Growing sweetpotato in a bag can provide a solution to how to maintain high enough soil temperatures to grow that crop on a hobby basis in a northern area. It also allows growing sweetpotato in areas with poorly drained soils. Follow the instructions below from Mr. Mitsuoki Sato, retired vegetable specialist from the Saitama Prefecture Agricultural Experiment Station.
An empty fertilizer bag is an ideal size for growing one sweetpotato plant. Any other sturdy plastic bag with a 20-30 kg capacity will also do. Pierce the bottom of the bag in several places. Mix one part compost with two parts soil. Compost could be cattle manure and/or leaf or lawn compost. It is better not to use soil that has been used for growing vegetables as this will likely contain too much nitrogen for the sweetpotato plant to properly produce tubers. Fill the bag about 80 percent full of the soil mixture. Place the bag on bare soil that receives plenty of sun. Run a rod from the top of the bag, through the bottom, and into the ground to prevent the bag from falling over.
Plant one sweetpotato slip in the bag. Planting material should be about 30 cm long, with 2 or 3 leaf joint nodes underground. An easy way to plant is to carefully bend the slip into a "J" shape to be able to push it underground with your thumb. In northern areas, a fast growing variety will be necessary in order to have a decent harvest before frost comes. Water only if the bag's soil becomes too dry.
In the Tokyo area, about 35 degrees N Latitude, the time to plant is late May to early June, but planting can be done whenever the soil is warm enough. Soil temperature must be greater than 18 degrees Centigrade, but less than 35 degrees (65-95 degrees F), for the plant to grow. However, the temperature should average between 20 and 25 degrees Centigrade (68-77 degrees F) for the tubers to form well in autumn. In the fall, large variations in day and night temperatures will encourage production of fine tubers.
When the sweetpotato's leaves die from the first frosts, it is time to harvest the tubers. The crop can be converted to a sweetpotato "bonsai" by cutting back the sweetpotato vine, removing enough soil to expose the tubers, and trimming back the bag. Alternatively, the mass of sweetpotatoes can be carefully replanted in a large planter as described in sweetpotato "bonsai".
Remember, when growing sweetpotato in a bag, the bottom of the bag must be pierced in several places, and the bag must be placed on bare soil. This not only allows water to drain from the bag, but also allows the sweetpotato feeder roots to extend into the ground and search for additional water and nutrients.
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