Tuesday, December 29, 2015

More Desert Gardening

I have come to know the Anza-Borrego Desert fairly well over the last 15 years. There is a lot more complexity and variety than I originally expected. The following photos illustrate some of the landforms and ecotypes that make up this desert.

Rocky hills and mountains

Desert Washes - this one is Fish Creek

Alluvial fans and bajadas

Steep walled slot canyons

Canyons with year-round water

Desert Transition Chaparral at higher elevations

Borrego Valley
As I mentioned in my last post, my philosophy of gardening is to use plants that would normally be found where the garden is located. I need to add a phrase to that: "...or reasonably nearby." In my Encinitas garden I use plants from other parts of California and Baja, as long as I can create suitable conditions for them without extraordinary measures. 

The same principle applies in my Borrego Springs garden. My garden is located in the Borrego Valley. I could restrict myself to plants that are native to the valley where the dominant plant community is Creosote Bush Scrub, but that would eliminate a number of plants that I want to grow, including some Baja species. So my task is to try to replicate the conditions in which these other plants would normally occur. Desert wash species such as Smoketree (Psorothamnus spinosus) can do just fine in the valley if I put it in deep, loose sand and give it an occasional flood. Plants found higher up in the rocky hills can do okay if I surround them with lots of rocks. This method doesn't work perfectly because my knowledge of what plants want isn't perfect. But it has been reasonably successful.

In this post I want to continue the discussion of trees, this time focusing on the Palo Verde group (genus Parkinsonia, formerly classified as Cercidium). The most common Parkinsonia in this area is Blue Palo Verde (P. florida). It is a handsome, fast growing, relatively trouble-free tree. One of the special adaptations of all Parkinsonias is the smooth green bark on the trunk, branches and twigs, which is the origin of the name Palo Verde. The bark contains chlorophyll so the tree can conduct at least some photosynthesis even when all the leaves are gone. As the tree ages its lower trunk loses the green color and becomes a more typical rough, brown bark. The blue-green leaves are small, like those of Ironwood and many other desert plants, which helps conserve water but cuts down on photosynthetic productivity. Green bark helps make up for this shortcoming. Here's one of the Blue Palo Verdes in my garden.

This tree is approaching 10 years old. Blue Palo Verdes are not terribly long-lived, but their longevity is improved by not giving them too much water. Although they will accept any amount of water you want to give them, the result in often excessively fast growth, brittle limbs, and short life span. I recommend weaning them off supplemental water after the first year. Blue Palo Verdes accept pruning well, but they do have some spines so use gloves when handling. One of the best features of Blue Palo Verde is the flowers which come out in late spring or summer.

Here are the seed pods, which are edible but reportedly not as tasty as Mesquite.

There is a hybrid variety called 'Desert Museum' which many people like because it has no spines. I'm not big on hybrids but this one does seem to be a good choice.

The next species of Parkinsonia is Little Leaf Palo Verde or Foothill Palo Verde (P. microphylla) which grows in Imperial, Riverside and San Bernardino Counties and Baja. It differs from Blue Palo Verde in having even smaller leaves and slightly smaller stature, but otherwise they are quite similar. 

The flowers look very similar to Blue Palo Verde, but the seed pods look a little different.

Both of the above species have produced seedlings around my yard after a winter of good rainfall.

The last Parkinsonia that I have in my garden is called Palo Brea (Parkinsonia praecox). It is a Baja species that does not occur in the wild north of the border. Jon Rebman says it occurs from the Sierra San Francisco south to the Cape Region and a few spots in South America. I like it because it has a slightly different appearance from the other two and works well in Borrego Springs. In my experience it has not been a prolific seed producer, so it would appear that its chances of become invasive are low.

One member of this genus that I do not recommend is Mexican Palo Verde (P. aculeata). It is more weedy than other Parkinsonias and could become invasive in California deserts, although it is said to be better behaved near the coast. Pacific Horticulture Magazine says, "Since both its growth and seed production are markedly stronger where temperatures are high, in some parts of the world where it has been introduced, it has become a weed species that can overwhelm weaker indigenous plants." It has already become a noxious weed in Australia. Unfortunately, it is popular with some landscapers and homeowners because it is even faster growing than Blue Palo Verde. Its fast growth makes it brittle, poorly shaped and short-lived, but apparently some people don't know or don't care about that. It could also hybridize with native Palo Verdes and introduce undesirable genetics into local populations. The 'Desert Museum' hybrid Palo Verde that I mentioned earlier is reportedly a three-way cross between P. aculeata, P. Florida and P. microphylla. This kind of controlled and intentional hybridization may be a good thing, but random hybridization in wild populations is definitely not. If you want the look and performance of a Palo Verde, choose one of the others.

Parkinsonia aculeata (Mexican Palo Verde)     Photo by Carolyn Martus
 That's all for the Palo Verdes in my garden.

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