental beds can prevent four o’clock seeds from germinating, but this is usually overkill for a home garden situation. Hand-pulling and hoeing, combined with more aggressive deadheading shears during the bloom season, are enough for most gardeners. The real key is consistency: one season of letting the plant go to seed unchecked can set you back several years in terms of the seed bank in your soil.
When Not to Cut Back
There are a few situations where you should leave your four o’clocks alone rather than reaching for the shears. If the plant is stressed from drought, transplant shock, or a pest infestation, cutting it back adds further stress and can push it into decline. Address the underlying problem first, let the plant recover for a week or two, and then trim if needed.
If you’re growing four o’clocks specifically to attract hawk moths (the large, hummingbird-like moths that pollinate the fragrant evening flowers), avoid shearing during the peak moth-activity period in your area, which is typically mid to late summer. The moths are drawn to the flowers’ strong fragrance and long tubular shape, and heavy shearing removes the bloom sites they depend on. A lighter touch, removing only the most wayward branches rather than shearing the whole plant, lets you maintain some control without eliminating the pollinator resource.
Similarly, if you’re growing four o’clocks in a naturalized or cottage garden setting where a loose, sprawling habit is part of the aesthetic, midsummer shearing defeats the purpose. In these settings, the fall cutback after frost is really the only pruning event you need. Let the plant do its thing during the growing season and clean up the remains once winter arrives.