Growing potatoes in your greenhouse may not seem very exciting or even particularly worthwhile. After all, you can buy a bag of spuds for a few dollars. But imagine growing high-value fingerlings such as Russian Banana, Rose Finn, or Blossom. These can be hard to find in a grocery store, especially during winter. And they make very tasty salads.
Growing potatoes in your greenhouse will provide your favorite varieties all year long, and you don’t need much space to do it. I grow mine in one-gallon or three-gallon plastic bags (available at a local hydroponics store), which I fill with screened garden soil and compost. Using flexible bags makes it easy for the potatoes to grow freely without the walls of a pot distorting their shape.
Your crop can be easily started from seed potatoes. Inspect these carefully and cut away any signs of rot. Larger tubers can be cut in half as long as there is an eye on each piece. But be sure to allow at least 24 hours for any cut surfaces to harden before planting, or the tubers may rot in damp soil. Some growers also allow the tubers to sprout before planting.
As for the planting method, simply put three inches of soil mixed with compost into each bag and bury the tubers about an inch down in it. Plant one tuber in a one-gallon bag and two or three in a five-gallon one. Then set the bags in a warm part of the greenhouse and water the contents well, but don’t saturate it. As soon as green shoots have reached three or four inches high, add more soil and compost, leaving an inch or so of sprout showing. For cleaner potatoes, you can add aged, damp straw or shredded leaves instead. Continue this process as the sprouts grow, until the surface of the planting medium is about an inch below the bag’s top. Also, be sure to feed your plants with a high-phosphorus fertilizer, such as 5-20-0, which root crops prefer.
After the plants come into flower, you can early-harvest some “new” potatoes by carefully removing a few from around the edges of a plant. Eventually, the plant tops will die back, and the entire crop can be harvested. To do this, simply upend the contents of each bag into a container and pick out the potatoes. The growing medium can then be spread on your outdoor garden.