The
past few years have been sobering for anyone concerned with our planet’s future. In
Southern California, the effects of four years drought can be seen in nearly every
garden and wild place. Record-breaking
heat and winds have also plagued us, compounding the effects of the drought. Climate change is happening right now – and
we’d best be planning for more to come.
In
2014, we gave a talk on Climate Change and the Southern California Garden. We
discussed the climate models, their predictions for Southern California and the
implications for local gardens. Two
conclusions are clear: 1) overall temperatures – and the number of high heat
days (> 95° F; 35° C) – will increase in S. California over the next
century; 2) the frequency of extreme precipitation years (both drought and greater than normal precipitation)
are also likely to increase. For more predictions see our 2014 talk: http://www.slideshare.net/cvadheim/climate-change-2014.
The
climate predictions have clear implications for garden design and planning. In fact, we’re surprised the media isn’t teeming
with articles on the subject. The good news?
We can take steps to mitigate effects
of climate change in our homes and gardens. The sobering news? We need to take action now rather than putting it off until next year or the next decade. This winter will likely bring needed rains to
our part of the country; and as we discussed last month (http://mother-natures-backyard.blogspot.com/2015/08/preparing-for-el-nino-winter-rains.html), wet years are the perfect time to
plant.
The
question is, which plants to choose? The
current drought has spawned much interest in drought-tolerant plants. While drought tolerance is important, it’s not the only characteristic needed to survive
the next century. Our S. California
garden plants must adapt to higher
temperatures, more extreme heat days
and greater precipitation extremes –
and that’s just a start. While only the
future will tell, we suspect that some drought-tolerant plants in local gardens
will not make it through the El Niño years or periods of prolonged high
temperatures. So what plants are likely to be long-term survivors?
The
past decade has provided an interesting natural experiment in gardens and wild
lands alike. Those who manage natural
areas have a front row seat to the action; our wild lands clearly demonstrate
which local plants can survive drought on rain alone - and which cannot. And if we have a strong El Niño winter this year,
we’ll see which plants can take not only the drought, but also the subsequent floods.
At
the Gardena Willows Wetland Preserve and gardens – and the Preserves and
gardens at California State University at Dominguez Hills – we hope and pray
for an El Niño winter. Clay soils are the proving grounds for a plant’s ability
to tolerate winter flooding. Since both
sites have clay, they will provide a good test of El Niño survivability for a
variety of California native plants. We look forward to the results –positive
and negative. Those plants that survive both the drought and
the floods will likely be long-term winners as our climate changes. And
that’s particularly important when choosing long-lived plants like trees and
large shrubs.
Our
gardens must provide more shade in the
future. Hardscape features like
patios, arbors and awnings are one way to create shade. But ‘living shade’ – that provided by trees,
shrubs and strategically placed vines – has additional benefits. Plants cool the surrounding areas by
evaporative cooling; that’s why the shade under trees feels cooler than the shade
under a patio roof. Plants also release
oxygen, clean the air of contaminants and provide habitat for numerous
creatures. If chosen carefully, shade
trees and shrubs provide colorful flowers, edible fruits and a green oasis in
summer.
Ideally,
shade-producing plants are long-lived – 50 or more years is ideal. That means that shade trees planted now must be tough enough to thrive in the
climate of 50 years and more in the future.
We’re betting strongly on California natives, including some trees and
shrubs that already grow in the Los Angeles Basin. Locally
native survivors are appropriate for our soils and provide key habitat for
local creatures. So we should use the local natives when appropriate.
But
our shade trees/shrubs may also need to come from further afield. Plants that thrive in other mediterranean
climates, including citrus trees like Blood oranges, lemons and tangerines, are
one good option. They provide tasty fruit
and require only occasional deep summer water. But we should also consider
trees and shrubs from California regions that currently experience heat,
drought and occasional floods.
