If you have a greenhouse or polytunnel in the garden you’ll be aware of pests. The damp but warm environment in the greenhouse or polytunnel can often be a haven for pests and encourage diseases. And they can cause a lot of damage and more than a little heartache.

It’s fair to say keeping pests out of the polytunnel and away from the plants is a conundrum which taxes many gardeners. We certainly receive lots of questions from customers on this. Here’s the most common questions we get asked along with our answers. Let’s start with the bane of many gardeners - aphids.

What are aphids?

Aphids are very small (less than quarter inch) insects which suck sap and liquids from plants. Unfortunately, they are very common and there can't be many, if any, gardeners whose greenhouse plants haven't been blighted by these pests. Greenfly and blackfly are probably the most well-known varieties of Aphid. Plants attacked by aphids will be weakened and at serious risk of fungal disease. Once aphids infest a plant, they reproduce at an incredible rate so they must be treated and eliminated immediately you notice them.

How do I know my plants have aphids?

You’ll realise pretty soon but tell-tale signs include:

• Leaves and stems will be coated with a sticky residue. This residue can get everywhere and cover other surfaces in the greenhouse.
• The almost sugar like residue can attract a fungus called sooty mold. This black stuff can quickly cover a plant or stem.
• Leaves will become misshapen and turn yellow.
• Always check the undersides of leaves as that is the most common place to find aphids.

How do I get rid of aphids?

As soon as you discover aphids on plants in your greenhouse or polytunnel you need to act. There are plenty of sprays and homespun solutions available for you to try. One or a combination of these treatments should work:

• Ready packaged sprays – available from supermarkets, DIY stores and garden centres. There’s lots to choose from. You pay your money and you take your choice.
• Water - sometimes spraying cold water on the plant’s leaves will be enough to get rid of the aphids.
• Neem oil - one of the most effective treatments. Keep a bottle of neem oil in your polytunnel for all year-round protection.
• Insecticidal soaps - available from some garden centres and work the same way as oils.
• Washing up liquid - mix with water and spray every couple of weeks.
• Garlic spray- definitely one from the traditional gardener’s handbook. Cloves of garlic soaked in vegetable oil and then mixed with water (10:1 ratio water/garlic) can make an effective treatment.
• Milk and water - eight parts water to one-part milk and applied from a spray bottle is an organic solution which is gaining favour with some gardeners.

However, if you’re looking for a faster and simpler solution to keep aphids out of the polytunnel there are lots of chemical sprays, most of which are very effective, available from DIY shops and garden centres. A word of warning. When applying a spray, especially chemical ones but including organic ones , always test on a small area of the plant first and monitor for any negative reactions before applying more liberally.

Enough about bugs and pests. Why not treat yourself to some wonderful new plants or shrubs? We have thousands for you to choose from. Click here to see today's selection.

Return to the Ask the Expert page for more gardening tips and advice