How Things Work: Plant Drought Tolerance ~ Tk-123456

Mother Nature's Backyard - a drought-tolerant S. California garden (Fall)

Last
month we discussed shade plants suitable for the next 50-75 years (http://mother-natures-backyard.blogspot.com/2015/09/sustainable-gardening-trees-for.html).
Climate change is forcing us to seriously re-think our plant choices. The
climate models suggest that long-lived garden plants in our area will need to
tolerate more heat, more drought and years
with greater than average precipitation (El Niño conditions). That’s asking a lot of a plant!

We
thought you might enjoy learning why some plants are more heat-, drought- and
flood-resistant than others. We’ll try to keep the discussions straight-forward
and relevant to home gardeners. But we will discuss aspects of plant anatomy,
physiology and ecology that are relevant to the topics. If you don’t care about
the details, skip to the end of the articles, where we summarize the bottom
line for gardeners.

What is drought
tolerance?

After
four years of serious drought and water restrictions, most California gardeners
are concentrating on drought tolerant solutions. But what makes one plant drought tolerant and
another water-loving? The answer is
more complex than you might imagine.

Drought
tolerance is broadly defined as the ability of a plant to withstand periods
with no or little water from precipitation or irrigation. While this definition is helpful, not all drought tolerant plants are equally
drought tolerant. In fact, drought tolerance can range from modest to
extreme (more on this later). So you can’t
assume that all drought tolerant plants need the same (small) amount of water.

Coastal Sage Scrub plants are well-adapted to the

wet winters and dry summers of S. California

The
timing of drought is equally important
as the severity, particularly for California native plants. Some drought tolerant plants need a little
water year-round, some need occasional summer water, while others require a
period of absolute drought in summer and fall. The details of a plant’s drought
tolerance depend to a great extent on the
type of drought conditions the plant normally experiences in the wild. The more our gardens mimic a plant’s native
conditions, the better that plant will do.

In
Southern California, drought tolerant plants evolved under several types of
conditions: 1) areas with very low yearly
precipitation, occurring almost entirely in winter (Mojave desert and other
places with less than 10 inches (25 cm.) of winter/spring precipitation a year);
2) areas with very low yearly
precipitation, but with some summer precipitation from ‘summer monsoons’
(Sonoran desert; parts of Baja California; dry mountain foothills of San Diego
county and, to a lesser extent, Orange & Los Angeles counties); 3) areas with
adequate seasonal (winter/spring) rainfall,
followed by a long (6 months or more)
yearly dry spell (mediterranean
climate areas like the Los Angeles Basin).

The Mojave Desert has low precipitation that occurs in winter/spring

So,
why worry about the differences between these three types of drought regimens? Do they have any relevance for the California
gardener? The answer is yes. The mechanisms plants use to survive very low
precipitation vs. seasonal rainfall can be very different. These differences are key to plant survival,
particularly in places with year-to-year rainfall variability. And these differences will likely become increasingly
important if precipitation becomes more erratic. We’ll discuss this further at
the end of the article.

Strategies of drought
tolerant plants

But
first, how do drought tolerant plants differ from their thirsty cousins? Plants employ several basic strategies to increase their drought
tolerance. These strategies are: 1) to avoid drought; 2) to postpone dehydration and; 3) to withstand severe dehydration (desiccation) and
rehydration. Within each strategy, plants
employ a range of mechanisms to make themselves more drought tolerant.

Annual wildflowers like Purple Clarkia avoid the drought

Some
plants simply avoid drought by limiting
their lifespan to periods when the soils are moist. Winter/spring annual grasses and wildflowers
are good examples. Plants from both low-
and seasonal-precipitation climates have adopted this avoidance strategy; that’s
why annual wildflowers grow in both types of desert (in ‘good rain’ years) and in
the western regions of Southern California.
It also explains why desert and coastal wildflowers can each be grown in
the other climate: all that’s required is enough moisture at the right time.

Mature
seeds are very dry, with a 90% to 95%
loss of moisture in many cases (you’d be a mummy at this desiccation level). The seed embryo enters a state of suspended
animation and remains so until moisture becomes available. Some embryos can remain viable for 100’s of
years; others (often the smallest seeds) need to germinate quickly or they will
die. But most seeds can survive at
least a few years of drought – so this is a pretty good survival strategy in
dry places with variable yearly precipitation.

Other
plants produce underground propagules (in this case, embryonic plants) inside bulblets
(new bulbs) and cormels (new corms). The
parent plant may die during the dry season; but the small, more drought
tolerant offspring are likely to survive.

