Gardening with Chip

Sweet potatoes from Lower Columbia River gardens? Here are some of one of our gardeners’ techniques.

I read a news item this week from Dr. Carol Miles, a WSU vegetable research scientist. She does research on new potential crops for the western Washington region and got her start in Cowlitz Extension some years ago. She has recently been working with sweet potatoes and is convinced that, in the right locations and the warming from climate change, they can be grown commercially.

In Columbia County, quite a few gardeners have experimented with sweet potatoes with success. That said, most were in the warmer part of the county. However, in a garden with full sun, I think sweet potatoes can be grown in the Cathlamet/Clatskanie area as well.

They typically harvested about a pound or more sweet potatoes per square foot in their Warren garden. Yields from their four-by-eight beds have ranged from 34 pounds to 45 pounds per bed. They have gotten similar results in the OSU Master Gardeners Demonstration garden at the Fairgrounds.

Sweet potatoes are grown from “slips,” which are shoots that sprout from the sweet potato. Slips need to be started indoors in late January or early February. To get slips, they cut sweet potatoes from last year’s harvest in half, widthwise. They put the cut end down in a shallow container on some half-inch clean round rocks. They fill the container to cover the tubers up to about a half inch high above the cut surface.

They start the slips, at first, inside their house since sweet potatoes like warmth. They place the “slip” tray where it gets decent sun. You need organic sweet potatoes to start with, but not all untreated tubers sprout consistently. Organic sweet potatoes are more likely to sprout since most commercial sweet potatoes are treated with a product that stops them from sprouting. Some tubers will rot before good slips are formed. Watch carefully and discard tubers that start to decay.

If all goes well, small “slips” start to grow. When the slips are one to 1.5 inches long, they cut or gouge them out of the sweet potato and place the base of the slips in a shallow vase in water until they root well. Once they get good roots, they pot them up into two-inch-by-two-inch containers in a nice, loose potting mix. After a little settling in, they move the slip containers out to their greenhouse in trays that hold water. They put the tray on a heating mat to supply bottom heat. If the slips start to root out the bottom of the containers, they repot them into larger containers.

Sweet potatoes need loose soil. It is hard to overemphasize that requirement. If the soil isn’t loose, the sweet potato tubers won’t grow large. Prepare the four-by-eight-foot raised beds located in full sun when the soil is able to be worked easily, adding some fertilizer and lime at that time. You can put a soaker hose or drip tube that will water the sweet potatoes in the summer on the surface. Then cover the beds tightly with black plastic. This warms the soil and keeps the weeds down. Start to harden off the slips by giving them several hours per day outdoors in indirect sun several weeks before transplanting. Slips may lose leaves at transplanting. Usually, they re-sprout leaves.

When it comes time to plant (usually in early May), their method is to plant three slips per bed, evenly spaced down the center of the bed. They cut holes in the plastic to transplant the slips. One gardener plants eight slips per bed. The planted beds are covered with row cover to provide extra heat and to reduce transplant shock. Row covers are removed in mid-June.

Consistent irrigation is necessary throughout the growing season. Vines will grow in a tangled jungle with some stems getting 10 feet long. Deer eat the foliage and field mice (voles) like the tubers, so be prepared.

They harvest in late September. You have to dig carefully and gently as sweet potatoes can be quite brittle when first dug. Dry them in a bin. Separate out any damaged tubers and eat them first. Sweet potatoes store best between 50-65 degrees. Don’t let them freeze. They typically hold through winter.

Take excess produce to the food bank, senior centers, or community meals programs. Cash donations to buy food for the food banks are also greatly appreciated. This is especially true now, since many of the federal food funds that helped the food banks have been cut hard. Sign up for Gardener training in either county. Very helpful information sources are your local Extension offices. The Columbia County Extension is 503-397-3462; the Wahkiakum County Extension is 360-795-3278; and the Cowlitz County Extension is 360 577-3014. The Extension Service offices offer their programs and materials equally to all people. Advice on future garden topics is welcome by emailing me at chip.bubl@oregonstate.edu.