Cover garden soil in plastic to warm it for planting

Q: I have some tomatoes and pepper plants in pots. They are about 18 inches tall. Is it too early to plant them in my raised bed garden? It’s been too cold lately to plant.

A: You’re right, it’s been unusually cold. Both the air temperature and soil temperature are excessively cold for good plant growth of warm-season plants such as tomatoes and peppers. These are tropical plants, what we call “warm season,” and grow best at temperatures from 60 to about 95 degrees. They need enough new leaf, stem and root growth to get established and stay ahead of damage from disease or insects.

How do you know if the soil temperature is warm enough? It is important that day temperatures are warm but nighttime soil temperatures can be cooler, down to about 45 F. A soil thermometer stuck several places in the soil, 1 to 2 inches deep for transplants, will give you a good idea of its temperature. I use an inexpensive AcuRite soil thermometer to do this.

Garden soil, covered in plastic for a few days before planting, helps warm it up. The plastic should be sealed tightly against the soil. Then cut slits in the plastic where you plant. Covering the soil in plastic raises the soil temperature quickly. Covering the soil for two to three days is usually enough.

I prefer tomato and pepper transplants to be about 6 to 8 inches tall when I put them in the ground. At this size, they can handle a little bit of abuse, and they are not so large they suffer from noticeable transplant shock after planting.

Expect large plants like yours to go into a few days of shock before they start growing again. Warmer temperatures help them to recover faster.

If plants are really big, I pinch the tops back so the top and roots are in balance. It’s inevitable to have some root damage from planting. Pinching the top back helps to compensate for this root damage.

Transplant shock is an adjustment period the plant goes through when removed from a container and planted into a garden soil — from a protected environment in the nursery or greenhouse to a more hostile garden environment. All plants go through transplant shock. Smaller plants adapt quicker to a new environment than larger plants.

Q: I was reading about pheromone traps for insect control on fruit trees. Can I use pheromone traps to reduce the need for chemical spraying and which insects are attracted to them?

A: Pheromone traps release insect pheromones, or “scents,” that attract other insects needed for their reproduction or survival. Pheromone traps are used to either determine when spraying insecticides is most effective or, in some cases, to reduce the need for spraying insecticides altogether. A different pheromone trap is required for each insect.

Pheromones are not available for every insect but a different pheromone is required for each insect you want to control. Two insects I have successfully caught in pheromone traps include the peach twig borer (wormy peaches) and coddling moth (wormy apples and pears). I have been successful using them in what is called “mating disruption” and totally avoided the need for spraying chemicals to control these two pests.

The trap is made from cardboard with a sticky bottom surface combined with a rubber lure impregnated with a chemical pheromone. Buy the winged traps and not the Delta traps and the highest concentration of pheromone you can buy in a lure. The trap combined with a fresh lure is hung in the branches of trees you want to protect.

Hang traps with the lures in the trees beginning in about April. Replace the old lure with a fresh one about every 30 days. The sticky surface of the trap should be replaced when it gets dirty or full of insects. I will offer a class on pheromone traps in early April. Watch for it on Eventbrite.

Q: What type of grapes should I grow to make raisins? Is there any special trick to making raisins or do you just dry the grapes?

A: Traditionally, Thompson seedless is used for raisins, but you can use any seedless raisins (usually table grapes). Whichever taste of grapes you like make good raisins. Allow them to dry on the vine (unless birds or ground squirrels are a problem) or remove them from the vine and dry them by themselves.

Remove the berries from the bunches and remove the stem from the berry. If there is a secret to drying fruit in the desert, it is to control the temperature used for drying.

I made a solar dryer a few years ago, but the air temperature in the dryer was too hot. I ended up using this dryer but put it in the shade rather than in the sun.

Cover the drying grapes with cheesecloth or any breathable material that helps keep birds and dirt off of it. Temperatures should be below 140 degrees, but suggestions will vary. Generally, the lower the temperature and high wind movement the better. Think low temps and a fan.

So drying them in the shade in open air in the summer is about perfect here. Just avoid high temps (above 140 degrees) and dry them as quickly as possible.

Q: Is it too late in the year to replace a flowering plum tree? Another tree in my yard is starting to blossom.

A: Whether to plant a new flowering plum tree or not has nothing to do with its flowering time. The best time to plant is just before new growth begins in the spring which is a week or two before flowering. But there is nothing wrong with planting any time during the spring. It just so happens planting time also corresponds to flowering time as well.

Another great time to plant is during the fall months when temperatures are beginning to cool, when it is not yet cold, and before leaf drop. In the Mojave Desert, the spring begins around the first week of February, give or take, and the fall begins toward the latter part of September.

I’m curious about why you are replacing a flowering plum tree. If this was because of borers that tunnel into the limbs and trunk, then you are fine planting in the same hole. These insects do not get into the soil but spend their entire life above ground.

However, if you are replacing this tree because of overwatering problems then it’s best not to plant in or near the same hole. Root diseases like collar rot stay in the soil and could pose a future problem for most trees and shrubs planted there. Plant several feet from this area.

Q: What type of fertilizer do you suggest for table grapes? Mine have been in ground three years now, and the production has been less than I hoped. I have been using compost and worm castings twice a year.

A: The problem may not be the fertilizer or how much you are applying but how the vine is pruned. There are two general methods of pruning grapes. Some grapevines are pruned using the spur method while others are pruned using the cane method.

Grapes that should be pruned using the cane method may produce little or no fruit if they are pruned using the spur method. However, grapes that require spur pruning will produce fruit if they are cane pruned.

The difference between the two is the amount of last year’s growth left attached to the vine. In spur pruning, last year’s growth is cut back severely, leaving less than an inch remaining. In cane pruning, last year’s growth is cut back so that about 8 to 10 inches of growth still remain attached to the vine.

Bottom line, if you are not sure how to prune your grapes, leave last year’s growth 8 to 10 inches long. Last year’s growth will be a different color than the older parts of the vine. Sometimes it’s reddish-brown and sometimes it’s yellowish-brown. When the fruit emerges, re-cut the cane to a better length.

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

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