Sweet potatoes

In the Garden

Ralph Purkhiser, Purdue University Master Gardener

     One of the last summer crops to be planted is the sweet potato.  Since sweet potatoes are very cold-sensitive, one should wait until the soil temperature at the depth of four inches is above seventy degrees.  However, since tuber formation begins in only fifty to sixty days, there is plenty of time for a good crop, even if the slips are planted as late as early July.

     Sweet potatoes are only distantly related botanically to potatoes.  The plant is actually most closely related to bindweed and morning glories, which is obvious if you ever see a sweet potato in bloom.  However, that rarely happens when there is more than 11 hours of daylight, and by that time of the year, temperatures in our zone have cooled too much for the vines to set blooms.  The blooms are usually pink to light purple and have the same horn shape as the blooms of morning glories and bindweed.

     Although many people refer to the tubers as yams, true sweet potatoes are not at all related to yams.  True yams are rarely grown as a home-garden crop.  Sweet potatoes are native to tropical areas of the Americas, but although Columbus and his men tasted them on his first voyage, that was not the first exportation of sweet potatoes.  There is evidence that there was contact with Polynesian tribes much earlier and sweet potatoes have been grown in the south Pacific for centuries.

     Sweet potatoes are not picky about the soil, although they do best in soil that is slightly sandy and a little on the alkaline side.  They are susceptible to aluminum poisoning, and since aluminum is usually found in acetic soils, lime should be added in such areas.  That is not usually a problem in southern Indiana, where the amount of limestone in the soil tends to sweeten the soil.  Sweet potatoes rarely need any fertilization. 

     Warmth is a friend to sweet potatoes, which will thrive in hot, humid conditions.  To achieve a little extra warmth in the root zone, many produces plant the sweet potato slips on ridges.  Another common cultivation method is to cover the ridges with black plastic and plant the slips through slits made in the plastic.  This not only makes the soil warmer, it will keep weeds from growing between the plants.  At harvest time, one needs only to cut off the vines and remove the plastic.  The tubers will be easy to dig out of the ridges.

     Sweet potatoes usually do not require irrigation.  The critical time for moisture is during that tuber-formation period, fifty to sixty days after planting.   Once the tubers have formed, the plants require very little moisture.  Although most of the sweet potato varieties are cultivated for the tubers, the tender shoots and leaves may also be eaten as greens.  They may be eaten raw in salads or cooked.  Sweet potatoes are not usually bothered by insects and are not affected by disease, so they are easy to raise without chemical sprays.  In the past, the biggest problem I have had with sweet potatoes is voles, which will eat the tubers.

     My plan for sweet potatoes this year is to raise them in an old stock tank filled with compost and leaf mold.   I hope the metal tank will keep the voles away and harvest should be very easy.

     The vines of sweet potatoes are also very ornamental.  Indeed, there are some varieties that have been bred for their ornamental aspects.  Most of the tubers produced by the ornamental sweet potatoes are less sweet and lack the texture of the garden sweet potatoes, but they are edible.  Whether you are growing them for beauty or nutrition, I urge you to grow some sweet potatoes this summer.  You won’t be disappointed.