Drought Tolerance and Native Plants

The current drought has inspired many Californians to pull up their lawns, their thirsty garden shrubbery, and to become more receptive to alternative landscapes. Happily many re-landscapes are including our native plants, or if not natives, drought tolerant plants from other Mediterranean or desert regions of the world. There is however a common public misconception that because our plants are adapted to drought that they can be planted and once established left on their own for mother nature to care for.

So let's start with this test question.  Choose the correct answer For the following statement: California native plants require....
A.) no supplemental irrigation 
B.) supplemental irrigation a few times during the summer dry period 
C.) regular weekly or twice a month irrigation during the dry season 
D.) I don't know

Don't you wish "I don't know" was a test option when you were in school? Well for today's question D.) "I don't know" is the correct answer.

---

A plant's tolerance to drought - or in a garden setting the lack of supplemental irrigation - is conditional upon a great many factors but principally by the three which make up the magic triangle of horticultural:  Provenance, Soil Type, and Climate. Actually there really is no such thing as a magic triangle of horticulture. Google it for yourself. I just made it up to serve my purpose here. These considerations are very important however and I will address each of them in more detail shortly. For now let's just state that while a majority of our native plants do have lower water requirements than conventional landscape plants they are not all created equal in their water thriftiness.

But before we get into our discussion just remember this: Watering your native plants too much or too little will either kill them or at best they will look really really ugly. Native plants are beautiful and if you are growing really ugly plants in your yard it gives California native plant gardening a bad rap. So we will all strive to provide just the right amount of supplemental water so our native plants will thrive and look as gorgeous as they should look!

So, how much and how often do you water native plants? Without understanding a little about your plant's provenance, your soil, and local climate there is no simple answer to this question. You pretty much have to figure this out for yourself and this process is called gardening.

But I want to help and not be snobby about this so let's get back to the magic triangle of horticulture noted in the diagram above and let's start with...

Provenance
Provenance refers to an object's place or source of origin, or better stated, where did it come from? Why is this so important? A native plant's inclinations and preferences are encoded in their DNA by their provenance or where they evolved. So when we move plants from, say, their cool and moist place of origin to a hot and dry region, they need some help to be happy in their new environment.
Do keep in mind that a plant's provenance is not the nursery where they were grown or where you purchased them. The Irishman is not going to tan better just because he was born in southern California southern California. Right?

Here are two maps I have selected to help illustrate my point. You ask what point am I trying to make? OK, fair enough. Where our native plants come from, their provenance, (their Provenance) makes a big difference on how drought tolerant they will be. So now back to the maps. You can click on the map to view them larger. Do this, study the maps and the legends, and then we will continue. (a big thank you to Oregon Climate Service and NOAA)

---

So now you have memorized the maps. Good. For an example, let's use a very popular native landscape tree, the coast redwood (Sequoia sempervirens). Now these plants, even though they grow in the fog belt of California's pacific northwest, are quite adaptable and widely grown around the state in parks, along some freeways, and in many larger home gardens. Now they grow wonderfully with little to no supplemental irrigation near the central to northern coast (within their local provenance area) but bring them inland and they will require a whole lot of water to be happy and beautiful, i.e. not really really ugly. Remember, no ugly native plants. And if you live anywhere east or south of the central and north coast ranges and want to save water...do not plant sequoias. Choose something more local to your region.

For another example of how these maps and knowing a plant's provenance can help us to better coordinate our garden and maintain our plants, we will look at some plants in my garden. Beautiful aren't they?

Here in this view of my garden are a hybrid buckwheat (Eriogonum molle x fasciculatum), a silver sage brush (Artemisia ludoviciana), a salvia cultivar (Salvia 'Allen Chickering') and a desert mallow (Sphaeralcea ambigua). Now, these four plants thrive in my garden, all are drought tolerant, but only one is "local" or native to my part of California, i.e. of local provenance. The other three are imports, tourists, or immigrants, if you wish, who originated from other places in the state. Before I get into any specifics about the differences in these plants I want to answer the question which I am certain you have been wanting to ask. "How can we know where a plant comes from?" Well fortunately there are many good folks in the state who work very hard to maintain data bases of California plants. My favorite and easiest to use for this topic is Calflora. I have linked the silver sage and the desert mallow above to the Cal Flora website. Click on them and you will see a map which shows where in the state they have been documented growing naturally. You can then compare these sites with the temperature and precipitation maps to get a better idea of how suitable to your region a given plant might be.

