Southwest Yard and Garden

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

cenchrus sandburs and stalk photo from weeds.nmsu.edu 2

goatheads puncturevine photo from weeds.nmsu.edu

Both are annual weeds with vengeful, spiny seeds, but sandbur (top) is a grass and goatheads (bottom) are a flowering, broadleaf species. Photos from the NMSU website here.

Question: Can you help me battle my weeds organically? I’ve got sandspurs and goatheads. Are there soil conditions weeds hate?

Helen B., Las Cruces, NM 

Answer: Ouchie, that’s a nasty duo of weedy enemies. Most readers can commiserate all too well. Sandbur is a grass of the Cenchrus genus, also commonly referred to as “stickers” or “sandspurs.” Goatheads (Tribulus terrestris), also known as “puncturevine,” have tiny yellow flowers; delicate, compound leaves; and spiny seeds that are even meaner and tougher than sandburs. Many people mistakenly call sandburs “goatheads,” and I understand why. Both have spiny seeds. Both get all tangled in your socks and shoelaces when you’re not looking. Here’s the quickest way to tell the difference: place one of each seed type on the floor and then step on them both while barefoot. They both hurt, but pay attention to the depth of the pain and the length of time that pain persists. The goathead spines feel like they punctured all the way up to your knee and the spot of contact may ache for over an hour. You’re welcome.

Question: The Texas red oak, live oak, and pecan trees in my yard were looking bad going into the fall, as were my neighbor’s honey locust and maple. When I searched for problems online, I found different possible pests and diseases for each tree. Can you help me narrow down the possibilities?

Richard V., Hobbs, NM

Answer: This is Part II of the column on diagnosing tree problems. Last week, we learned that water stress and weed whacker injury are the most common tree problems in our landscapes, and that the rooting area necessary for large trees to survive and grow is much bigger than most folks realize.

southwestjanRed arrow is pointing to a porcupine munching on tree bark way up in the canopy of a cottonwood at the Albuquerque Botanical Garden in February 2018. (Photo credit M. Thompson)We also touched on the reasons why symptoms are rarely sufficient for conclusive diagnosis of a tree disorder. This is partly because symptoms may point to secondary or tertiary problems. Many—but not all—insect pests and pathogens are more likely to attack trees that are already stressed.

Plant stressors can be broadly divided into two categories: biotic and abiotic. Biotic stressors are caused by living or once-living organisms, like insects, bacterial and fungal pathogens, and animal pests.

Nature's Notebook is a national, online program with the USA National Phenology Network that uses amateur and professional naturalists to record plant and animal observations in a given location over time. The steps for becoming a volunteer are straightforward. And you can set your backyard as a location or pick a public space and get a group to sign up together (Visit Nature's Notebook here).

mulberry tree with girdled roots in torcGirdling roots on this mulberry tree in Truth or Consequences may be the underlying problem causing canopy dieback no matter how much extra water is applied. (Photo credit M. Thompson)Question: The Texas red oak, live oak, and pecan trees in my yard were looking bad going into the fall, as were my neighbor’s honey locust and maple. When I searched for problems online, I found different possible pests and diseases for each tree. Can you help me narrow down the possibilities?

Richard V., Hobbs, NM

Answer: Local tree experts agree that the number one tree “disease” in New Mexico is drought. And the number one pest for trees in New Mexico is humans with our weed whackers and mowers.

That being said, last summer over the phone with the Hidalgo County Extension Agent, I diagnosed Afghan pines with water stress. Luckily for those poor trees, I had a trip planned to visit Lordsburg that month, so we scheduled a site visit, and though the problem was technically water stress, it wasn’t what I expected. The homeowner’s soil was heavy clay, and the roots were staying way too wet.

Roots need water, that’s for certain. But it’s also true that they need oxygen, so before anyone waters their trees (along with all your other landscape plants), stick your finger in the soil or use a tool to dig down a few inches. If the soil feels moist, don’t water quite yet.

Roots also need space to grow. The rules differ by tree species and soil type, but the larger the tree trunk and canopy, generally, the larger the root zone needs to be. The International Society of Arboriculture defines the critical root zone (aka critical root radius) for a given tree as the area equal to a 1-foot radius from the trunk base for every 1 inch of trunk diameter. Trunk diameter measurements should be taken at 4.5 feet above ground (or thereabouts, depending on tree age and whether there are huge, knobby lumps in the trunk). As I mentioned back in June, the quickest way for me to estimate tree trunk girth without a measuring tape is to visualize a whole pizza that’s the same size as the trunk diameter—personal pan pizzas tend to be 6 inches and a large pizza is usually around 15 inches. So, if your tree trunk is a medium pizza size, you can guestimate that the trunk is 12 inches in diameter and translates to an approximate 12-foot rooting radius. That’s a 24-foot diameter of rooting area for a model tree to have room to breathe, but in order for the tree to continue to grow without failing, it will need even more space.

snowmeltTwo cups of snow scooped up from the front yard at the NMSU Agricultural Science Center at Los Lunas on January 3rd melted down to a little over 1/3 cup of water, but this isn’t always true because temperature has a big impact on snow density. Photo credit M. Thompson.

Question: We got 4 inches of snow just before the New Year that’s still melting a week later. I’m glad the established trees and shrubs are getting some water but is it enough to hold off on watering manually this month?