Wild hyacinth (Dichelostema species) produce cormlets

to survive drought

As
veteran California gardeners know, bulbs, corms & tubers are a bit more
vulnerable to prolonged drought than are seeds. Some tissue water is generally
required to maintain viability, so a prolonged drought (like the present one) can
kill some native bulbs and bulb-like perennials. This also explains why native bulbs are more
common in wetter western S. California than in the deserts; desert soils are
often too dry, for too long.

A
second class of strategies involve desiccation avoidance; mechanisms that
keep key plant tissues hydrated for as
long as possible. The most important tissues, of course, are those that
produce new, post-drought growth (dormant buds); but roots and other important
organs are also selectively spared. Most drought tolerant plants – and particularly
those from Mediterranean climates - use this strategy.

There
are a large number of ways to postpone dehydration and Southern California plants
often employ several. That’s one reason
why local native plants may be better long-term choices than non-native plants. They simply avoid desiccation better than their non-native counterparts. In short, they are prepared for our erratic
and often unpredictable climate.

Some
plants delay desiccation by storing extra
water during the rainy season. Many local plants do this to some degree; but
some can stockpile a good deal of water. A classic example is the cactus, which stores
water in its fleshy stems, then uses it as the plant dries down in summer. Other plants have fleshy leaves (succulents)
or roots modified to store water.

Perennial
plants that die back to the ground in summer are examples of this latter
strategy.   They remain alive – albeit at
a low-functioning level – due to water stored in bulbs (example: native
onions), tubers (example: the Manroot or Marah
macrocarpus) or other fleshy roots and rhizomes (examples: Yerba mansa;
Douglas iris). The underground storage
structures can be impressive in size.   Tubers
of mature Manroot plants can be several feet long and weigh up to 500 lbs. (225
k.); they can sustain plants through years of drought.

Other
plants, particularly the herbaceous perennials of the S. California Coastal
Dunes/Bluffs, Coastal Prairies and Coastal Sage Scrub, have smaller storage
roots. But most store enough water to
get a plant through the normal yearly dry season. In gardens of the future, winter rains may
not be sufficient. Supplemental winter/spring irrigation will likely be needed
to allow these plants to store water during dry winters.

Since
plants lose water primarily through their leaves, many drought tolerant plants
have modifications that decrease leaf water
loss, either directly or by decreasing the temperature of leaves. Waxy surfaces, dense hairs, light coloration,
thickened ‘skin’ (epidermal) cell walls and fewer stomata (the pores through
which most water is lost) all help decrease both leaf temperature and water
loss. Many local plants exhibit these
modifications; they are part of what make our native plants so unique and attractive.

Leaves of Coast liveoak (Quercus agrifolia) are adapted

to long dry periods.

Many
evergreen chaparral shrubs and the local live oaks have sclerophyllous leaves. These
leaves are relative small, tough, thick and leathery – picture a liveoak leaf
and you’ll get the picture. Such leaves
are well adapted to mediterranean climates.
Not only do they conserve water, they also discourage herbivory (being eaten), a distinct advantage in harsh
climates, where growing new leaves comes at a price.

Other
leaf modifications can also be seen in locally native plants. Some plants, like the Salvias (Sages), produce two
sets of leaves: a set of larger, fleshy leaves for winter/spring growth and
a set of smaller, drier leaves to get the plant through summer/fall. Other locally native shrubs simply lose their leaves in summer/fall. They decrease their life processes to very low
levels by maintaining low – but survivable – hydration levels in the buds that
will form new leaves. A well-known
example is California encelia (Encelia
californica), which normally loses all its leaves in summer (unless it gets
a little summer water).

Spring (left) and summer/fall leaves: Munz' sage

But
there are even more subtle ways of delaying desiccation. Carbon dioxide supplies the carbon needed for
photosynthesis, a process that occurs, at least in part, during the day (sunlight
provides a direct source of energy) [1, 2].
Since carbon dioxide gas enters
the leaves through the stomata (leaf pores), most plants keep their stomata
open during the day. But having carbon dioxide enter - and water vapor exit -
through the same pores creates a problem in hot, dry climates. What’s a plant to do? Open the stomata (and lose water) or close
the stomata (and decrease the photosynthesis need for growth and other plant
functions)?

Some
plants decrease water loss by keeping
their stomata closed during the hot part of the day – or even all day. How is this possible? Some plants only partially close their stomata
during the driest times – a compromise that usually leads to decreased growth
during hot, dry periods.

California native grass - Alkali sacaton (Sporobolus airoides)

Other
plants, particularly the water-wise grasses, have a complex anatomy and
physiology that allows them to keep their stomata partially closed, but still
photosynthesize efficiently and grow normally.
Not surprisingly, such plants are more common in hot climates. To learn
more about these plants, known as ‘C4 plants’, we recommend references 2-4, at
the end of this article.

