Twenty-one varieties of sweet potatoes were planted which could be grown by the home gardener. Plant characteristics along with pest and disease problems were observed and recorded.
A public field day was held on November 11 when people toured the garden and tasted eight varieties of sweet potatoes. Due to the inclement weather only about 65 – 75 people came to the tasting vs the ~120 who attended in 2005. Rating sheets were available at the field day tasting. These were tabulated and used to pick the top varieties based on flavor and texture. (See the section on Public Tasting Results.) When selecting which sweet potato varieties to grow, home gardeners can consult these tasting results as well as the plant characteristics and pest and disease information.
The public will find that sweet potatoes are easy to grow but require a lot of space for the vines. The soil needs to be properly prepared so that it is friable, providing roots the environment in which to enlarge. To insure that the slips can be planted in late April or early May it would be advisable for people to grow their own slips. However, slips ordered by individual type rather than as an assortment from Sand Hill Preservation might arrive early enough in May for production by/in October. Assortment orders are filled after all of the individual orders have been filled and shipped. Very few states are allowed to ship slips to California.

Soil Preparation and Planting

The McClellan Ranch team uses organic gardening methods for the vegetable trials. Preparations for spring planting began in early November 2005 with the sowing of fava beans as a cover crop for the winter. Row covers were placed over the beds to prevent seedling damage by birds. The row covers were removed when the fava plants were large enough to withstand any damage by birds. See the Fava photo in photo section. On 25 February, before the flowering fava beans developed seed pods, the stalks were cut down, finely chopped and spread on top of the soil. A layer of straw and goat manure from the local 4H project was then applied. In early April, the beds were rototilled again to prepare the soil for planting.
The first five varieties of sweet potato slips (Beauregard, Diane, Golden Sweet, O’Henry, and Oriental Yam) were planted on May 13. These were purchased from a grower in the Central Valley (CV). One variety, Hawaiian Purple, was planted the following week by Ed Lo who had grown the slips from an organic potato he had purchased from Kitazawa Seed Company in Oakland, CA. The fifteen varieties that had been ordered from Sand Hill Preservation Center (SHPC) in Calamus, Iowa were planted on June 24, much later than we had hoped to be able to plant them. The Sumor sweet potato, after several months of storage, can be a near substitute for Irish potatoes per SHPC.
Per SHPC, never place your slips in the refrigerator if you are not ready to plant them when they arrive in the mail. Put them in a container with wet soil or wet peat moss. (We put ours in a container with wet Super Soil because that was what was available.) Don’t store the slips in jars of water as they will get mushy and rot. If necessary, you can keep them in the container of wet soil for up to 10 days before setting them out.
Per SHPC, sweet potatoes are very sensitive to cold – both as roots and as young plants. At temperatures below 55º F, fungi grow on the sugars in the root and they quickly rot. Most slips grow more roots in transit. If the weather is nice and warm when the slips arrive, they should be immediately transplanted and kept very wet in the garden for a week or so to let them establish themselves. Once established, they take off and do not need to be watered as much. As the plants grow one needs to take care that the plants don’t set down roots away from the main plant. Check for this by gently lifting the plants every once in a while to keep them from setting down roots along the vine. If they put down roots, and they will in moist soil, then the yield can be decreased to next to zero. The only place you want them to root is at the spot where you planted the original plant.

Plant Characteristics

The plants made extensive runners/vines. When the soil is deficient in nitrogen, as it was at the MG Sunnyvale Training and Demonstration Garden, the vines are minimal. See an example in the photo section. Some varieties were prone to send down roots at the joints. The team had to check for that condition and move the vines to stop the rooting. Roots at the joints can be the cause of the main roots not enlarging to make potatoes. Some varieties can be trained on trellises if the home gardener has trellises. One of the team members planted slips of the Hawaiian Purple sweet potato and used trellises since he had limited space. He had excellent results. He also picked small/new sweet potato leaves, sautéed them and brought them to the project for members to sample. They were very good.

Pest and Disease Problems

Various pest problems were encountered, primarily gophers and rats. At one point one of the sweet potato varieties appeared to have been attacked by the birds so we retrieved a row cover from the storage shed in the barn and covered that partial row until the plants recovered. We had not put row covers over the slips when they were planted. The gophers chewed the roots off below the ground on some of the plants. Later the gophers chewed off parts of the enlarged roots. The rats (or other critters – squirrels, raccoons, or opossums) chewed through the vines of a couple of plants above ground but did not appear to have consumed any of the vines. Squirrels were being caught by many of the nearby gardeners. One gentleman had caught 15 in his tomato patch by 2 September. Another gardener caught a raccoon and a large opossum.

No insect control was performed other than leaving some volunteer amaranth to attract flea beetles. Only two gophers were caught during the season using the new cinch traps. This compared with the 16 caught the previous year with Macabee and Victor traps. The gopher runs tended to be located under the plants such that the plant would be demolished if we did enough digging to set the traps.

Harvesting Results

A trial dig was done on August 26, day 105 for the Central Valley (CV) sweet potato slips planted on 13 May at McClellan Ranch. The first plant dug was a Beauregard. The plant had a LOT of vines that had to be moved aside to find the spot where the plant emerged from the ground. No one could believe that there were so few potatoes under the plant with such extensive vines. There were 7 potatoes (by stretching the definition a bit). The total weight was almost a pound. The largest potato was about 6 ounces. The smallest "potato" was less than one ounce. It is debatable whether the 2nd largest potato/fingerling would be worth cooking. These potatoes were elongated versus the Beauregards grown at the MG Sunnyvale Teaching and Demonstration Garden which were round. See photo section. The Sunnyvale group had planted some of the slips our project received from the CV.

