Southwest Yard and Garden

This column comes through the Grant County Extension Service out of New Mexico State University.

Southwest Yard & Garden by Dr. Marisa Thompson

swyardgarden1Tree growth is slowing as temperatures fall (photo credit M. Thompson).

Question: I’ve seen conflicting advice online on whether or not to fertilize trees now to promote healthy root growth.

Camille R., Albuquerque

Answer: Don’t fertilize anything in fall because we want growth to slow with dormancy, and the salts in fertilizer will either just sit there and be unhealthy for soil/roots or actively damage roots. When carefully selected and applied, fertilizers can help boost a plant that’s already putting on a flush of top growth, like in the spring and early summer. In our areas, applying fertilizer now may extend late-season growth, and that new tender growth is particularly susceptible to cold injury.

cherries in bloom sept 30 2018 stacia jacobi garcia 2Ranier cherry blossoms in Albuquerque at the end of September. Photo credit Stacia G.

Question: Do you know why my Ranier cherry tree is blooming now (September 30)? It doesn’t look so good, but earlier this year it produced a decent amount of cherries. - Stacia G., Albuquerque, NM

img 0033It’s too late to deadhead zinnias now and get new blooms, but cutting flowers earlier in the season can really increase flowering time and number of new blooms. Photo credit M. Thompson.

Southwest Yard & Garden by Dr. Marisa Thompson
Amendments to Soil Constitution

Question: Should I amend my soil when planting a tree?
- Common Question from Tree Lovers Across the State

Answer: The short answer is no. The less-short answer is still no, assuming you’re planting a tree that’s recommended for your area. The recommended tree species are the ones with roots that are well adapted to our native soils, so they’re more likely to live long, sustainable lives.

Before I get into the weeds on why soil amendments, including fertilizer, are not recommended when planting trees, let me first explain that this week I’m talking about landscape perennial trees, shrubs, flowers, grasses, cacti, succulents, and vines. I’m not addressing the needs for annual flowers, garden vegetables, or commercial fruit trees. Another important distinction is that non-native, non-adapted trees may benefit from modifying the planting site, but even then only in the short term.

Why not amend the soil around your tree? There are a bunch of reasons depending on the problem—or perceived problem. The number one issue we see around the state with dying trees is just plain water—where and how you apply it. Another common reason trees don’t make it is that they’re planted too deep, so amending the soil wouldn’t help with that issue. For more on how to plant a happy tree and figuring out the whole watering conundrum, visit the links on this week’s blog post: https://nmsudesertblooms.blogspot.com/2018/10/amendments.html 

We want secondary (smaller, finer) roots to grow out laterally 2–4 times the height of the mature tree. They’re the ones doing all the water and nutrient absorption work. Amending the soil in the planting hole restricts the outward growth of roots beyond the edges of the hole and encourages circling roots to form. Research on trees and soil amendments has shown that by adding organic matter to your soil backfill, the initial results are good. But that interface between the amended soil and the native soil is bad news. Water doesn’t move well across the interface, and neither do roots. With time, the roots just stay in the original hole area and spiral in there, so you end up with a potted tree with reduced growth rate, constant water and nutrient stress (because those lateral roots never fully developed), and heightened vulnerability to pests and diseases.

My favorite answer to most horticulture questions is asking more questions. What are you trying to achieve by amending your soil? What type of soil exists in the tree’s native habitat? What kind of soil do you have? What is the soil history in the planting spot? How will you be watering the tree? What’s wrong with your soil, again?

Let’s take an example of a Texas red oak tree (Quercus buckleyi) in a 15-gallon container that was donated by the Los Lunas El Jardin Encanto Garden Club and planted this week here at the NMSU Agricultural Science Center at Los Lunas. This tree is native to sandy soil areas of West Texas and the Pecos River Valley. The USDA Web Soil Survey of the planting site shows that the soil at our planting site is a Glendale clay loam (check the blog, address above, for a little video I made on how to use the Web Soil Survey tool to know your soil better). Sure enough, on NMSU Extension Horticulture Agent Jeff Anderson’s suggestion, I filled the hole with water a few days before planting to get an idea of the water-holding capacity, and it was still mucky at the bottom today. This will become a problem if the tree is overwatered, and trying to increase organic matter by amending the soil might make it worse. A major reason for adding organic matter when planting is that it increases the water-holding capacity of soil. That’s why it is recommended for veggies and annual flower plantings in sandy soils where the water drains too quickly.

