Potatoes: You need good compost if growing in bags or pots

Wednesday, February 29, 2012
10:32 AM

Question: For the third season I have grown potatoes in bags. But each year I get masses of foliage, the flowers bud but never flower. The crop bears no resemblance to the picture on the bag packages. To date I have had only one and a half pounds, per bag but I would have expected much more, according to the pictures. I planted a single potato in a 10inch pot that had little foliage. They are very different. The tall ones were King Edward. I water well – can you help? (J Starling, Wrentham)

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Growing potatoes in bags has become increasingly popular over the past decade, as the ‘grow your own’ bug has taken off among people across the country.

Many people have gone on to buy one of the popular potato-growing kits, which are often sold with a selection of tubers to grow, and are perfect for the purpose. They are reasonably cheap to buy, and offer good value for money. These kits are fine if you like packages but there is nothing wrong with using an old potting compost sack to start with.

Potatoes don’t require much to grow a good crop, and it can easily be done using grow bag compost. Don’t try growing your potatoes in a grow bag as they are not deep enough they are meant for growing tomatoes etc.

One thing you need to buy in any case when growing potatoes in bags is compost. 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) – you can use grow bag compost for this as I always have plenty of old compost bags in the garage (they are useful for so many things). Stab a few holes in the bottom of the bag with a garden dibber or a sharp knife to allow excess water to drain out.

The advice is the same whether you chose to grow the potatoes in pots or bags. Start out by putting 15cm-20cm of good quality potting compost in your container. Alternatively you can use a multipurpose compost, mixed 50/50 with sharp sand and/or some well rotted farm yard manure or home-made 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. You can also mix in chicken manure pellets with your compost, but be careful not to overdo things.

Some people advocate growing first earlies or second earlies when you’re growing potatoes in containers, but there is nothing to say you can’t use main crop potaoes if your container is big enough.

Place one, three or five chitted seed potatoes on top of the compost, depending on the surface area of your containers. As a rule of thumb use one tuber for a 25cm wide container, three tubers in a 40cm container or five tubers for a dustbin-sized container.

Now, cover the tubers with another 10-15 cm of your compost 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 5cms below the top of your container.

This emulates the act of ‘earthing up’ soil grown potatoes, and will protect your tubers from light, and the potato plants from frost. Both are essential for the success of your crop.

During this period of time you need to keep your potatoes well watered. You can apply a liquid plant food to the water as you go along, but only if you haven’t already added plant food to the compost. Over-feeding can have fatal consequences for your crop.

It is important to take care with watering when growing potatoes in containers. Soil in containers dries out a lot quicker than on the ground, but at the same time you could end up over- watering the crop. In the height of summer you may well have to water twice a day, once in the morning and once in the evening. Try to water straight into the soil, as the water will just run off the top of the plant, and spill on to the ground. The time to harvest your potatoes varies according to variety. For first and second earlies you can start harvesting when the plants stop flowering. Take the top off the plant, remove the compost, and take out as many potatoes as you need. The rest you can leave in the soil for up to four weeks.

If you have grown maincrop potatoes, the time to harvest them is when the tops of the plants drops and withers away. This will usually happen in the latter half of September. Take the potatoes out of the soil, and leave them to dry, before storing them away for winter.