Sweet Potato

A rooted stem cutting of Ipomoea batatas can be growing new leaves within a week, which is why so many gardeners meet this plant in a jar of water on a windowsill before they ever plant it outside. The same species gives you two different plants depending on why you grow it: chartreuse, purple or variegated cultivars sold as trailing foliage for hanging baskets and window boxes, and the plainer-leaved field types grown for their edible tuberous roots. Both share the same care, the same vigorous sprawling habit, and the same intolerance for frost.

Ornamental Vine or Food Crop: Know What You Bought

Nursery cultivars such as 'Margarita' (lime green, heart-shaped leaves) and 'Blackie' (near-black, deeply lobed leaves) are bred for foliage color and density, not for large storage roots; they still form small tubers, but they are an afterthought. Field varieties sold as seed roots or slips put their energy into spindle-shaped edible roots and carry plainer green leaves. If a label does not say which one you have, watch the growth habit: ornamental types branch low and mound before they trail, while production varieties send out long runners with fewer side shoots.

Light and Placement

Give it full sun to light afternoon shade. In full sun the ornamental cultivars hold their leaf color best: Margarita stays lime rather than fading toward plain green, and Blackie deepens closer to black. Indoors, a bright south or west-facing window with several hours of direct light keeps a windowsill cutting from stretching and going leggy. Outdoors it tolerates part shade but grows more slowly and flowers less there.

Soil and Planting

Plant in loose, well-drained soil; heavy or compacted clay produces long, stringy edible roots and invites root rot, which sets in when soil stays below about 10 degrees C (50 degrees F). Set slips or rooted cuttings roughly 30cm (12 in) apart, in rows 90 to 120cm (3 to 4 ft) apart if you want the vines to fill the space between them within a season. For edible types, a soil pH of about 5.8 to 6 keeps scurf, a fungal disease that blotches the skin, in check; rotating beds every few years does more against this disease than any spray.

Watering

Water newly set slips daily for the first two weeks while roots establish, then settle into a weekly soak, about 1.2cm (0.5 in) of water a week is enough once the vine is running. Established plants have moderate water needs and wilt visibly, rather than crisping, when they go too dry, so a drooping mat of leaves in the afternoon heat is a cue to water rather than an emergency. If you are growing for tubers, cut back sharply on water in the final three to four weeks before harvest; wet soil late in the season splits and cracks the roots.

Feeding

This is a plant that punishes generosity with nitrogen: soil too rich in it produces a tangle of leafy vine and few or no usable roots. Work 2 to 5cm (1 to 2 in) of compost into the bed before planting rather than feeding through the season, and add bone meal at planting if you want to encourage root formation. Container and hanging-basket specimens grown purely for foliage are the exception and can take a diluted balanced feed monthly through summer, since there is no tuber harvest to protect.

Propagation from Cuttings and Slips

This is the plant's party trick. Cut a trailing section with two or three leaf nodes, strip the lower two-thirds of the leaves, and stand the cut end in a glass of water; roots appear from the nodes within about ten days, and the cutting is ready to pot or plant out two to three weeks later. To grow slips for an edible harvest, suspend a whole tuber half-submerged in water or bury it in moist sand; sprouts form along its length, and each one that has grown a few roots of its own can be twisted or cut free and planted as its own start.

Pests, Diseases and Common Mistakes

Scurf leaves dark, papery blotches on the skin of stored roots; it lives in the soil, so rotate beds and start from disease-free slips rather than treating it after the fact. Voles, mice and deer treat a bed of forming tubers as a pantry, and a physical barrier such as hardware cloth or a raised bed does more than any repellent. The most common mistake with ornamental cultivars is planting them in shade and rich soil expecting the vivid nursery color to hold; the foliage fades and the growth turns leggy instead. The most common mistake with edible types is leaving them in the ground past the first light frost, which shortens their storage life even when the cold has not touched the roots directly.

Harvest and Overwintering

Dig edible roots around the time of the first fall frost, using a spading fork well clear of the crown to avoid stabbing the tubers. Let them air-dry on the ground for two or three hours, then cure for ten to fourteen days at 27 to 29 degrees C (80 to 85 degrees F) and high humidity, a loosely sealed plastic bag with a few slits works without a dedicated curing room, before storing at 13 to 16 degrees C (55 to 60 degrees F) in darkness. Outside its hardy range the whole plant collapses at the first real frost; gardeners in cold climates either bring a favorite ornamental vine indoors as a rooted cutting before the cold arrives, or treat the season's growth as an annual and start fresh from new slips or cuttings the following spring.