Joe S., Belen, NM 

Answer: There’s a rule of thumb that 10 inches of snow equals 1 inch of water, but it really depends on how wet the snow is, and that depends on temperature. Drier snow can be expected at lower temperatures and vice versa. I found some fun calculations and snowmelt discussions online. (Just like snow weight, fun is relative.) “In general, colder temperatures make snow fall less densely and lower the rain-to-snow ratio, resulting in more inches of snow per inch of rain… If 3 inches of rain are expected but the temperature drops suddenly to 5 degrees Fahrenheit, 120 inches of snow will fall” (Richard Graham, www.sciencing.com). That’s compared to 30 inches of snow required to get 3 inches of water when temperatures are close to freezing.

Until this past monsoon, I’d have said we’ll never expect 3 inches of rain on a single day in New Mexico. And I would have been wrong. On August 16, some areas of Doña Ana County got over 2.5 inches of rain. On July 24, more than 3.5 inches of rain was recorded in Santa Fe and Colfax Counties. Dr. Curtis Smith and I drove back down from Raton in that wild rainstorm. And some reported almost 4 inches of rain on August 24 in Grant County! When I searched back in previous years, there were lots of times when some lucky part of New Mexico got that much rain on a given day.

Question: I have a large monarch butterfly garden, and I gathered zinnia seed heads to plant next spring. Zinnia seeds appear to have two distinct morphologies. The ray flower seed is shield-shaped and the disk flower seed is smaller and flatter. Which of the seeds is viable? I have researched and found some who say only the ray seed, some say only the disk seed, and some say both. Please let me know which zinnia seed is most viable and why.

Tim P., New Mexico

swyg01Zinnia showing off both disk and ray flowers at the NMSU Agricultural Science Center in Los Lunas. Photo credit M. ThompsonAnswer: I know exactly what you mean. I’ve had this question before myself. Zinnias were one of my first garden successes when I moved to Las Cruces in 2009. I too gathered the seed heads and I too wondered which part was the true seed that should be saved. That year I just saved the whole shebang. In the late spring I crunched up the entire dried flowers and sprinkled them around the garden. It worked, so I kept that routine right up to today.

Question: I love the poinsettias I bought this year, but one is already starting to droop pretty badly. How do I keep them looking good through the season?

Elizabeth S., Santa Fe

Answer: Did you know you can purchase locally-grown poinsettias at plant nurseries across the state?

poinsetta1Poinsettias were already showing off their color potential at the end of October. (Photo by M. Thompson)I interviewed poinsettia growers in Albuquerque, Santa Fe, Las Cruces, Radium Springs, and Estancia to find out more about poinsettia production in New Mexico, how to get my hands on one (or a few) this season, and how to take care of them at home.

The City of Albuquerque Parks and Recreation Department grows its own poinsettias for holiday displays at the Albuquerque International Sunport and various City Hall buildings. I remember being struck by the vibrancy of the gorgeous Sunport poinsettia planters last December. This year, I got to visit the city’s greenhouses twice to check on the growing process. When I first visited in late August, each plant was looking puny, but promising. By the time I made it back by to check on their progress just two months later, they’d grown by leaps and bounds.

by Dr. Marisa Thompson

chollaCholla infested with cochineal scale. (Photo by S. Moran)

cholla2Waxy white clumps produced by cochineal scale on cholla. (Photo by S. Moran)Question: What’s the white cottony stuff growing on my cholla cactus? And should I do anything about it?

Albuquerque resident via Bernalillo County Extension Horticulture Agent, Sara Moran

Answer: Although it looks like cotton fibers, that stuff is actually a fine wax produced by adult cochineal scale insects, and little black specks may be their nymphs. It’s common around these parts on cholla (Cylindropuntia spp.) and prickly pear (Opuntia spp.) cacti. The similar-looking white beards on other cacti, like the Peruvian old man (Espostoa lanata) and Peruvian old lady (Espostoa melanostele), are normal, healthy modified tissues, not an insect product. The white waxy coating made by cochineal scale on landscape chollas and prickly pears helps protect these true bugs from predators—and insecticides.

cottonwood color comparisonThe same vista, 20 days apart, showing the bright colors all turning dark brown down in the bosque at River Park in Los Lunas. (Photo by Marisa Thompson)Question: Why are some cottonwood trees turning brown rather than yellow this fall? Sometimes a portion of the tree (often the lower portion) turns brown while the crown does turn yellow. I think I have seen this in other years, but this fall it seems more pronounced. Is it weather related? Moisture?

Wes B., Albuquerque, NM

Answer: Explanations for why leaves change color the way they do can be related to the species or cultivar, temperature fluctuations, seasonal day length changes, and potentially the soil moisture levels too. A general rule is that while temperature tends to affect the intensity of leaf color, it’s the shorter days and longer nights that trigger overall color change.

Chlorophyll is the green pigment that starts the process of photosynthesis by helping convert sunlight, carbon dioxide, and water into sugars that travel through the plant to other branches or roots where the plant uses them as food. Normally, chlorophyll breaks down and is remade continually throughout the season, but in the fall when the days get shorter, veins that carry water and nutrients to the leaves start to close off, so less chlorophyll is made and more and more chlorophyll breaks down. As the green color fades, other pigments that were there all along, like yellow and sometimes orange, can be seen.