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.

Sunday, December 6, 2015

A Desert Garden

Many people think that southern California is all desert, but that isn't really true. The confusion comes from the seasonally arid nature of the Mediterranean climate which, to people who come from other areas, feels a lot like desert weather. However, we do have true desert in San Diego County, about 90 miles east of the coast. The differences between the coast and the desert are dramatic, in terms of climate and ecology.

Borrego Springs
 In April 2001 my wife and I bought a very rundown old house in Borrego Springs. We have fixed it up and use it as a weekend getaway. One of my primary reasons for buying this house was to have a native desert garden. I have been working on this garden for amost 15 years and I'm now ready to start blogging about it. Thus, I am departing from the "Encinitas" nature of this blog and expanding into the more general discussion of gardening with California native plants throughout the state. First, here's a couple of early shots of our house in Borrego Springs...

 It sits on an acre lot with about 10 acres of vacant lots around it. Plenty of room to do some gardening. There wasn't a lot of landscaping when we bought it and the house was in very rough condition. What it did have was lots of rocks (always good), and some really large, mature barrel cactus (Ferocactus cylindraceus). There was also a big, old Ocotillo (Fouquieria splendens) in the front yard and three palms (I think two are native Washingtonia filifera and one is W. robusta).

 The biggest problem area was the west side of the house which had very compacted, bare soil. I think the previous residents must have parked heavy vehicles on it. To be able to garden there, I had to bring in some loads of loose sand to create artificial dunes. Then I was able to plant. My gardening philosophy there as well as in Encinitas is to use plants that would normally be found growing wild in that area. I think this just makes sense because those plants are going to be the easiest to grow with the least amount of human intervention. They are also going to fit best into the local ecology, and they will "look right" in that area.

I started with a number of trees. The first is a Desert Ironwood (Olneya tesota). This tree is found in a number of locations in the Anza-Borrego area as well as parts of Arizona and Baja. It is a very attractive tree in the Fabaceae (Legume) family, slow growing but worth the wait. The one I planted was a 15 gal. about 2 ft. tall. It is doing very nicely, still not huge but quite healthy and happy. I planted it right on top of one of the new dunes so that it would have plenty of room for root growth and very fast drainage.

Ironwood is an interesting plant because its wood is so dense that it will not float. In Baja the wood is used to carve curios for tourists to buy. It also makes very long-lasting firewood, but that seems like such a waste. It has very small, grey-green leaves for water conservation, and it will drop its leaves in very dry periods (usually summer). It will leaf out in response to winter rain. It has spines so caution should be used in working around it. It is usually found on alluvial fans and bajadas that are dry in summer but have subsurface water in winter. It blooms in summer with very pretty flowers that few people see due to the summer heat. Below is a photo (sorry, it's not in very good focus) of my Ironwood in bloom. Now that this tree is well established, it requires almost no supplemental water and minimal care.

Here's a shot of a mature tree in the wild in Borrego Palm Canyon.

Another tree I planted was a Desert Willow (Chilopsis linearis). This tree is almost the complete opposite of Ironwood. It is in the Bignoniaceae (Begonia) family. It's origin was probably tropical, and it looks like it. It has long, bright-green leaves. The leaves resemble those of willow species (genus Salix) which led to the common name. It drops its leaves and goes dormant in winter. It is typically found in desert creeks or wet areas where there is year-round water. It blooms from late spring into summer with large, extravagant flowers of pink, purple and white. It has some resemblance to Oleander and can be used as an effective substitute. It's biggest drawback is the fact that it is deciduous in winter; that's when most people are in the desert and they want their plants to look good.

The other important thing to note about Desert Willow is that it must be kept watered, unlike most other desert plants. To accomplish this, I planted mine in a shallow basin in the sand, not on one of the dunes. I put a bunch of rocks in this depression to mimic a stream. When I think it needs water, I put the hose into the rocks and flood the basin. The rocks help keep the soil moist for a long time, so I don't need to water it all that often, maybe once a month. This is a great tree for using greywater. If you could run an outlet hose from your washing machine out to a Desert Willow, it would be the happiest tree in the world.

One more tree that I planted early was Smoketree (Psorothamnus spinosus). This is a truly iconic tree of the desert with a totally unique appearance. Its grey-green, almost leafless stems really do look a bit like smoke from a campfire when seen in the distance. Smoketrees grow in desert washes where the sand is very deep and there is a dependable supply of subsurface water. below are some Smoketrees in the Split Mountain wash.

I took a big risk with the Smoketree that I bought because it is a notoriously tricky tree to get established. It has a deep taproot that helps it find water in desert washes, and this taproot can get damaged easily when in a container or in moving it from container to ground. The ideal way to grow Smoketree would be from seed, but that's not easy either. The seed must be scarified and/or soaked before sowing. This usually occurs in a big storm that tumbles the seeds in the sand of a wash. Surprisingly, a lot of smoketrees pop up on road shoulders in Borrego Springs when there is a mature tree nearby. The seeds get scattered out on the road and get run over by cars which scarifies them. Then winter rain triggers germination. Below is a photo of one such tree.

My big risk was that I bought a 24" box tree, and it was expensive. Fortunately, it has done well. As with the Desert Willow, I did not plant it on a dune. I just felt it would do better on flat ground. Below is a photo of it in my garden.

Smoketrees have terminal spines. That is, the stem ends in a very hard, sharp point. 

On the plus side, the flowers are beautiful. Like Ironwood, it blooms in late spring or summer and many people never see it. Also like Ironwood, it is in the Fabaceae (Legume) family. In fact, very many of the native trees and shrubs of the desert are in this family, including Palo Verde, Mesquite and Indigo Bush. They all have typical "pea" flowers and seed pods. I'll highlight more of these in future posts.

That's all for now.