The
California deserts, particularly the Mojave and Sonoran Deserts, fit that
description. While some desert plants
are small, a surprising number of large shrubs and trees grow in the desert foothills
and along desert arroyos and seasonal streams.
Many of these plants are already used
in home gardens throughout the Southwestern U.S. Perhaps we should also consider some for
local use?
There
are several potential drawbacks to using California desert trees/shrubs in the
Los Angeles Basin. One is our lack of experience with them,
particularly in gardens whose drainage is less than perfect. Desert soils, including those from desert
waterways, tend to be sandy, rocky and well-drained. Not all soils in western Los Angeles County
are blessed with excellent drainage. But is perfect drainage an absolute
requirement, particularly if we limit summer irrigation?
We
suspect that some desert shrubs will survive just fine in the soils of the Los
Angeles Basin. For example, we’ve seen
desert Paloverde trees thriving in local neighborhoods with clay-loam
soils. But we need more experience with
desert plants in local gardens.
Fortunately, native plant nurseries like the Theodore Payne Foundation,
Grow Native Nursery (Rancho Santa Ana Botanic Garden) and Tree of Life Nursery
are now offering some of these plants for us to try. We need to begin these trials now – the information
may be essential to future gardens.
Another
potentially serious drawback is the introduction
of new (non-local) species into wild areas.
This is a serious issue; it’s the reason why many native plant groups, including
the California Native Plant Society, advocate growing plants from locally
native seed sources. Those who live near
wild areas should be good neighbors
in their plant choices. But the issue of
plant introductions is particularly knotty when considered from the perspective
of future climate change.
Plant
distributions are already shifting due to changes in temperature and
precipitation. Some plants are ‘moving
up the mountains’ as the valleys get hotter; other natives are struggling with
the drought. Dramatic climate changes
require a serious re-thinking of our concepts of locally native species. How
does one define the ‘local native plant community’ when conditions no longer
support its continued survival? Should
we try to sustain local communities with supplemental water? These are questions that challenge the most
capable scientists and Preserve managers.
 |
| Toyon (Heteromeles arbutifolia) pruned as multi-trunk tree. |
So
what is a responsible gardener to do?
Plant local natives as a first choice, particularly if you live within a
mile of native vegetation. And choose species that have the least potential
for becoming invasive. We’ve
included several plants on the list below with some trepidation. These include the native mesquites, which
have become invasive in parts of Africa and Australia. They are wonderful, drought- and heat-tolerant
plants; and they don’t present a problem in areas with limited
precipitation. But we’ll need to watch them closely if we
plant them in the Los Angeles basin.
The
plants on the list below were chosen for their potential as shade trees or
large shrubs. However, the shade
provided varies from dense shade to light/dappled shade. In general, desert trees are more open; they
provide filtered shade. But remember that
desert plants tend to have denser foliage in gardens than in the wilds. Also note that some desert plants are armed
with thorns; that can be an issue until the plants achieve tree-size.
The
list below is based on empiric observations (of local species and some desert
species) in a range of soils from sandy to clay. These plants appear to do well in a range of
precipitation regimens; they also tolerate summer heat (the desert species
being the most heat-tolerant). In choosing plants, we assumed they might receive
occasional supplemental water in drought years. And some potential candidates were
excluded due to large size and/or roots that invade water or sewer pipes.
We
view this list as a first attempt; we will continue to modify it as our
experience with these and other native plants grows. Please feel free to send us your suggestions,
recommendations and experiences in growing native trees/shrubs in our changing
environment. We will add them
to our collaborative list.
