Moisture Needs

Once established, sweet potatoes may be considered drought-hardy, and will produce a fair crop even under quite dry conditions. But supply them with ample moisture for best yields. They need about 3/4 inch of rain or irrigation per week while small, and up to 2 inches of water per week when growing vigorously in hot weather.

Occasional leaf wilting during the hot part of the day is no cause for concern. But if the plants do not revive by early evening, you may wish to irrigate.

Excessive moisture after a prolonged dry period may cause the roots to crack. These cracks usually heal over by harvest time, resulting in a less appealing appearance but with no loss to eating or storing quality.

Sweet potatoes do not ripen or mature, so the time for harvest is judged by root size. At about 110 to 120 days after transplanting (if you can wait that long) you may wish to check on root size. To do so, carefully dig away some of the soil from several hills near the center of the row the end plants may develop faster. When a majority of the roots have reached a size you desire, they should be dug immediately. A light frost which only slightly damages the vines won't cause problems.

Dig roots carefully to avoid injury. A shovel usually works better than a fork. After digging, place the roots in containers. Keep separate any badly injured or cut roots for immediate use.

A yield of two pounds a plant is good.

Optimum conditions for curing and storing sweet potatoes are rather exact and should be maintained as nearly as possible. ("Curing" is a means of preparing sweet potatoes for storage.) Cure roots 6 to 8 days at about 85 F and 85% to 90% relative humidity. Temperatures should never exceed 90 . A high relative humidity can best be maintained by loosely wrapping the containers in polyethylene film. Don't cure in the sun as the roots may sunburn.

After curing, store the roots in a cool place (55 to 60 F) with a high relative humidity (85%). An unheated area of the basement or a root cellar might be okay.

If storage temperatures drop below 50 F for an extended time, chilling injury may develop and the roots may spoil. Temperatures above 65 will likely result in sprouting and pithiness. If the relative humidity is too low, shriveling will result. However, if proper conditions are maintained you may expect your sweet potatoes to keep well and provide good eating through winter and into spring.

If you wish to produce your own "seed" for the following year, begin when the plants have developed vines 1 1/2 to 2 feet long. Cut a segment 8 to 10 inches long from the tip of a vine and transplant it into a row which has been fertilized as previously mentioned. Plants should be set about a foot apart and 2 to 3 inches deep, making sure to cover at least 1 node. Water with 1/2 cup of starter solution or water.

You can expect a 10-fold increase with most varieties. So if you want 100 plants next year, have 10 hills of vine cuttings. Fertilize and cultivate your cuttings as suggested for the regular crop.

If the majority of roots from the vine cuttings are 1 inch in diameter or larger, harvest them at the same time as your main crop. Roots produced for sprouts should be cured and stored the same way as roots for table use.

Producing Sprouts

If you have produced roots from vine cuttings and wish to produce your own sprouts, several methods may be used depending upon the facilities at hand. If a heated frame is available, place disease-free "seed" roots near each other but not touching, and cover them with 1 1/2 to 2 inches of light sand 4 to 5 weeks before transplanting. Soil temperature should not exceed 85 F. Open the sash on warm sunny days to avoid excessive heat buildup.

When the sprouts are 6 inches tall (usually a week or two before transplanting) turn off the heat and leave the sash open, except for very cold nights. Water the bed as necessary to keep it damp but not waterlogged.

Alternatively, you may wish to bed roots in a cold frame. Put your cold frame in a sunny place protected from Wind, Allow an extra week to produce sprouts without heat, but otherwise the procedure is about the same.

If neither a cold frame nor a heated bed is available, you can bed sweet potatoes in your garden. Choose a sunny spot protected from wind and make a raised bed 6 to 8 inches high and 1 1/2 to 2 feet wide, 6 to 7 weeks before transplanting. Bed the roots as described and cover with 1 1/2 to 2 inches of sand. Then cover the bed with clear polyethylene film, securely anchoring edges of the plastic with soil.

When the sprouts begin to emerge, ventilate the plastic by punching 1/4-inch holes every 6 to 8 inches. After 2 to 3 weeks or when the sprouts are 3 to 4 inches tall, you may remove the plastic but replace it if a frost is likely. The sprouts should be 6 to 8 inches tall by transplanting time.

Remove sprouts from the roots by grasping them firmly one at a time and pulling sharply from the soil. With a little practice you should be able to pull sprouts without disturbing the bedded root. Transplant the sprouts as soon as possible. The bedded roots will continue to produce sprouts which may be used for a second planting, or you may wish to dig up the roots and destroy them.

Fortunately for the home gardener, there are relatively few serious disease and insect pests of sweet potatoes. Fusarium wilt, also called blue stem and stem rot, attacks certain varieties resulting in both a loss of plants and a yield reduction. It can be best controlled by growing resistant varieties and a rotation of four or more years' duration.

Scurf, also called soil stain, causes a dark "freckled" appearance to the skin of storage roots. The fungus attacks only the skin and causes no loss to eating quality. Control it by using only vine cuttings to produce the "seed" roots.

Pox or soil rot results in deep corky pits on the root. Once again, although the appearance is greatly affected, eating quality is not. Control pox by reducing the soil pH to 5.0 to 5.5, resulting in greatly reduced incidence and severity.

Black rot is occasionally a problem both in the garden and in storage. Affected areas begin as small black, nearly round spots, but under favorable conditions they enlarge and may nearly cover the root and extend well into the flesh. Usually you can control it by using disease-free sprouts, not planting in the same area of your garden each year, and using clean containers for storage. Some fungicides are effective in controlling black rot.

Rhizopus rot or soft rot attacks the roots after harvest, usually entering through a wound. It results in total loss of the root. Control soft rot by carefully handling the roots during harvesting, thus avoiding as much as possible any injury to them, and by curing the roots properly as soon after harvest as possible in order to heal wounds.

Fusarium root rot or surface rot is usually not noticed until after some months of storage. It appears as a circular spot, black or dark brown, sometimes slightly sunken, and only extends about 1/8 inch into the root's flesh. If infected areas are pared away the remainder of the root may be eaten. There is no known means of control, but immediate and proper curing helps in reducing the incidence. The occurrence is sporadic and seldom reaches serious proportions.

Nematodes may cause serious losses to sweet potato yields. The infected roots become misshapen and cracked, and galls may be observed on feeder roots. At present the home gardener has no effective way to control nematodes. Many varieties are resistant to one or more races or species of nematodes. Your local Extension Office is the best source of information if nematodes become a problem.

Although several species of insects and soil grubs attack sweet potatoes, they rarely result in serious losses. Damage is usually confined to a small area of a root and may be pared away prior to cooking. If serious losses occur, consult your Extension Office.

I have tried to outline the how's and why's of sweet potato growing for the average gardener, but experience is the best teacher. You may find it advisable to change some of the recommendations.

Sweet potatoes are likely to produce a fair crop even if you make a few mistakes, so feel free to experiment with changes after you have a little experience. If you fail in your attempt to grow sweet potatoes, find out why and try again. After all, not everyone finds buried treasure on their first try.