How to articles
How to deal with pests and plant illness
One of the first things you need to know when you start gardening is that plants aren’t alone in the garden. There’s a whole host of bugs that like to set up home in your flower beds and borders, as well as a lot of viruses that aren’t good for your plants.
But don't panic. It's actually it's a good thing to have a few pest and bugs in your garden. They'll support a wide range of helpful insects that will make your flowers bloom.
However, should a certain pest or disease get out of control and start damaging your plants there are lots of ways to deal with them – instead of just using harsh chemicals.
How to prevent problems happening in the first place:
• Choose plants that are suitable for your garden – so if you’ve got a sunny patch, use plants that like a sunny position.
• Use pest and disease resistant varieties of plants and vegetables – you can find this out by checking the labels.
• Help your plants grow strong with feed, so that they can shrug off pest and disease attacks.
• Grow a wide range of plants and mix them together, so that if any pest or disease does set in, it has less chance of spreading to plants of a similar type.
Think ahead and set up traps and barriers (for things like slugs) where appropriate.
These are some of the basic principles behind pest and disease control in the organic garden which every gardener would be wise to adopt.
How to spot a problem
Get up close and personal with the plants in your garden when they’re healthy. That way you’ll know what they should look like, and be able to spot any potential problem earlier on.
Should you find something that’s a bit suspicious, like bite marks on leaves there’s a few culprits that might be responsible.
These are the ten most common problems and best ways to remove them:
Slugs and snails
Look for irregular holes eaten from the middle of leaves.
Damage: Young plants and seedlings can be eaten to nothing. Sumptuous leaf plants like Hostas can be left patchy and ugly.
Treatment: These fiends generally feast at night. A ring of material that’s uncomfortable to cross like sharp grit or broken egg-shells will sometimes discourage them. But if that fails, you can try a biological control containing nematodes (look for Nemaslug). It’s a natural, non-toxic product that’s safe for both you and the wildlife. Nematodes generally work for about six weeks – so one dose should protect plants. For any plants in pots use salt or Vaseline around the rim.
Avoid using slug pellets – they can lead to hedgehog and song thrush death.
Look for insects about 2mm long, full of sap, and green, black, yellow, pink, greyish-white and brown in colour.
Damage: Bad infestations can reduce the strength of a plant and leave it vulnerable to attack from other pests. Viruses can also be spread by aphids as they move from one infected plant to another.
Treatment: Boost the number of ladybirds, lacewing and hoverflies in your garden – they’re the natural predator of the aphids. You can do this by planting nectar rich flowers like buddleia, calendula, sedum, stocks, sweet william and wallflowers.
Another trick is to grow a patch of nettles with a small colony of these creatures. They’ll naturally increase their numbers over time, and when the aphids arrive you can cut back the nettles to allow the predators to hunt them out.
Lacewing houses can also be bought for the garden to give them shelter in the winter.
Look for irregular notches taken from the side of the leaves. If a plant suddenly dies and appears to have no roots, this is the work of the weevil larvae.
Damage: The effect on your plants will depend on how far the roots stretch underground and the number of grubs eating it. To start with the growth will slow, the plant may then begin to wilt and finally die – as it no longer has roots to sustain it.
Treatment: Adult vine weevils mainly come out at night. They can’t fly, and so have to walk everywhere. They are quite easy to spot and step on. Alternatively, you could use a biological control that contains nematodes (look for Nemasys H). It comes in powder and is watered on in late summer – while the ground is still warm and the vine weevil larvee is small.
You can also try products like Bio Provado, and for new plants Levington Plant Protection Compost. The drawbacks of these chemical controls are that they can kill useful ground beetles.
Look for purplish-black blotches on the leaves, which start to yellow and fall early. In bad cases the stems can also become affected.
Damage: Really badly affected plants can die, as they’re too weak to survive the winter frosts.
Treatment: Prune out any damaged branches during the spring, and clear up any infected leaves during the winter. This will help minimise the spread.
It’s important to keep roses as healthy as possible so they can withstand attacks. Try using mulch with a good hand deep layer of manure around the base of the plants in the autumn and spring. It is also worth searching out roses that show resistance to blackspot.
Look for white powdery mould on the leaves, stems and buds. This fungal disease is encouraged by the plant being dry at the roots, with damp still air around the top of the plant.
Damage: It doesn’t look very nice and can cause leaves to drop early. If the plant is settled in, this is a relatively harmless disease. However, you should take care of younger plants as it can seriously weaken them.
Treatment: Remove all the dead leaves in autumn to prevent the spores from surviving. Use compost in the spring and autumn with manure to prevent the roots drying out. You can also try pruning the plants so they have a more open shape – meaning air can move through the branches.
Look for tiny white creatures clustered under the leaves of houseplants and in greenhouses. If disturbed, they will fly up into a cloud.
Damage: Like most sap-sucking insects the whitefly will weaken the plant and make it susceptible to further attacks from pests.
Treatment: Tap the leaves and as a cloud of whitefly flies up, suck them up with a small vacuum cleaner. Whiteflies are also attracted to yellow paper, so you can try hanging strips of sticky yellow card which the whitefly stick to.
For a more drastic approach try using a biological control containing the parasitic Encarsia in your greenhouse or conservatory.
Look for leaves that have been stripped and tiny black balls of poo – left behind by caterpillars.
Damage: Young leaves can be eaten away and more mature plants are left with nasty holes.
Treatment: You can either leave them for the birds, or pick the caterpillars off by hand as soon as you see them. For really bad cases, try using the biological control with Bacillus Thuringiensis in it – this stops the caterpillar feeding altogether.
Look for stems and flowers covered in a velvety brownish-grey material.
Damage: It’s a really common fungus, which produces spores that are always around in the air. They attack plants through areas of damage. To start with, small pale brown or white patches will appear, then the flowers will rot.
Treatment: Controlling this fungus is difficult because it’s so widespread. Affected areas must be cut out and disposed of or burnt.
Look for orange-brown spots developing on the undersides of leaves. These can often go unnoticed until the disease has taken hold, and spots appear on the other side of the leaves. So keep your eyes peeled.
Damage: Badly affected plants can become really weak, and are unlikely to survive winter frosts.
Treatment: Remove and bin all the discoloured leaves.
Red Spider Mite
Look for leaves covered with hundreds of little dots. Badly infested plants can become covered with fine webs.
Damage: Sucking out the sap weakens the plant. If the infestation is bad, so much sap can be taken that the leaves can dry up and fall off. House and greenhouse plants are usually the ones affected.
Treatment: Red spider mites breed in hot and dry places. If you can increase the humidity around the plant you decrease the pest's reproduction rate. In greenhouses and patio areas try wetting the floors to cool them down.
For a houseplant, you can make a frame out of an old coat hanger and cover with a plastic bag. This will increase the humidity. Just remember to put holes in the bag to let the air move around.