California Native Trees & Large Shrubs for the Next
Century
Preliminary List –
September, 2015
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Trees
and Tree-like Shrubs
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Name
|
Origin
|
Learn more about this
plant
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Catclaw acacia
Acacia/Senegalia greggii
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Sonoran Desert foothills
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Bigberry manzanita
Arctostaphylos glauca
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Santa Monica and San Gabriel Mountain foothills
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Mountain mahogany
Cercocarpus betuloides/ Cercocarpus
montanus var. glaber
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San Gabriel and Desert mountains
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Little-leaf mountain mahogany
Cercocarpus montanus var. minutiflorus
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Foothills of Orange & San Diego Counties
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Desert willow
Chilopsis linearis ssp. arcuata
|
Mojave Desert
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Summer
holly
Comarostaphylis diversifolia
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Chaparral, Los Angeles County incl. Southern Channel Islands
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Desert olive
Forestiera pubescens
|
Mojave Desert mountains
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Tecate cypress
Hesperocyperus forbesii
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Desert mountains (San Diego county)
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Toyon
Heteromeles arbutifolia
|
Los Angeles Basin
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Island ironwoods
Lyonothamnus floribundus
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Southern Channel Islands
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Baja birdbush
Ornithostaphylos oppositifolia
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Desert chaparral, San Diego County & Baja California
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Paloverde
Parkinsonia
florida
|
Sonoran Desert
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Single-leaf pinyon pine
Pinus monophylla
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Desert mountains
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Honey mesquite
Prosopis glandulosa
|
Sonoran Desert
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Velvet mesquite
Prosopis velutina
|
Sonoran Desert
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Catalina Island cherry
Prunus ilicifolia ssp. lyonii
|
Southern Channel Islands
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Coast liveoak
Quercus agrifolia
|
Los Angeles Basin
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Island oak
Quercus tomentella
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Channel Islands
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Blue (Mexican) Elderberry
Sambucus nigra ssp. cerulea
|
Los Angeles Basin
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Jojoba
Simmondsia chinensis
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Desert
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Mission manzanita
Xylococcus bicolor
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Verdugo mountains, Southern Channel Islands
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Large
Shrubs (*some may be pruned as small trees)
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Coast quailbush
Atriplex lentiformis ssp. breweri
|
Los Angeles Basin; desert
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Mulefat*
Baccharis salicifolia
|
Los Angeles Basin
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Baja fairyduster
Calliandra californica
|
Baja California, Mexico
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Buckbrush
Ceanothus cuneatus
|
San Gabriel Mountains
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Desert lavender
Condea (Hyptis) emoryi
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S. Mojave & Sonoran Deserts
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California coffeeberry
Frangula californica
|
San Gabriel Mountains
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Sawtooth goldenbush
Hazardia squarrosa
|
Los Angeles Basin
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Wolfberry
Lycium andersonii
|
Mojave desert foothills
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Baja desert-thorn*
Lycium brevipes
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Channel Islands; Sonoran Desert
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California boxthorn
Lycium californicum
|
Los Angeles Basin
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Fremont’s barberry
Mahonia fremontii
|
Mojave Desert Mountains
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Nevin’s barberry
Mahonia nevinii
|
Inland Los Angeles Basin foothills
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Laurel sumac*
Malosma laurina
|
Los Angeles Basin
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Bladderpod*
Peritoma (Isomeris/ Cleome) arborea
|
Los Angeles Basin; desert
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Desert peach*
Prunus andersonii
|
Desert mountains
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Desert almond
Prunus fasciculata
|
Desert mountains
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Desert apricot*
Prunus fremontii
|
Desert mountains
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Hollyleaf cherry
Prunus ilicifolia ssp. ilicifolia
|
Santa Monica and San Gabriel Mountain foothills
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Scrub Oak*
Quercus berberidifolia
|
Los Angeles Basin
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Channel Islands scrub oak*
Quercus pacifica*
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Southern Channel Islands
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Hollyleaf redberry*
Rhamnus ilicifolia
|
Los Angeles Basin
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Lemonadeberry*
Rhus integrifolia
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Los Angeles Basin
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Sourberry
Rhus trilobata
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San Gabriel Mountains
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We
welcome your comments (below). You can
also send your questions and suggestions to: mothernaturesbackyard10@gmail.com