There are no diseases that particularly single out plants grown in containers, with the exception of damping off – the fungal disease that affects seedlings, which we met in Chapter 2. As long as plants are kept well-watered and suitably fed (i.e. not stressed) then container culture should be very healthy, particularly if your potting compost was a sterile mix.
There are, however, pests that are particularly problematic in containers.
The Big Bad is vine weevil. Adults are small beetles with long noses and big feet, and they themselves don’t present too much of a problem, although they eat distinctive notches out of plant leaves. Unfortunately, their offspring eat through plant roots, and are much more destructive. Control the adults to prevent damage from their larvae.
Vine weevils can be controlled with a pesticide, but there are none that are safe for use on edible crops. When you’re filling your container, try covering the base with a few sheets of newspaper (or an old cotton t-shirt, or something similar), which will let water drain through but provide a bit of a barrier to vine weevil adults trying to find their way in. If you notice any adults when you’re moving or watering containers then either kill them or relocate them far away if you’re too squeamish. Put them in a bowl on the bird table – the local birds might enjoy a treat.
The only sure-fire way to control an infestation and kill the larvae in containers growing edible plants is to invest in the biological control. Nematodes (tiny worms) parasitize the vine weevil larvae and kill them off (vine weevil larvae are pale with darker heads, and curl into a ‘C’ shape). Biological controls have to be applied at the right time, and in the right conditions, so check the instructions before use.
Red Spider Mites
There are two different kinds of red spider mite (and neither of them is red all of the time). They do similar damage, but one prefers life under cover and one is more outdoorsy. For container plants they are more of an issue indoors, where they enjoy the low humidity levels. They are sap-sucking insects that feed on plants and gradually cause the leaves to turn golden. You may notice the webbing they create to move around before things get that bad.
Red spider mites can be deterred by keeping the humidity high (try standing containers on gravel trays that are kept filled with water). Infestations do respond to pesticides, and there are options available for organic gardener – usually sprays based on soap or fatty acids.
There is also a biological control, which is a good option for larger areas like greenhouses that are particularly prone to spider mites.
Of course, if you have spider mites on indoor plants you could try giving them a holiday outside (if weather conditions permit) and see if that helps. Affected leaves should be removed; badly infested plants should be isolated or disposed of before the pests spread to others.
Outdoor containers can become home to an ants nest. They’re not that much of an issue for the plant, but they do indicate that the compost is far too dry – and their tunnels will make it increasingly free-draining. Keeping the compost wetter may encourage them to move on, but you may need to repot the plant into fresh compost to remove the nest.
If you have chickens then take the plant out of the pot and let them loose; chickens love ants and particularly ant eggs, and will deal with them in short order. (I am talking about ants that don’t sting or have mild stings; in other parts of the world I know there are ants that are more problematic and would need to be dealt with more carefully.)
Slugs and snails
Slugs and snails are no more of an issue for plants in containers, except for the fact that they can often find nooks and crannies in which to while away the daylight hours and stay close to the plants for stealth raids every night.
Windowboxes seem largely safe from slugs (but not snails), but free-standing containers can be protected by standing them on gravel or something rough, or using rough mulches. You can also use copper mats underneath, or copper tape around rims, as slugs and snails don’t like crossing copper barriers.
If you have real problems with slugs then there is also a biological control you can use, and environmentally-safe slugs pellets that are not poisonous to wildlife. For more ideas about slug control, listen to episode 33 of The Alternative Kitchen Garden Show.
And don’t forget that we have already covered fungus gnats and aphids in Chapter 2!
This blog post was written by Emma Cooper and was published on The Unconventional Gardener website. If you're reading it elsewhere you may want to navigate away from plagiarised content.