On 9 September some of the CV Golden Sweets were harvested and enough Dianes (aka Garnets) to cure for the tasting. See photo section.

Subsequent digs of the CV sweet potatoes were more productive. On October 14 all of the Beauregard and Hawaiian Purple sweet potatoes were dug bringing the harvest for those two varieties to 21 pounds from 15 plants and 14 pounds from 12 plants, respectively. Enough of the other 4 CV potatoes were dug to cure for the tasting. The other plants were left in the ground so that we would have plants for the public to see on the field/tasting day. Many plants had no enlarged roots.

A trial dig on 30 September (day 98 for SHPC slips) of two plants of Ivis White Cream (90 day) yielded no usable sweet potatoes so we went on to the dig SHPC Gold Star (90 day). At first we thought that we weren't going to get any potatoes from Gold Star but it turned out that there was one large potato (11 oz) among the roots of the first plant. The second plant also had one large potato (10 1/2 oz). The third plant had one small potato (1 oz). We decided to wait 2 weeks before digging the other varieties which had longer times to maturity.

On October 21 (day 119 for SHPC varieties) one or more plants of each variety were dug. Again, some plants dug had no enlarged roots. It was decided that the sweet potatoes were not large enough to cure for the tasting. We would leave the rest of them in the ground to show to the public. Sweet potatoes grow when there is a lot of heat which we were no longer getting.

On the day of the tasting, while leading a tour of attendees (see photo section) Ed did demonstrations of digging. Surprisingly, the Ivis White Cream and the Sharp had edible sized sweet potatoes much to the delight of the witnesses. The remaining sweet potatoes were dug 17 November. Most of the SHPC plants that were dug did not have any roots that were large enough to bother weighing. The table near the end of the report gives information on number of plants and total weight of each variety harvested.

Harvesting and Curing Tips
Per SHPC, when harvest time occurs, make sure that the sweet potatoes are not subjected to cold soil. Even if there is no frost, soil temperatures under 55º F will cause surface fungi to take hold and the crop will either rot in the ground or in storage. After they have been dug, they should be carefully laid out in a warm, dry area or placed near a furnace vent for a few days to cure. Don’t store them in a cool area. It is ideal if it is about 65º F. Roots stored where it is too cool will quickly rot.

It is important to cure the sweet potatoes to increase the flavor and storage time. When the roots are dug they should lie in the sun for a couple of hours. All the sources had different recommendations for the balance of the curing. The team members who cured roots used different methods, i.e., (1) put roots in a paper bag with a wet paper towel, close bag, and store/cure in a hot attic for ~2 weeks then store them in a cool room at ~60º F; (2) same as (1) except store/cure in a large plastic container where it gets a lot of sun (interior temperature ~85º to 90º F) for 7 to 10 days; (3) same as (1) except store/cure in hot humid bathroom for 10 days. See references at the end of the report for more extensive information.

Public Tasting Results

The tasting was held at McClellan Ranch Park on Saturday, 11 November 2006 from 11 AM to 1 PM. Due to the weather, the tasting was held in the Conference Room in the main building. Only eight of the twenty one sweet potato varieties were available for the tasting. The Welcome Table and display table were set up under the carport of the
building. Fifty nine people turned in Rating Sheets from the tasting. The results were tabulated and shown in the table below. Tours of the garden were led by team members.

McClellan Ranch Team

The public tasting is a team effort. In some years Master Gardeners outside the team assist with the tasting. This year visitors were greeted at a Welcome Table that included photos of previous activities at the McClellan Ranch Project plus photos taken during the season.

References with More Growing and Curing Tips

Pam Pierce, in Golden Gate Gardening, ISBN 157061136X published June 2003 by Sasquatch Books, 615 Second Ave., Seattle, WA 98104, has a section on sweet potatoes where she describes how to grow slips from a sweet potato purchased in early November and how to treat the potatoes when you harvest them. She says to cure them in a warm, humid place (about 80ºF) for two weeks. Then wrap each root in newspaper and store at 55º to 60ºF taking care not to injure the roots, since this will encourage decay.

Sally Jean Cunningham has a section on sweet potatoes in Great Garden Companions ISBN 0875967817 published Feb. 1998 by Rodale Press, Emmaus, PA. In the section on Growing Basics, she has a paragraph on growing slips which is somewhat different from Pam’s method. Sally Jean also recommends working bonemeal into the soil at planting to supply the phosphorus needed by the sweet potatoes.

Also see:

• Successful Organic Gardening Vegetables, by P. S. Machalak and C. R. Peterson, published by Rodale Institute Press, Emmaus, PA, Dewey decimal #635.05.
• Organic Gardening, December 2005 – January 2006, page 28 “Sweets for the Holidays” by Matt Ernst.
• Wikipedia has a 4 page article with photos at Sweet Potatoes.
• Kitazawa Seed Company, P. O. Box 13220, Oakland, CA 94661-3220; Telephone: (510) 595-1198
• Sand Hill Preservation Center, 1878 230th Street Calamus, Iowa 52729; Telephone: (563) 246-2299