Slow and steady wins the tree race. Trees that grow too fast are more likely to develop structural problems and have other issues. Fertilizers are not recommended at the time of planting perennial plants because we want the plant’s energy to go into root establishment at a healthy rate to support the aboveground growth. Hold off on fertilizing for at least a few years while your tree is setting down roots. Even then the best practice is “Test, don’t guess.” Having your soil tested can help you decide which nutrients are deficient, if any. Furthermore, a nutrient deficiency isn’t necessarily remedied by adding more. For example, calcium in tomatoes can be deficient in the fruit, causing blossom end rot, but calcium levels in our soils are usually adequate to extreme, so adding calcium won’t help—water is the key in that case. NMSU Extension Guide A-114, “Test Your Garden Soil” http://aces.nmsu.edu/pubs/_a/A114/welcome.html , has more info on soil testing.

Nutrient toxicity is a potential issue to consider too. Soil amendments are not regulated, so fertilizers can be extremely high in salts. According to a helpful Colorado State University Extension publication on soil amendments for vegetable gardening http://extension.colostate.edu/docs/pubs/garden/07235.pdf , “Plant-based composts are low in salt. These may be applied at higher application rates, more effectively improving the soil. Plant-based composts are typically higher in price.” By “plant-based composts,” they’re talking about compost made from kitchen scraps. Contact your local city government or county Extension office (http://aces.nmsu.edu/county/) to find out about compost options for your community. Also, ask about composting workshops. The Bernalillo County Extension Master Composters volunteer program is another helpful resource and fun group to work with. Or check out NMSU Extension Guide H-110, “Backyard Composting” http://aces.nmsu.edu/pubs/_h/H110/welcome.html , to learn how to make your own compost.

As Community Forester for the City of Las Cruces Jimmy Zabriskie said when I asked if he ever recommends using soil amendments when planting a tree, “Save the money for mulch.”

Stay tuned for more on the goodness of mulch in future columns.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu Please copy your County Extension Agent http://aces.nmsu.edu/county/  and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms http://desertblooms.nmsu.edu/  and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/ .

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505- 865-7340, ext. 113.

Southwest Yard & Garden by Dr. Marisa Thompson

NuMexTwilight. This ornamental 'NuMex Twilight' chile plant produces colorful fruit all season long (photo credit The Chile Pepper Institute).

Question: What’s the difference between different colors on peppers? I’m getting red fruit and purple fruit on the same plant.

Mary S., Santa Fe, NM

blue grama grass rsFigure 1. Blue grama (Bouteloua gracilis), the state grass of New Mexico, photographed in November 2017 near Carson National Forest (photo credit M. Thompson).Question: Do you have recommendations for native grass species for residential landscapes and how to care for them?

- Otero County Extension Master Gardeners

Answer: Bigger ornamental grasses provide beautiful backdrops, hedgerows, and screens in our landscapes. Smaller species help fill in between broadleaf plants and make great garden borders. Both big and small grasses provide texture, contrast, and grace, all with minimal maintenance and zero fertilizer. In winter, when many of our ornamental plants have shed their leaves, grasses can be delightful, even though they're dormant and brown.

blossom end rot rsBlossom end rot on a tomato picked at the NMSU Agricultural Science Center at Los Lunas on August 21 (photo credit M. Thompson).This week, when trying to select a question for the column, I looked through the 22-year archive for this gardening column online (http://aces.nmsu.edu/ces/yard/archives/) to find issues that pop up again and again at this time of year. I selected these three columns written in 1996, 1997, and 2000 by my predecessor, Dr. Curtis Smith, retired NMSU Extension Horticulture Specialist.

pomegranates rsPomegranates split wide open before they were ripe on this backyard tree in Socorro (photo credit V. Gonzales).Question: I have a pomegranate bush, which produced about 30 pomegranates last year. Initially, the pomegranates seemed to be very healthy. However, as the season progressed, prior to ripening, they all began to split open. This year's crop has already started doing the same thing. Is there anything that I can do to prevent all of them from following suit? If not, perhaps you can suggest something that I can do for next season.

