There are plenty of plants that don’t grow easily in Tasmania but some of these are worth a try — especially with the forecast of a hot summer ahead. Lately I’ve seen plenty of marginal plants for sale including all sorts of citrus and bananas.
While bananas are too big a commitment for me, I am willing to give sweet potatoes a go after seeing plants growing well in a friend’s garden. That lush and productive vine was growing in a raised vegetable bed and although the crop was small it did produce. Right now there are sweet potato plants for sale in garden centres.
Normally sweet potato (Ipomoea batatas) likes to sprawl across the ground. It’s part of the morning glory family (Convolvulaceae) with small white or pink morning-glory-like flowers. The heart-shaped leaves are also pretty.
I’ve grown sweet potatoes before when I gardened in a warmer climate but even there the vine was often destroyed by frost before the crop was ready to harvest. I hit on a technique then that worked well and I am now employing it here. I’ve bought a very big black pot (50cm across) and filled it up with several bags of premium potting mix. In has gone the sweet potato plant, which was already growing well, surrounded by a comforting layer of mulch, and a tripod of stakes.
It is possible to grow sweet potato from sprouted tubers but these advanced plants give a head start and the black pot should help keep the plant warm. I’ve planted an orange variety but they also come as white and purple. Sweet potatoes are sometimes called yams and thought of as a tropical vegetable from the Pacific Islands. Not so. Sweet potatoes are native to Central or South America. They spread across the Pacific to New Zealand perhaps more than 1000 years ago. Sweet potato is often called by the Maori name kumara.
When sweet potato is grown as a ground cover it forms tubers as it spreads and forms roots. In a pot or raised bed, either allow the vine to sprawl over the edges or train it up a tripod at least 2m high. Don’t let it form roots into the soil. These techniques restrict the number of tubers but those that grow have a chance to mature.
Cold is the enemy of the sweet potato. Cold winds or a cold spell set the vine back while a frost or very low overnight temperatures will kill above-ground parts. To give the sweet potato plant every chance to thrive, select the warmest part of the garden for example against a north-facing masonry wall or at least beside a sheltering fence so the sweet potato vine can grow for the necessary 16 to 25 weeks to form a crop. Mine is sitting on top of brick paving, which warms up during the day.
Whether they are in the ground or in a pot, keep the plants growing well with regular watering and ensure they’re free of competing weeds. Once they smother the ground their foliage tends to act as living mulch. Excess leafy growth from sweet potato vines can be pulled up through summer and autumn and fed to the chooks.
Apply complete slow-release fertiliser at planting. An occasional liquid feeding helps pot-grown plants keep growing well.
Once the weather gets cold or frosty or the leaves start to yellow, remove the top growth and search for tubers. Take care if digging as sweet potato tubers are easily damaged. In a pot, harvesting is as easy as tipping out the potting mix, picking up the tubers, brushing off the soil and heading for the kitchen.