A quiz I wrote a few weeks ago was so popular with readers that I thought I was going to ask you more questions, but this time it was about what to do in the fall.
Q: Are the cooler autumn months the best time to plant trees and shrubs?
A: It depends on whether. Divide the available trees and shrubs into two groups: those that are hardy and those that are hardy. The best time to plant hardy trees and shrubs is in autumn, around the end of September to mid-November.
The warmer air and soil temperatures in autumn and spring give these plants two “springs” to establish themselves before the summer heat. The worst time to plant trees and shrubs that are sensitive to winter is autumn, as there is a risk of frost in December, January and February.
This recommendation depends on our winter and our landscape. Sometimes we have winters that don’t freeze at all. Sometimes we have winters that barely drop below freezing. Occasionally we get harsh frosts with winter temperatures well below freezing into our teenage years and even below.
I recommend trees and shrubs, which are important design features of your landscape, to withstand temperatures of up to 20 degrees, 12 degrees below freezing point. The chances are good that established trees and shrubs will recover from rare frost events. In rare cases, our landscapes can get colder.
Q: What is the best fertilizer for roses in autumn?
A: The worst months for roses growing in the hot desert are the 3½ hot summer months. Our 3½ months of extreme heat are brutal for landscape roses, and they need a chance to recover once the air temperatures begin to cool.
The early planting months in autumn are also the best time to fertilize roses with the same fertilizer that is used in spring: high levels of phosphorus and potassium with lower amounts of nitrogen. The instructions on the fertilizer bag recommend the maximum amount of fertilizer to be applied.
When your roses have dark green leaves, plenty of growth, and flower buds ready to burst, reduce the recommended amount by half. Some rosaries use Epsom salts – for the magnesium content – as part of their fertilizer mix.
Roses love compost and wood chips, not stone. Roses thrive best when surrounded by corrosive wood chips, with compost mixed into the soil.
Realize that there are two types of compost, including rich composts that are full of manure. Rich compost is typically recycled animal or human waste that has been composted.
One 1 cubic foot sack of compost will treat four standard roses. Rich compost does not need fertilization. Other types of compost with a lower nutrient content will likely do so. Apply all fertilizers and rich compost no closer than 12 inches from established plants.
Q: Is spring the only time to deeply fertilize trees and shrubs?
A: Not necessarily. Inexpensive Venturi-type fertilizer injectors like EZ Flow are growing in popularity and are probably the best way to fertilize hardy landscape plants. The old-fashioned way of using granular fertilizer on a regular basis, depending on what you’re growing, is probably the most cost-effective method. Both methods work.
If you are using a fertilizer injector, there is no need to deep fertilize trees and large shrubs. However, if you’re using the old-fashioned method of hand applying granular fertilizer, then deep fertilizing of landscaped trees and shrubs is a method that may or may not interest you.
The deep fertilization of trees and shrubs was initially developed for mixed landscapes in which trees and shrubs grew in lawns. The method avoided killing the lawn in places like dog urine does.
If your trees are in mulch rather than a lawn, dripping a handful of fertilizer where the drip jets are and the fertilizer doesn’t need to be placed deep is good enough. Water from the drip jets helps transport the fertilizer to where the roots are actively growing.
Q: If you were to buy an insecticide that would control as many pests as possible in a landscape, which one would you buy?
A: Probably dormant or horticultural oil. It’s an oil that is used in the middle of winter, after the leaves of deciduous plants have fallen, that suffocates many insects and prevents them from becoming a problem months later. It is a preventative use, so the results are usually not noticed when using it.
A second insecticide I would use is just old soap and water mixed in a spray bottle. Neither is technically organic by definition, but both work.
The resting or horticultural oil is sprayed all over the plant from top to bottom after the leaves have fallen off and pruned. Spray on a warm winter day without wind.
These types of oils are used to control soft insect outbreaks such as aphids, scale insects, mites, whiteflies, and others. It doesn’t control those crazy-looking leaf-foot plant beetles or maggots that feed on roots in the ground. But like many organic sprays, it’s an indiscriminate killer: it kills both the good and the bad.
I had to include the second insecticide, soap, and water mix because it’s so effective. It also doesn’t distinguish good insects from bad insects: it kills everything you spray.
I was taught years ago that it kills by drowning the insect. But the insect’s waxy outer layer is also damaged.
A tablespoon of liquid soap mixed with a gallon of water is a very toxic combination when sprayed directly on harmful insects. But as with many natural sprays, nothing remains after spraying, no residue. Frequent repeat sprays are needed if insect problems persist.
Q: Is now a good time to sow damaged areas of my lawn?
A: Now until around the end of October or possibly mid-November (if it stays warm) and the lawn is a cool fescue type. But if your lawn is made up of Bermuda grass, buffalo grass, zoysia, or a warm season grass that uses less water, then you’ve missed the time to repair the dead spots. If yours is a warm season lawn, it should be sown, planted, or fertilized by late July or possibly even early August so it can be planted by fall.
But you’re not too late to fix the dead spots. You still have time to re-seed your lawn with a perennial ryegrass (I said perennial ryegrass and not annual) for the winter months and then repair your warm-time lawn in spring around April or early May.
Perennial rye is much nicer and mows better than annual rye grass. But both will work. Annual ryegrass is for the budget conscious and perennial ryegrass is for those whose focus is on beauty, not price.
Q: When can I start pruning?
A: Light cuts can be made at any time of the year, even in midsummer. Buckle your secateurs on your pants or belt every time you go out into the garden. However, severe pruning should only be done when the plant has stopped growing. Most of the plants have stopped growing by mid-autumn, but the leaves have not fallen off.
What do I mean by light section? Light pruning is just cutting or pruning stems or branches with a small diameter. Pruning with hand scissors can be done anytime, just not excessively.
Do not open the tree to sun damage. Heavy pruning is when something larger than hand shears, like pruning shears or a saw, is used to cut. This should only be done during the cooler winter months and preferably after the leaves have fallen.
You can prune lignified plants before leaves fall when the plant has stopped growing. Pruning will revive a tree and grow it if done too early. But after the leaf falls, you can see the (limb) architecture of the tree and make better, faster decisions.
October and November are the seasons when the plant has stopped growing but has not shed its leaves. It is safe to start pruning now if you can see what you are doing.
Bob Morris is a horticultural expert and Professor Emeritus from the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.