I did not link the other two plants as being of hybrid origin they have a more complicated provenance. Cultivars and hybrids are not listed on Calflora so you have to know the parentage to find out their provenance. The Salvia is a hybrid between Salvia leucophylla and Salvia clevelandii. In my garden when the weather turns hot and soil moisture is low,  even when this Salvia's leaves are curling from a lack of soil moisture, the foliage of the silver sage, which grows in a canyon about 12 miles from us (local provenance), and the desert mallow (desert provenance) are full and lush. If you look up Salvia leucophylla on the Calflora website you will find that it grows in the inner and outer south coast range, areas which, though it is quite dry and warm during the summer, are moderated by the local coastal influence. Salvia clevelandii from San Diego County grows mostly inland from the coast but at higher elevations in the Cleveland National Forest. So though my Salvia is nicely drought tolerant it requires more water in my warm inland valley area. In case you are curious about the buckwheat, the mom of this hybrid is from an island off the coast of Baja California (Eriogonum molle). However this selection growing in my yard is more like its dad, the common and very drought-tolerant California buckwheat (Eriogonum fasciculatum), and thus while it is not as resistant to drought as the silver sage or the desert mallow it is more so than the salvia. Making any sense?

The take home point here is that in general if you want to use less water in your landscape choose plants from your local region, desert plants, or if you can afford to, move closer to the coast.

---

Soil Type
Now, in my garden the soil is quite sandy and this brings us to the second of our Very Important Considerations: soil type.

Now learning about plants and geography is fun but studying soil is, well, pretty dull by comparison so I will keep this section really short. With my sandy soil having relatively large particles it does not compact and accepts and drains water quickly. Having large spaces between soil particles there is also less water retention and more air-holding capability. If I were growing my same plants in a heavier soil with more tightly packed moisture-holding clay or silt I would not have to water the Salvia as frequently.  In fact, I would have to be wary that I do not drown the plants by filling in those teeny tiny air spaces with excess water. Too much moisture, especially in the warm months when our native plants are not accustomed to getting water, provides a great environment for root rot fungi, the bane and grim reaper of native plants.

So our take home point here is, the lighter and sandier your soil the more water you may need to provide your plants and the more frequently you will need to provide it. The heavier your soil the more slowly you need to apply water, and  you need to be cautious not to over water your natives.

I could not find an author to credit for this illustration so to whomever made the soil particle drawing in this section, it is very nice. Thank you.

Climate and micro-climates are the last of the Very Important Considerations of the magic horticultural triangle. Thank you clipartpanda.com for this happy sun art work.

The region where you live and garden is affected by the local climate. I know, duh. But we Californians with our abundant water resources have been loathe to accept or even consider this.  California is blessed with lots and lots of varied climates and the climate where you live and garden will have a strong bearing on how drought tolerant your plant will be. Additionally, no matter where you live your home and property also has micro climates, smaller regions where the bigger climate is varied by local conditions. Think the north and south sides of your home. Micro climate variation is also created by the shade of a tree or large shrub, and swales which receive more water from rains and cool air settling at night. Slope direction and sites protected from prevailing winds also create varied micro climates. I know we are not supposed to have leaky faucets, but even these create micro climates.

To illustrate, let's say you want to have a cactus collection at your home in San Luis Obispo. Well, this cool and often foggy coastal climate is not too ideal for desert plants. However, selecting a south- or west-facing slope for your plants, just as you would for your solar panels, will provide the warmest and driest site for your sun-loving succulents. Or take that flannel bush (Fremontodendron) that you have tried and tried to grow and repeatedly killed. Being that it is sensitive to summer watering try planting it on the well-drained and warm south- and west-facing slopes. And those water guzzling native willows, alder, redwood sorrel or creek monkey flower which you just have to have in your garden but which you also know you don't have any business trying to grow in your region? Maybe they should be relegated to the moist and more humid north side of your house or garden shed. You can also sneak them a little extra water late at night when no neighbors are watching.

Now there is nothing better than failure to teach us valuable lessons, right? For another example of the influence of micro climate here for all the world to read is my "I should have known better" lesson.

---

This is a specimen of Ceanothus 'Wheeler Canyon' which grew wonderfully in my garden for about 6 years and which I had planted on the east side of my house. Being cheap and lazy, when I installed my irrigation system I put the whole section on one valve, so when I ran it all the plants received about the same amount of water. As you can imagine, the soil nearest the house is shaded all afternoon and thus holds moisture much longer than the same soil 10 or 15 feet out. Thus, even with my well-drained-soil watering for my native plants in the outer hot and dry part of the garden, I over watered the Ceanothus and lost it to the grim reaper of native plants, root rot.

So the take-away lesson here is that micro climates provide us with opportunities that can and should be used to our advantage to get the most water savings from the plants in our gardens. And the more you garden with your local climate in mind, the less water your plants will need....and success will likely come more easily to you.

There is a popular food movement to buy local. So to get the most bang for your water buck and have a beautiful landscape, know your soil, garden within the limits of your climate, and grow local.

Happy Gardening...