Dudleya virens - succulent native to L.A. Basin

Another
group of plants, known as the Crassulacean
Acid Metabolism or CAM plants,
store carbon at night and use it for photosynthesis during the day [6, 7]. This allows them to open their stomata at
night and close them during the day. Not
surprisingly, CAM plants evolved in dry climates, where closing stomata during
the day gives them a distinct advantage in terms of water loss. CAM plants
include cacti, succulents, bromeliads, some sedums & euphorbias, agaves
& yuccas, aloes and other plants we think of as ‘desert plants’. Since CAM photosynthesis is less efficient,
some CAM plants switch to CAM only when needed – in hot, dry weather.

Plants
that survive really arid conditions need a variety of ways to conserve
water. Most CAM plants exhibit other desiccation
avoidance mechanisms such as fleshy water storage organs, rudimentary leaves,
waxy cuticles, light color, etc. But
what makes these plants very drought tolerant can also make them vulnerable to
over-watering, particularly during the dry season. We’ll
discuss this problem in more detail next month (November, 2015).

A
third way plants delay desiccation is to develop
roots that make efficient use of available water. Nearly all California native plants take up
water entirely through their roots. So it’s
not surprising that S. California natives have several types of root adaptations,
allowing them to make best use of our variable rainfall.

Some
plants have net-like roots in the top several feet of soil, allowing them to effectively
use scant rainfall. Even the limited amounts of water produced by fog drip can
be taken up efficiently by these small, shallow roots. Most plants native to the L.A. Basin have at
least some shallow roots. Other plants
have long roots that can access water deep in the ground. Even drought tolerant perennials, like the California
poppy (Eschscholzia californica),
have a long tap root.

Many
of our native perennial grasses have roots that are three to six ft. (1-2
meters) long, many times longer than the 6 inch roots of grasses from rainy
climates. It’s no wonder our native
grasses are more drought tolerant; they continue to access water deep in the
ground, while non-native lawn grasses dry to a crisp. It also explains why we water native grasses
differently than the non-native sod grasses.
The short roots of most non-native lawn grasses require shallow, regular
water; the natives survive on occasional, deep irrigation.

The
native shrubs and sub-shrubs (for which our region is famous) are even more
impressive in terms of their root architecture. Most have a combination of net-like, shallow
roots and deep roots, allowing them to take advantage of both shallow and deep
water reserves. Typical Coastal Sage
Scrub shrubs have roots that are three to six feet long. Chaparral shrubs, and the native trees, are
even more impressive. Their roots can
reach water at depths of 20 to 60 or more feet below the surface. Is it any wonder these plants remain green,
even in summer?

Why is water important
for life?

Nearly
all S. California native plants survive drought by either avoiding or delaying
dehydration. These are good strategies
in our Mediterranean climate. They allow
plants to function in dry times – albeit at very
low levels – by remaining partially hydrated.
Which brings us to the underlying question: why is water needed for
survival?

First,
many of the chemical reactions of life require water (actually, it’s the chemicals
dissolved in water; but the water is essential). Second, and equally important, water is critical for maintaining the
architecture of tissues and cells. A
wilted perennial clearly demonstrates
this role at the whole plant level.

But
adequate hydration is even more important at the cellular level. It’s needed to keep a cell’s membranes
intact. It also insures that cell
components remain in their proper positions.
Without water, fluids leak into places they shouldn’t be, cell components
stick together, and chemical reactions don’t occur normally. In short, serious dehydration is usually incompatible
with life on earth.

Can plants survive being
really dry?

So
how do some plants survive being really dry?
Some local native plants produce special chemicals that allow cells and
tissues to retain their proper architecture and function relatively normally
even when fairly dry. These chemicals
take the place of water – up to a point.
And there’s the rub. In fact, very few adult plants, worldwide, are
able to withstand severe dehydration and
rehydration. Which is curious, since
all seed-producing plants possess this ability early in life (remember, seed
embryos are essentially living ‘plant mummies’).

Drying
out to very dry levels, then quickly rehydrating when moisture is available, is
extremely destructive for most adult plant cells. We’ve all seen the effects of severe
dehydration on garden and house plants.
There’s no way a dried out coleus is going to spring to life again, even
if you water it! Trust me on that!!

But
a few plants have special mechanisms that allow them to do so, and many are
called ‘resurrection plants’. They are
mostly small and simple – the ferns and mosses figure prominently – but they
truly can rehydrate and function after severe dehydration. The key to this miracle is the production of
even more specialized chemicals. The molecules of these chemicals literally replace
the water molecules, allowing the cells to retain their proper architecture
while entering a state of suspended animation. Upon rehydration, the water replaces the
‘compatible solute’ molecules and the cell returns to relatively normal
functioning. This is truly one of Mother Nature’s miracles!

