Growing Vegetables in Containers – What Went Wrong?

18 April 2014, written by Ann Marie Hendry

Using containers to grow vegetables can be very convenient but there are a number of pitfalls that can result in disappointing harvests.

If you suffered setbacks last year when growing in containers, you’ll be keen to avoid making the same mistakes twice. Everyone has occasional failures – for me it was forgetting to drill holes in the bottom of a new container I was preparing recently. By the time I realised what I’d done my plants were struggling, and when I tipped them out the stink from the bottom of the pot made my eyes water! With that in mind, here are the most common problems and ways to avoid them...

Choosing a Suitable Container for Your Plants

While it’s theoretically possible to grow any vegetable in a container, it’s important to choose the right type of container for your plant.

For root crops, a deep container is a must if you want carrots to grow long and straight (although you can grow small round types such as Paris Market which are suited to shallower pots) and to enable earthing up for a good crop of potatoes. Apples and other tree fruits need a lot of space for their roots – a half barrel is probably the minimum you can get away with, even for those grown on dwarfing rootstocks.

Tomatoes and peppers often do extremely well in growbags as long as they’re kept moist with planting holes cut out of the plastic to minimise evaporation, and herbs such as rosemary or thyme positively thrive in shallow troughs and window boxes, as they don’t mind being kept a little dry.

Ensuring Good Drainage in Vegetable Containers

Don’t make the same mistake I did! If your container didn’t come complete with drainage holes, get a drill and a big drill bit and make a few. How many you need depends on the size of your container, but I try to space them evenly across the base about a hand’s width apart. Next, cover the holes with a layer of broken crocks, gravel or small stones - it’s a great way to use up some of those little stones you inevitably turn up when digging the garden!

Water needs to be able to drip (or even pour) away easily from your container to prevent the roots rotting in stagnant water. If the pot is flat-bottomed it’s a good idea to raise it off the ground just a little– you can use whatever you have to hand, such as bricks or small bits of wood.

Waterlogged container causing lettuce to wilt

Retaining Moisture in Garden Pots

Like any normal garden bed, containers need to be free draining, but also water-retentive. Moisture is lost much faster from a container than it is from the ground and some pots, such as those made from clay/terracotta, are naturally porous and will evaporate moisture through the sides – lining the pot with heavy-duty plastic with drainage holes punched in the bottom is one solution for moisture-loving plants. Water will also quickly evaporate from the soil surface in a container, so mulching with bark or gravel can help to limit this.

A sheltered position that receives some shade for part of the day is usually best for containerised plants, especially leafy crops such as lettuce, spinach and rocket that are prone to bolting in full sun. Wind can also have a drying effect, and if your garden is exposed it’s important to make sure that pots are either heavy enough to withstand the wind (especially when growing tomatoes or other tall crops), or can be secured to the ground in some way.

To take advantage of any rainfall and minimise the amount of watering you need to do, avoid siting containers next to walls as these can throw a ‘rain shadow’ that can lead to plants not getting enough water, even during heavy rain.

Even with these measures, containers will still dry out relatively quickly and may need to be watered twice a day in very hot weather. Alternatively, it might be worth trying the wine bottle idea for crops in pots in our article How to Look After Your Garden When You’re On Vacation.

Feeding Plants in Containers

Finally, plants tend to exhaust the soil in containers quite quickly – it doesn’t take long for nutrients to leach away along with the water. Annual vegetables benefit from a balanced organic liquid feed which can be watered in, but this needs to be done regularly as the nutrients in the feed will wash out again all too readily.

Perennial plants in particular can suffer from nutrient deficiencies when grown in containers, as each year the soil becomes more and more depleted. At the start of each season it’s a good idea to scratch away the top layer of soil and replace it with rich compost, perhaps mixed with well-rotted manure or a granulated fertiliser. The nutrients will gradually filter down to the plant roots and feed your plant over a long period of time.

With all the planning and care that plants in containers need, you might wonder if it’s worthwhile growing crops in pots this year – but the convenience of having food right outside your door, and with minimal weeding too, more than makes up for it.