- V. Gonzales, Socorro, NM

Answer: My first thought was that splitting is a good sign. My dad knew his pomegranates were ripe when they split. He took great pains to grab the newly split fruit before the ants found them. One year, he even rigged a hammock over the driveway to catch ripe fruit that fell while he was at work because the split ones would explode when they hit the ground.

Splitting before the fruit ripen is a real problem, though. Over the past year, I've gotten this same question from gardeners in Bernalillo, Doña Ana, Otero, and Sierra Counties. Resources from multiple Cooperative Extension programs across the country point to consistent irrigation as the key to keeping pomegranates from splitting too early.
Pomegranates are a low-water-use tree, to be sure, but it's irrigation irregularity that is likely to be the issue. This is because the whole fruit gets bigger and bigger after bloom, but if at some point it is stressed for water, the outer peel may stop growing. Then, when watered, the seeds inside continue to expand and can rupture the leathery peel if it's too tight. This happens with other fruit as well; I'm specifically thinking of tomatoes.

Tree problems are often difficult to diagnose because the symptoms pop up long after the damage was done, so it's hard to know exactly what started it all. Having a regular irrigation schedule for your pomegranate trees can help ensure consistency and reduce early fruit split. Here are recommended guidelines for irrigating established pomegranate trees: Water the entire root zone (at canopy dripline and several feet beyond) to a depth of about 2–3 feet every 7–10 days when temperatures are getting above 90°F. When daytime temperatures are cooler, but still getting above 70°F, back off and irrigate to that same depth every 2–3 weeks. In the winter, irrigating once a month is recommended, but continue to water to the same depth and out beyond the dripline to encourage those powerful, but tiny, lateral roots. If you are like me and you accidentally wait too long between irrigations, don't try to overcompensate with extra water. Pomegranate trees that are water stressed may take up too much water too quickly once it's available, and this can instigate early fruit split.

It's always a good idea to check the soil moisture before watering to be sure you are not overwatering. During monsoon season, you may be tempted to skip an irrigation, but go out and see how deep the rain soaked in before making your decision. Conversely, if we get over a half inch of rain or so, you may very well need to adjust your schedule. Always apply water directly to the ground (preferably mulched ground). Do not spray or sprinkle water on the foliage, flowers, or fruit, because doing so practically invites pathogens, and pomegranate flowers are very sensitive to high humidity.

Splitting fruit was a challenge reported in pomegranate research trials performed by the University of Arizona. The actual cause of splitting was not determined, although it was speculated that uneven watering could have had something to do with it. Researchers noted that 1) after splitting, fruits were more likely to be further damaged by the leaf-footed plant bug and 2) younger trees had more problems with splitting fruit than older trees. They studied 32 cultivars of pomegranates planted at three different elevations: 164, 2,490, and 3,700 feet above sea level.

Conducting pomegranate cultivar trials for residential landscape use across New Mexico is a high priority for me. Pomegranate trees (Punica granatum) are native to the region that stretches from present-day Iran, Afghanistan, Pakistan, and northern India. Through the millennia, they were cultivated to grow in warmer regions of the Mediterranean. Most of the cultivars currently available at nurseries in the U.S. are from that Mediterranean lineage. Therefore, it makes sense that they perform well in California, Arizona, and southern New Mexico. Reference books I checked listed USDA Plant Hardiness Zones 7–10 or 6–8, depending on the pomegranate cultivar. Commercially available cultivars thrive in southern and central New Mexico. Potentially, by testing other cultivars from the original native range, which has much colder winters and lower precipitation than the Mediterranean cultivars, we will find a few that are well adapted for colder parts of our state. Stay tuned!

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the NM Desert Blooms Facebook page (@NMDesertBlooms)

Please copy your County Extension Agent (http://aces.nmsu.edu/county/) and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, office: 505- 865-7340, ext. 113.