Desert Horticulture Podcast: tomato planting, pheromone traps, flowering plum, raisins and grape poor production

Join me in this episode of Desert Horticulture. I will talk about planting tomatoes in Cole soils, using pheromone traps for fruit trees, making raisins, when to plant flowering plum tree, and how to correct grapes that are producing so well. These topics and more in Desert Horticulture.

Poor Grape Production Maybe Not Fertilizer

Q. What type of fertilizer do you suggest for table grapes. Mine have been in ground 3 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. I am thinking you have a great that may require cane pruning but you spur pruned it instead.

            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 eight or 10 inches of growth remains 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.

Re-Planting Flowering Plum - Why Did it Die?

Q. Is it too late in the year to replace a flowering plum tree? Another 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 of the year. 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 in 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.

How to Make Raisins

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, then 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 it. Temperatures should be below 140F, but suggestions will vary. Generally, the lower the temperature and high wind movement the better. So, drying them in the shade in open air in the summer is about perfect here. Just avoid high temperatures (above 140F) and dry them as quickly as possible.

Using Pheromone Traps in Fruit Trees

Q. I was reading about pheromone traps for insect control on fruit trees. What insect pests occur here that I can use with pheromone traps and reduce the need for chemical spraying?

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 thirty days. The sticky surface of the trap should be replaced when it gets dirty or full of insects. Watch for a class on pheromone traps offered by me here in Las Vegas in early April on Eventbrite.

Planting Tomato in Cold Soils

Q. I have some tomatoes and pepper plants in pots.  They are about 18" 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 like tomato and pepper. These are tropical plants, what we call “warm season”, and grow best at temperatures 60 – 65 ° F to about 95 ° F. 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 2 to 3 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 big, I pinch the tops back so the top and roots are in balance. It’s inevitable to have some root damage due to 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.

Desert Horticulture Podcast: Which Grapes, Why Western Redbud, Control Bermudagrass and Frost Blankets

Join me in another Desert Horticulture Podcast. This time our topics are which grapes to plant in the Mojave, why choose a Western Redbud over an Eastern Redbud, chemical control of Bermudagrass using Fusilade, and why pull those frost blankets over your plants when  you don't need to.

Desert Horticulture Podcast: Making Cold Soil Warm, Snow and Cold, Apple Tree Not Flowering and Planting in a Mimosa Hole

Join me in Desert Horticulture. I discuss how to make your garden soil warmer when its cold, what to do if your apricot is flowering and its going to snow, why do fruit trees produce little fruit one year and lots the next, why doesn't my apple tree flower and can I plant in the same hole where I removed a silk tree? These topics and more on Desert Horticulture.

Making Soils Warmer in the Spring

Many people contacted me whether they could plant seed or put transplants into the ground because of this unusually cold weather. My answer is the same to them as it is here. It depends. Soil preparation and soil temperature are very important for successful seed germination, preventing plant diseases and encouraging root growth of transplants.

Fluffing Up the Soil

Soil preparation and increased soil temperature are interrelated, which we tend to forget. “Fluffing up” the soil with amendments like compost allows warm air to enter the soil. These soils also cool off rapidly at night but can be planted earlier than soils not “fluffed up”. Amended soils warm up quickly, particularly if they are in full sun. This means amendments like compost are rototilled into the soil or “double dug” into these future growing areas.

Preparation of the soil before planting, the location of the garden spot in relation to the sun and the type of seed or transplant affects what and how soon we can plant after cold temperatures pass.

Warm Season Plants

 Warm season plants come from the tropics and include tomatoes, peppers, potatoes, squash and melons, annual flowers like impatiens, marigolds, petunias, geraniums, salvia, celosia and zinnias. These require warm soil temperatures, 60-70 F, at the time of planting.

Cool Season Plants

Cool season plants like peas, beans spinach, radishes, beets, pansies, snapdragons, sweet alyssum, California poppy, godetia, larkspur, violas, and bachelor's buttons can handle cooler air temperatures and soils, 40-50 F, and so can be planted earlier than warm season types.

Variability Within the Groups

Within the cool and warm season groups different plants, and even varieties within these plant groups, vary by quite a bit. If you aren’t sure, check seed packets for the best soil temperature for germination or online sources for the best root growth of transplants. Use a soil thermometer, with its tip pushed into the ground about 1 inch deep, to measure soil temperatures and to gauge when to plant seed and transplants.

