Sweet Potato
Ipomoea batatas
Grown from slips, not seed, and completely unrelated to the potato.

Slips, not seed or tubers
Sweet potatoes are grown from slips - rooted sprouts drawn from a stored root. Buy them, or produce your own by suspending a sweet potato half-submerged in water, or bedding one in damp compost, six to eight weeks before planting out. Twist the sprouts off when six inches tall and root them in water.
A tropical crop - wait for warmth
Set slips out well after last frost, once soil is at least 65F. Cold soil stunts them permanently. Black plastic mulch to warm the ground is genuinely worthwhile in zone 6b/7 and can add weeks to the effective season.
Soil
Loose, sandy, well-drained and not rich. Heavy clay produces small misshapen roots; too much nitrogen produces magnificent vines and almost no crop. Ridges or raised beds improve both drainage and root shape.
Space for the vines
They run six to twelve feet and root wherever a node touches damp soil, which produces lots of small tubers instead of a few good ones. Lift and reposition the vines occasionally, or grow them where the sprawl does not matter.
Water early, ease off late
Regular water for the first six weeks while roots establish, then much less. Heavy watering close to harvest causes splitting.
Pests
Sweet potato weevil is the serious one in the south but rare this far north; deer and voles are the practical problems here. Deer graze the vines heavily, so fencing may be necessary.
Harvest before cold soil
Dig before soil temperatures drop below 55F and definitely before frost, which damages the roots even if the vines are cut. Dig carefully with a fork well away from the crown - the skins are extremely delicate at harvest and every scrape is an entry point for rot. Handle them like eggs.
Curing is what makes them sweet
Freshly dug sweet potatoes are bland and starchy. Cure at around 85F and high humidity for ten days - a closed box with a damp towel, or a warm bathroom - which converts starch to sugar and heals the skin. This step is the entire difference between a good sweet potato and a disappointing one.
Storage
After curing, store at 55-60F. Never refrigerate: below about 55F they suffer chilling injury, develop hard centres and off flavours, and rot faster. Cured and properly stored they keep six months or more, improving for the first month.
- Days to harvest
- 90-120
- Plant
- Slips, after soil hits 65F
- Spacing
- 12-18 in
- Row spacing
- 36-48 in
- Sun
- Full
- pH
- 5.5-6.5
- Cure
- 10 days, 85F, humid
- Storage
- 6+ months