K-State Research and Extension

Summer is right around the corner and many are looking to produce more food at home these days. The sweet potato is a highly nutritious staple crop that can be easy to grow. Last year we discussed harvest and curing but in this edition I will cover “starts” and planting. Nurseries are usually taking orders for planting stock in early/middle spring and shipping out the first week of June. So I thought it would be a great time to cover the basics of growing this superfood. The roots of the sweet potato are its most commercially-important and commonly consumed plant part; although, every part of the plant is edible (stems, leaves, flowers). Fresh roots can be consumed raw, boiled, baked or fried, among other preparations.Roots of the sweet potato develop beneath soil and on average produce 4-10 edible roots per plant (2-3 lbs). However, yield is variable and largely dependent on specific cultivar, propagule, and other factors like soil type. 

Sweet potato display by Extension Master Food Volunteer, Judy Masonis. from recent “Garden to Plate” activity.

Slips 

In place of true seed, a new sweet potato root crop is propagated by transplanting stem cuttings, i.e. the sprouts produced by roots produced in the previous season. Horticulturists and commercial nurseries refer to the stem cuttings as “slips”. Fresh slips are produced commercially in soil,shallow trenches are planted with roots produced in the previous year. Small scale growers can produce slips in small pots with roots covered in potting soil or sand, and of course the tried and true method of suspending a submerged root end in clean water with toothpicks. When the slips grow to approximately 1’ in length, they are harvested and directly transplanted. When sprouting slips from roots harvest in the previous year,growers will raise temperature in their storage area to 70-85° F and decrease relative humidity to 75-85% to induce pre-sprouting—this will help the process move more quickly

 Optimum length and quality of sweetpotato slips (photos taken day of harvest) 

Preparing Field and Transplanting

Sweet potatoes are very adaptable but prefer light sandy soil with high fertility. In organic systems manure applications of 2 to 4 tons/acre have been applied with success in furrows prior to bed shaping.The Midwest Vegetable Production Guide recommends a N-P-K of 30-75-250 lb/acre applied pre-plant (Creswell et al., 2018). In the Midwest slips are typically transplanted between early June and July. Regular rotation of planting areas is critical for avoiding pest and disease pressure in sweet potato.Although there are many trials that outline the pros and cons of different plant spacing, root production is typically carried out in the open-field on raised beds/ridges ~10” high on 48” row centers and 12”in-row spacing.This spacing requires 10,890 slips to plant an acre.Spear dry soilat 1 foot increments with a trowel, clean stick, or piece of rebar and pull soil to one side.  Place a slip in the hole at a depth of ~3 nodes, this should be roughly 5” (Thompson, 2014) and then remove the trowel to allow natural back fill.Make sure that you do not bury the leaves of the slip below ground level.  Irrigate immediately following transplant—some growers will opt to water in transplants with starter solution 8 oz/plant. During transplant establishment, prior to the development of vine canopy, be mindful to water the ridged beds whenever soil dries. Irrigation is still necessary but not as pressing once vine canopy and ground cover is established.Plants will wilt for several days following transplant but should recover within a week. 

Transplanting slips at 1’ in-row spacing with “rebar trowel”  

Wilted slips just hours after transplant 

Weed Control 

Black plastic mulch has been used extensively with sweet potato and will greatly decrease intra-row weed pressure. Without the use of mulch, careful cultivation with small scuffle hoe is often necessary in the row and between plants for the first few weeks after transplant. The inter-row or aisle space can be cultivated with less precise/ time-consuming methods, e.g. wheeled stirrup hoe, small mower, trimmer or harrow. As vines grow into the aisles,weeding may require the plants to be lifted and moved to permit last rounds of cultivation. Although the vines may shade out any competing weed growth a month or two following transplant, continued lifting of the vines can prevent the rooting that occurs at the above ground vine nodes, which can slow development of roots. Some growers may slowly run tractor-drawn disc or other cultivator implement to routinely lift vines in aisles and prevent this extra rooting.  

Planted field one month after transplant with large portions of aisle soil still exposed

Harvest  

Like we’ve discussed before, roots will reach mature size in approximately 110 days. It’s best to harvest when the soil is mostly dry. The roots from a plant at the end of the row can be dug out carefully and serve as an indicator of root size for other plants in the field—if done without too much disruption or damage the roots can be recovered and will continue to grow. When plants reach, maturity mowing or cutting vines close to soil line can make the harvest easier. Harvest before chance of frosts or freeze. Light frosts will turn foliage yellow; the first time you observe this change, it’s probably best to harvest roots as soon as possible. There are many mechanized options for harvest, however smaller grower will opt to dig with spading fork and can largely avoid damage to the delicate skins with hand tools. And remember to read my previous post of sweet potato curing, this process greatly improves shelf life and compositional sugars. 

Picture 6-Tender vines show first signs of frost damage, indicating it’s time to harvest