Soil Thermometer I use

Apricot Blooming and Snow is Coming!

Q. My neighbor with an apricot tree is concerned about the forecast of snow. She said her tree is in full bloom and was worried about the forecast of snow later in the week. Any advice?

A. There is nothing that can be done to lower the freezing temperature. The most susceptible parts of fruit trees to freezing temperatures are the flowers when they are fully open. Generally, open flowers can tolerate temperatures to freezing and nothing below that when they are fully open.

            Very young fruit and unopened flowers can handle temperatures slightly below freezing, only a couple of degrees lower, and then they are damaged or die. Commercially, sprinklers apply water to fruit if air temperatures dip slightly below freezing. Ice formation on the fruit acts as an insulator. But not for the flowers.

            A couple of days after freezing temperatures have passed, squeeze the base of the flower and see if there was fruit set or not. If you can feel a small swelling at the base of the flower, then the flower and future fruit made it through the freeze. If it feels flat and there is no bump at the base of the flower, then it didn't make it.

            The second whammy was the cold weather preceding the freeze. Honeybees were not very active because of cold temperatures and overcast skies. If there are no honeybees visiting the flowers, then there'll be no pollination and no fruit produced.

            I put a short YouTube video together demonstrating how to squeeze the flowers to check if there is fruit or not. It’s available on my Xtremehorticulture YouTube channel. Otherwise, just wait and you will eventually find out.

Light Fruit Load Due to Many Possibilities

Q. Yesterday my neighbor was kind enough to share a jar of apricot jam she made from her 2017 crop of fruit. She mentioned, however, that her trees in 2018 produced hardly enough fruit to go through the jam-making process. Is it possible that our warmish winter was a factor? I believe my neighbor's trees "came with the house," so she doesn't know if they are the low chill variety.

A. The weather was too cold for early flowering apricots and peaches in 2018 and again in 2019. In 2018 it was so cold and overcast that honeybees didn’t come out to pollinate the flowers, but temperatures were above freezing so the flowers didn’t freeze. The result was poor fruit set in many early apricots because of poor pollination.

            In 2019 it was also cold when early flowering fruit trees began blooming. These low temperatures, again, limited honeybee activity and resulted in poor pollination. On top of that though, freezing temperatures occurred at least 3 times in February. Open flowers and their potential for fruit production do not survive even the slightest freeze. If you had an apricot tree that was flowering during freezing temperatures, there is a strong chance it won’t produce much fruit this year.

            The chilling requirement for apricot to produce fruit was met in both 2018 and 2019. A lack of winter low temperature was not the problem. In 2019 poor fruit production will be a combination of low honeybee activity and freezing temperatures.

Getting to the Root of an Apple Production Problem

Q. My Fuji apple tree is six years old and has never had any flowers. Is it still maturing, or should I get rid of it?

A. Most of what you're talking about depends on whether your apple tree was grafted or not and which type of Fuji apple tree you have. If you bought a tree grafted onto a dwarfing rootstock, you should see spur and flower development by the third to fifth year after planting. If it is not grafted but growing on its own roots, then it could take 6 or more years before it starts flowering.

            Try reducing the amount of fertilizer applied to this tree by half. If the tree is growing well, then don’t fertilize it again. A young apple tree like yours should grow about 18 inches a year. If the average new growth is more than this, reduce or eliminate the fertilizer applied next spring by at least half.

            Fruit trees that were planted in soil amended with a good quality compost may not need a fertilizer application for 2 years after planting. It depends on the amount of new growth each year. The high nitrogen content of fertilizers, and some types of rich compost, may push new growth excessively at the expense of making flowers.

            When winter pruning apple trees, prune back last year’s growth to no more than 18 inches. Pruning back excessively long growth encourages fruit production closer to the trunk.

Mimosa Silk Tree Planting Hole

Q. Thanks for your article about mimosa trees and their short lifespan. Ours appears to have a disease problem and falls within that 15-20-year age span you mentioned. Can I plant a new mimosa tree in the same spot if mine was diseased and removed?

A. I double checked recommendations from plant pathologists before I got back to you just to make sure. You should not plant in the same hole, or in the same vicinity of a previously diseased mimosa tree. The disease organism can survive in the soil and enter a susceptible new tree if planted in that area. Trees resistant to this disease include redbud, oak such as Heritage Live Oak or Holly Oak, ornamental pear, honey locust and pine trees.

More info on this disease