“Don't let the tall weeds cast a shadow on the beautiful flowers in your garden.” — Steve Maraboli

I am typing this at 7,000 feet, at the foot of the 12,637’ San Francisco Peaks in northern Arizona near the southwestern edge of the Colorado Plateau. The “Peaks” are collectively the rim of a dormant volcano, which last erupted perhaps nearly a million years ago, or maybe it was just 400,000 years ago. Long before there were people in North America. At its maximum height it was probably more than 14,000 feet, but like Mt St. Helens, its top exploded leaving a shorter, jagged rim, and today each significant upward jag is a named peak, hence why it is referred to as the “Peaks."

Although long dormant, just east of the Peaks there are a series of cinder cones, with exposed lava flows that look like they might have oozed out of the earth yesterday. Actually, it was yesterday by geologic standards, less than 1,000 years ago. So recent that archeologists found remnants of a village entombed in the hardened lava, probably built and occupied by ancestors of today’s Hopi tribe. They would have witnessed the oozing of molten rock, moving slow enough that they could have easily escaped the 5- to 30-foot wall of lava, a process that had been shaping this landscape for millions of years. The San Francisco Peaks, comprised of layers and layers of basalt, hardened lava, are extremely porous; any precipitation that falls here gets absorbed like an enormous rock sponge.

Soil derived from eroded basalt includes all the minerals plants need for luxuriant growth. This landscape would be a jungle if it was not at 7,000 feet, where the winters are long and cold, and the growing season is a mere five months. As it is, the natural plant cover is dominated by ponderosa pines and quaking aspen with their roots tapping into that rock sponge reservoir, descending to pinyon pines and junipers at lower elevations, and then grasslands at still lower elevations.

At 7,000 feet, the high summer temperatures tend to be in the mid to high 80s, with a rare day maybe poking into the 90’s, and so very comfortable by human standards. Not surprisingly people live here, somehow tolerating seven months of winter with mostly freezing temperatures. With people have come their plants, and finally to the topic of this essay, weeds. Wanting to live among native plants, birds, lizards, and pollinators, we spend an inordinate amount of time battling weeds, thriving in those soils derived from prehistoric volcanic eruptions, eroded basalt with every mineral a plant could want. Weeds are very happy here.

The simplest definition of a weed is a plant or animal, not native to a region, that can propagate and spread on its own, and that is unwanted. My own definition adds a bit more by saying those plants that I consider weeds also displace native vegetation and or animals and in doing so disrupt ecological relationships between native plants, herbivores, pollinators, and soil microorganisms that have developed over millennia. They can also change ecological processes such as wildfire frequencies, creating a negative cascade resulting in an ecologically depauperate landscape. In my dictionary, weeds are not positive or even benign. If they were I would welcome their additions to local biodiversity. Rather, what I call weeds wreak havoc on nature. Back where those weeds came from, most often Europe or the Middle East, those same plants are not weeds. There they developed through the same millennia, coexisting with their own array of insects and birds and mammals and soil microorganisms, in an oscillating ecological balance. Most such plants when brought here need to be coddled to survive. Think apple or pear trees. However, some arrivals land here and find whatever was keeping them in check back from whence they came is now missing.  Now having access to that missing soil nutrient, or released from that insect or microorganism that was keeping them in check, their population explodes.

It seems like the worst weeds here in the western arid landscapes are either grasses or mustards. The mustards germinate early and then rob precious soil moisture from the many native wildflowers. Although cultivated mustard greens are palatable to humans, as are broccoli, cabbage, cauliflower, and Brussel sprouts, with the exception of cabbage white butterflies, (which occur naturally throughout the temperate portions of the northern hemisphere, (North America, Europe and the Middle East), few North American native wildlife species eat the non-native mustards. Even harvester ants, a key food source for many desert lizards, cannot sustain their colonies on mustard seeds alone. Across the low elevation deserts of North America, one of the worst of these weeds is Sahara mustard, a plant accidentally introduced to the Coachella Valley as seeds in the root balls of date palms brought from north Africa or the Middle East. In years when there are ample December rains Sahara mustard germinates quickly, sending down a long tap root.

The native desert wildflowers, however, through millennia of experience usually wait until January or February to germinate just to be sure there will be enough soil moisture to complete their life cycle. The problem is that by then the Sahara mustard is already well established and robs the natives of both moisture, sunlight, and access to pollinators. Few if any are able to complete their short life cycle and replenish the soil seed bank, a necessary deposit so their future offspring will be able to germinate when those infrequent spring rains return to the desert. Ant-eating lizard populations decline. Each fall desert naturalists look to the sky in hopes that rains will come and create carpets of wildflowers. I always hope those rains wait until January or February, when the natives have a chance to compete with the mustard.

Non-native grasses share a similar strategy with Sahara mustard – germinate early and steal all the available moisture from later germinating wildflowers. Similarly, the grasses provide little or no succor for pollinators, or other insects that are key components of the food web supporting lizards and birds. When we encounter large patches of another non-native grass, Cape needle grass, on our trail surveys our collected data show an immediate decline in lizard encounters. That is the same grass that Palm Springs residents admire when it turns their hillsides green in the spring. Those non-native grass weeds include fountain grass (escaping from suburban gardens), buffelgrass (introduced to support cattle on desert landscapes where cattle cannot otherwise find enough food), and brome grasses (also introduced to support cattle production). These weedy grasses have one more attribute – they fuel wildfires and thrive in a frequent wildfire regime. Native desert plants are not adapted to wildfire and often perish when wildfires occur. The heat and aridity that characterize deserts does limit what weeds that can become established, but those weeds that can tolerate what deserts offer can have a large negative impact across the desert food web.

Back here at 7,000 feet, the short growing season may limit some weeds from becoming established, but rainfall and soil nutrients allow other weeds to thrive. The main factor determining whether weeds become established here appears to be soil disturbance. Plow or scrape the surface and weeds thrive. Leave the soil alone and the native species remain intact. Unfortunately for my efforts at trying to restore a native landscape around our home, the land has had plenty of disturbance. From logging, to a ranch to a housing development, disturbance has been the norm. Weeds have been having their way with this landscape for decades. Time to get back to pulling weeds. The attached image is portion of our yard where we are, for now, winning our war with weeds.

Nullius in verba                                                                                                                                                Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe