Quote from Lee Garden Ninja on 2nd June 2025, 9:21 am

Hi @sarah924

Thanks for your question about you Californian lilac (Ceonothus). It looks at first glance like its been the victim of spider mites, one of the garden pests we have to deal with. The good news is that spider mite is not terminal, and most plants recover; you just have a few dead leaves and bare patches to put up with in the meantime!

Let me explain what spider mites are. Spider mites are tiny sap-sucking arachnids, closely related to spiders and ticks, that are a common pest in both indoor and outdoor gardens. The most frequently encountered species in the UK and many temperate climates is the two-spotted spider mite (Tetranychus urticae).

What damage does spider mite do?

Spider mites feed by piercing plant cells and sucking out the contents, which leads to a stippled or speckled appearance on leaves. As feeding progresses, leaves often develop a bleached or silvery appearance and may become dry, brittle, and curled at the edges. Over time, affected leaves yellow and drop prematurely, which can weaken the plant significantly. Infestations are particularly damaging in hot, dry conditions where the mites reproduce rapidly.

Some plants may become completely defoliated, which yours hasn't yet, which is a good sign, if the problem is severe and left unchecked. Spider mites are notorious for attacking a wide range of ornamental plants, fruits, vegetables, and houseplants including tomatoes, cucumbers, beans, peppers, roses, and even some trees like Ceanothus!

In addition to their physical damage, spider mites also create fine silk webbing across leaves and stems. This webbing protects them from predators and environmental hazards, and is often one of the first visible signs of an infestation. It can collect dust and further impede photosynthesis. High populations often congregate on the undersides of leaves, where they are sheltered from wind, water, and many chemical treatments, making them harder to reach with sprays or controls.

Organically managing spider mites requires a multi-step approach. First, maintaining good humidity levels around susceptible plants is crucial, as spider mites thrive in hot and dry conditions. Increasing humidity can help slow their reproduction. Regularly misting plants or using a humidity tray indoors can be helpful.

How to get rid of spider mites organically

Next, physically washing the mites off affected plants using a firm spray of water can dislodge many individuals and remove webbing. This is particularly effective for greenhouse or houseplants. Be sure to spray the undersides of leaves, where most of the mites reside. I prefer this method, wack some gloves on and soak the leaves and carefully rub off the mites and webbs by hand.

Introducing biological controls, such as predatory mites like Phytoseiulus persimilis, is a highly effective organic method for greenhouse use but may not work outdoors for you. These natural enemies feed on spider mites and can keep populations under control if introduced early and given suitable conditions. However, they are most effective in controlled environments and may not persist outdoors unless conditions are just right.

Plant health also plays a vital role in resistance. Well-watered, well-fed, and stress-free plants are better able to withstand some spider mite activity. Avoid overusing nitrogen-rich fertilisers, as soft, lush growth is more attractive and vulnerable to mite infestations.

I'd advise you give your Ceonothus a good liquid plant feed and ensure it doesn't dry out.

https://youtu.be/5BhGtCjT2TQ

It should survive and recover in time! So tell your daughter not to panic, now you can get back to weeding! 😉

All the best

Lee Garden Ninja