Potatoes: What can I do about long tops when growing in bags?
Question: I grow potatoes in vegetable garden bags and keep covering with compost to about two inches from the top. I find that they make really long tops which I have to stake. Why is this happening – and what can I do about it? (Mrs C Gaskin, Norwich)
Growing potatoes in bags and other containers is certainly a good way to grow them without having to have a garden. Over the past decade, as the 'grow your own mentality' has taken off among people grow kits have become available but it is still as easy to just use old compost bags and much cheaper.
However, the most important part is to have the correct variety of potato and it is best suited to growing first earlies. The good news is that there are lots of varieties to choose from. Potatoes don't require much to grow a good crop, and it can easily be done in growing bags. One mistake you don't want to make though, is buying a standard grow-bag from the garden centre.
An empty compost bag can easily double up as a potato growing bag, as long as it has a capacity of more than 20 litres (typical size of the smallest bags). Cut the top of the bag off completely, and empty out the contents into a suitable container.
Fold the bag down to about a third of its height, refill the bottom of it with loose compost (usually comes vacuum packed). Stab a few holes in the bottom of the bag with a knife to allow excess water to drain out. Another option could be to use a ton bag. If you live anywhere near a builder's yard, or you know a builder, you can usually get them for the price of taking them away, and they will play host to 9-12 potato plants. Again, fold down the sides of the bag, fill the bottom with compost, and plant your potatoes. You shouldn't need to make holes in the bag
Start out by putting 15cm-20cm of good quality potting compost in your bag. Alternatively you can use a multipurpose compost, mixed 50/50 with sharp sand and/or some well-rotted farmyard manure or homemade compost. Adding the organic material to the mix will make watering easier throughout the season, and add valuable plant food which will boost your crop.
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Place one, three or five seed potatoes on top of the compost, depending on the surface area of your containers. Now, cover the tubers with another 10-15 cm of your compost mix on top of the tubers. After a couple of weeks you should see potato plants starting to sprout through, if it is warm enough. As this happens, keep covering them in more compost, until the level of compost reaches 5cm below the top of your container. During this period of time you need to keep your potatoes well watered. Over-feeding can have fatal consequences for your crop and can cause the stems to grow long and weak. Make sure the plants are getting enough light as this will help keep the tops shorter. In the height of summer you may well have to water twice a day.
The time to harvest your potatoes varies according to variety. For first earlies and second earlies you can start harvesting when the plants stop flowering.