How to Control Spider Mites and Protect Your Greenhouse Crops

Spider mites may be tiny, but their impact in a greenhouse can be massive. These sap-sucking pests thrive in hot, dry conditions and can quickly turn your lush tomato or cucumber plants into a patchy, brittle mess. They’re especially sneaky, by the time you notice the tell-tale webbing, they’ve usually taken hold. But with a few clever strategies, we can control spider mites and protect our greenhouse crops without resorting to harsh chemicals.

Spot the Early Signs

The first clue? Pale speckling on leaves, usually starting at the lower canopy. Look closely and you might see tiny red or yellowish mites moving on the underside of leaves. If left unchecked, they multiply fast, leaving behind fine webbing and eventually causing leaf drop and stunted growth.

We check our greenhouse crops at least twice a week during peak summer, especially tomatoes, aubergines, peppers, and cucumbers, which are spider mite favourites.

Crank Up the Humidity

Spider mites love hot and dry. So we make life uncomfortable for them by increasing humidity. A simple tray of water, damp gravel paths, or regularly misting the air can help slow them down. We also keep plants well-watered and group them together to maintain a more humid microclimate.

For serious outbreaks, we even dampen the greenhouse floor or drape wet hessian sacks over support bars to boost humidity quickly.

Give Leaves a Good Rinse

We’re not afraid to get hands-on. A gentle spray with the hose, especially on the undersides of leaves helps knock mites and their eggs off the plant. We do this early in the day so the foliage can dry before nightfall and avoid fungal issues. A weekly rinse goes a long way in keeping numbers down.

Introduce Natural Predators

When we spot early signs of spider mites, we bring in the big guns, Phytoseiulus persimilis, a predatory mite that feeds exclusively on spider mites. These tiny allies are safe for your plants and incredibly effective in a controlled environment. You can release them directly onto affected plants and let nature do the rest.

Timing is key, introduce predators early, when spider mite numbers are low, for the best results.

Remove and Isolate Heavily Affected Plants

If a plant is badly infested, we sometimes make the tough call to remove it completely. Leaving it in the greenhouse risks the mites spreading to healthier neighbours. At the very least, we prune out the worst-affected leaves and bag them before composting to limit the spread.

Keep Airflow Moving

Stagnant air creates the perfect environment for spider mites. Keep greenhouse vents open as much as possible and use a solar-powered fan or two to circulate air. Good airflow also helps reduce fungal problems and improves pollination, win-win!

Clean Between Crops

Spider mites can overwinter in debris and crevices, so end-of-season cleaning is essential. We scrub down greenhouse walls, benches, and pots with soapy water, and clear out all dead plant material. It’s not glamorous, but it sets us up for a healthier season ahead.