Clean-up, Dormant Spray and Mulch
If you were troubled last year by fungal diseases like powdery mildew, rust, black spot, peach leaf curl, or others, there are steps you can take now to limit these diseases in your garden. And an added bonus is that these steps will also curb insect pest infestations. Once I started following these three steps, I’ve had significantly less disease and pest issues in my garden. Here’s what to do:
Step 1: Clean-up
Yes, your mother was right, cleaning up is good for your health and your garden’s too! By cleaning up garden debris, you get rid of places where insects and fungal spores can over winter and proliferate in your garden come the warmer days of spring and summer. Rake up clipping from pruning, fallen leaves and any other garden debris. Pull weeds and dead plants, cut dead wood out of shrubs and trees and throw away.
Step 2: Dormant Spraying
‘Dormant sprays’ are sprays you apply in winter when your deciduous trees and shrubs are ‘dormant’, ie: they’ve lost their leaves and have not started producing new leaves. January is a great time to apply these sprays in mild winter California, especially right after you’ve pruned your roses and fruit trees (see post ‘What to Do in the Garden: January‘).
There are two general types of dormant sprays: oil based sprays and fungicide sprays.
Oil Based Sprays:
These act by smothering fungal spores and over-wintering insects. The types of sprays, how they work, and how to apply are listed below. They control aphids, mites, scale, leaf rollers, other caterpillar eggs/larvae as well as powdery mildew and some other fungal diseases.
- Mineral based horticultural oils – These are highly refined oils derived from petroleum. They work primarily by smothering insects, their eggs and larvae, as well as some fungal spores. Oils pose few risks to people and other organisms, they dissipate quickly leaving little residue. Drawbacks are they can burn plants if applied when temperatures are high and they can stain some surfaces. Apply spray by mixing according to package instructions (or buy pre mixed if you only have a few shrubs or trees) and spray all surfaces of the tree/shrub thoroughly. Spray needs to come in contact with insects to be effective.
- Vegetable based horticultural oils – These are derived from plants. Cottonseed oil and soybean oil are examples. Apply as you would mineral based oils.
- Neem oil – Is derived from the seeds of the neem tree. Neem oil also works by coating plant surfaces preventing fungal spores from germinating and smothering insects. It also acts as an insect repellant. Thorough coverage of plant parts is important.
Fungicide sprays contain minerals that kill or suppress fungal spores.
- Lime/sulfur sprays – Control peach leaf curl, fire blight and scab (pears, apples), and anthracnose. Do not apply sulfur sprays to apricots. Mix according to package directions. Apply to well irrigated plants when temperatures are below 90 degrees.
- Plain sulfur sprays – Control black spot, powdery mildew and rust. Apply as above.
- Fixed copper sprays – Control peach leaf curl, fire blight, and powdery mildew but don’t withstand winter rains as well as Bordeaux mixture.
- Bordeaux mixture – Is a combination of copper sulfate, lime and water. It controls the same diseases as fixed copper but is longer lasting.
Note: Carefully follow all product instructions when using sprays. Wear protective clothing, rubber gloves, and mask. Dispose of containers and excess product properly. For more information on pest and disease control visit the University of California’s Integrated Pest Management website.
Step 3: Mulch
‘Mulch’ can mean any material used to cover the soil such as shredded bark or bark chips, straw, plastic, shredded leaves, etc. My favorite mulch is compost, either homemade or purchased. At the nursery or hardware store, bags of mulch may be labeled compost, soil amendment, garden soil, but all of them are composted plant waste like leaves, tree trimmings, grass, etc. While compost as a mulch may not last as long as say bark chips, I like it because it is already decomposed and, therefore, won’t rob nitrogen from your soil.
Mulch helps control garden disease by providing a protective layer over the soil and lessening fungal spores on the soil from splashing onto plants when it rains or is irrigated. Mulch also prevents sunlight from reaching weed seeds resulting in fewer weeds which can harbor insects and disease. It also adds nutrients to the soil and encourages beneficial organisms like earthworms. Finally, mulch preserves moisture and regulates soil temperature resulting in less frequent watering.
Spread mulch in a layer 3 – 4 inches thick around your trees and shrubs after you have pruned, cleaned up, and sprayed. Keep mulch 3-6 inches away from tree trunks and main stems of shrubs to prevent disease organisms and too much moisture on these plant parts.