Pruning can be done any time of year

A: My old college professor used to say, “The best time to prune is when the pruning shears are sharp.” There is a lot of truth to that saying.

Pruning does not have to be restricted to winter months. If there is a problem, then fix it when you see. Prune with a hand pruner 12 months of the year. Reserve tools, such as loppers, hand saws and chainsaws, for the winter months or dormant season.

The biggest danger in our desert climate is removing too much just before the heat of the summer. There is a delicate balance between leaving enough new growth for shading limbs and leaving too much, which can interfere with fruit production.

There is a pruning technique called “summer pruning” used to help keep trees smaller and redirect growth where it is needed. The term “summer pruning” is a misnomer because it’s not done during summer but during late spring just after new growth has begun to surge. In our climate, summer pruning is in late March, April and up until early May depending on tree growth.

To me, fertilizer is fertilizer. If you haven’t applied any yet, then do so now. It’s not too late. The issue I have is whether there is enough soil exposed under the tree. The bare area under the tree not covered with anything but mulch should be at least half of the diameter of the canopy. For instance, if the canopy is 10 feet in diameter, then the exposed or open area should be at least 2½ feet on either side of the trunk.

There is nothing wrong with fertilizer stakes. They are convenient and less messy than a bag of fertilizer. But they are more expensive per pound of fertilizer than buying it in a bag. If you choose to go this route, then follow the directions provided by the manufacturer.

Q: This is our second year growing tomatoes from seed. We are having similar issues as last year. Some of the older leaves are yellowing and then dying. We are fertilizing once a week with a liquid fertilizer. We take the plants outside for an hour to accustom them to natural light. We have had issues in the past with spider mites, but I doubt that is the problem in their short lifespan.

A: When growing tomatoes from seed in the spring, the usual failures are associated with cold soils. When soils are too cold, tomato seeds either won’t germinate, germinate very slowly or the seedlings turn yellow and frequently die. The soils must be warm for strong and simultaneous seed germination and strong plant growth.

It is too cold now for spider mite problems. Spider mites become a problem during hot weather and dusty situations. It’s a good idea to take plants outside to acclimate but leave them outside all day and bring them in only when temperatures are expected to drop below 45 F. Use hot caps if they are planted in weather that is still cold.

When I first germinated my own tomatoes from seed in the 1970s, I would put seedling trays on top of the refrigerator or television for good germination. The seeds germinate quickly, and I immediately put them under strong fluorescent lights.

Now, I always use heat mats to germinate warm season crops like tomatoes. They make a huge difference.

When seedlings are grown in cold soils, there is the ever-present danger of plant diseases in the soil attacking plant parts below soil because the plants are growing poorly. These soil-borne diseases also cause yellowing of plant leaves. But this yellowing symptom results from soil problems. The temptation is to water more often but, actually, this makes the situation worse.

Again, warm soils get around these problems. Plants such as tomatoes can tolerate cool air temperatures and even thrive in them if the soil is warm.

Another cause of yellowing is a lack of fertilizer, in particular nitrogen. Classic lack of nitrogen causes old leaves to become yellow. Unfortunately, it can also look like the yellowing that results from the soil diseases.

A lack of fertilizer can be dismissed quickly if spraying yellow leaves with liquid fertilizer causes them to “green up” in a day or two.

I think your problem is most likely from cold soils. Next spring, germinate seeds using a seedling heat mat. They can be bought from local nurseries and online stores that sell gardening supplies.

Q: What is the best way to get rid of powdery mildew on my euonymus plant leaves?

A: These are probably Japanese euonymus, which is notorious for powdery mildew when grown in shady spots. Control this disease on euonymus either with chemicals or without them. The best way is without using chemicals but requires more work initially.

Controlling powdery mildew without chemicals requires understanding the conditions which create the fungal disease in the first place. These conditions are high humidity, splashing water, shade and lack of air movement across the leaves. Applying chemicals circumvents the disease but doesn’t find a long-term solution to the problem.

Make sure plants are in a sunny location or else provide more sunlight to the plants. If they are not, consider moving them to a sunnier location. Young plants should move easily. Older plants may require replacement.

Water from sprinklers should not splash on the leaves. Adjust the sprinklers to prevent this. Use drip irrigation when possible.

These plants are nearly always pruned with hedge shears. This is a no-no unless planted in a hedge. Prune to create openings for air movement through the plant and across the leaves to dry them more quickly after they get wet.

As a last resort, use fungicides. Fungicides that are sold to control powdery mildew on roses should also work. Neem oil provides some control of powdery mildew.

Q: I picked up 10 acorns in Illinois that were on the ground. Will these acorns germinate and produce an oak tree? What are the chances this tree will survive our desert climate?

A: There are about 20 different kinds of oaks native to Illinois. Two of the more common oaks are northern red oak and white oak. Both are not native to the Southwest. They will grow here, but you will have trouble keeping them healthy as they get older.

We have native oaks in the Southwest. There also are oaks available from nurseries suitable to desert climates. In the long run, you are better off planting those than growing your own from Illinois.

But I also understand the fun of trying. If you go down this road, be prepared that you may have to remove them when they are no longer healthy.

If you found acorns on the ground, then most likely the seed inside the acorn is mature. However, the seed may not be “alive.” Put the acorns in a bowl of water. Use the ones that sink and discard the ones that float. The floaters are not alive.

If you pick these acorns up in the spring, plant them in the soil. If you pick them up in the fall, simulate a cold winter by putting the “sinkers” in a plastic bag along with a moist sponge for three months. Then plant them in soil. It doesn’t hurt to leave them in the fridge longer if the timing is off.

In early spring, plant one to four of them in a 5-gallon plastic container with good water drainage. Mix compost 50/50 with native soil and use soil mix for planting.

Plant the acorns on their side in this amended soil, about 1 inch deep, in mid-November. Water only when the container starts to feel light when you move or lift it.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.