A
similar process occurs when seeds dry out during seed development and rehydrate
at germination. So the genes needed to produce the compatible solutes and other
key chemicals are found in virtually all higher plants. Wouldn’t it be nice if ordinary plants could
turn these genes on in times of drought?
Rest assured that plant scientists are working on this and other ways to
increase plant drought tolerance. But
for now, our knowledge of drought tolerant plants can help us garden in ways
that are both water-saving and life-friendly.

In
summary, S. California native plants survive dry conditions by either avoiding
drought (annuals) or by preserving adequate moisture in their essential
tissues. The most common desiccation
avoidance mechanisms include: 1) storing water in specialized storage
organs for later use; 2) decreasing leaf water loss though a variety of leaf
modifications and; 3) accessing ground water with a combination of shallow and
deep roots.

Each
drought-tolerant plant is well-adapted to survive
its normal drought conditions, whether they include a bit of summer water
or not. Understanding the mechanisms –
and the drought regimen a plant normally experiences – can help us provide the
best conditions for our drought tolerant plants. In addition, they can guide us in our planting
and watering practices. Below are a few
ideas.

Native plant garden in summer: Garden of Dreams, CSU Dominguez Hills

Bottom line: drought
tolerant plants in the home garden

Not
all drought tolerant plants are the same. There’s lots of evidence to support
this – much of it relevant to California native and other mediterranean climate
plants. A few basic principles are important when choosing, placing and
maintaining drought tolerant plants.
These principles - which come straight from Mother Nature, herself – are
discussed below.

1.
Some
plants are more drought tolerant than others.
For the most part, desert plants are more drought-tolerant than those
from the Los Angeles Basin. But even
locally native plants vary widely in their drought tolerance. This means that you should understand a plant’s water needs before you purchase and plant
it. You also need to group plants according to their water needs. This is called Water Zone (Hydrozone)
gardening. For more on this topic see:
http://mother-natures-backyard.blogspot.com/2012/04/water-wise-gardening-tip-save-water.html.

Native plant gardens and nurseries are
good sources of information on plant water needs. So are the many new books and
internet resources on California native plants. We also provide water needs information for
all the plants we feature: http://mother-natures-backyard.blogspot.com/p/learn-more-about-native-plants.html.

If you don’t want to research the
water needs for a number of plants, consider choosing plants that naturally
grow together. These plants will have
similar soil, water and other needs. You’ll
only need to understand their common needs in order to water them
correctly.

2.
While
desert plants are drought tolerant, they aren’t for every garden. Before
you fill your garden with cacti and other desert species, consider the
down-side of desert plants. Remember,
desert plants are adapted to very low precipitation levels (often half the
average rainfall for the L.A. Basin). An
El Niño year in can produce four or more
times the amount of rainfall usually falling in the local deserts. Some desert plants cannot survive this much water.

In addition, desert soils are usually
very well-drained – sandy or rocky. If
you’ve done a soil percolation (perc) test (http://mother-natures-backyard.blogspot.com/2012/10/getting-to-know-your-gardens-soil.html)
you know whether your soil drains quickly, slowly or somewhere in between. If
your soil is sandy and well-drained, great.
You can plant desert natives and water appropriately for the desert from
which they originated (see #3).

But how will desert plants stand up to
El Niño years (or extra irrigation) in non-sandy soils? Only
time and experience will tell. We’ll
discuss flooding tolerance more next month; but we bet that some desert natives
will not make it through an El Niño winter.
This winter may provide an important test of water tolerance in desert
plants grown in the L.A. Basin.

Gardeners with slow-draining soils
should think twice about planting true desert plants. There are many plants native to the LA Basin
that are quite drought tolerant and can also survive El Niño conditions. Look for locally native plants with wide
irrigation tolerances. We’ve provided a
list at: http://www.slideshare.net/cvadheim/california-native-plants-with-wide-water-tolerance-ranges

3.
Local
(L.A. Basin) native plants are adapted to a Mediterranean climate – wet
winter/spring and dry summer/fall. Plants from the
Mojave Desert have a similar precipitation cycle. But plants from the Sonoran Desert, the
deserts of Baja California and the dry foothills of San Diego County are
adapted to both winter and summer
(‘summer monsoon’) precipitation. They need some summer water; and in our area,
you’ll need to supply it.

The same is true of the ‘water-wise’ natives
from Central and Northern California, which need even more water. Remember, a ‘drought-tolerant
plant’ from Northern California is not the same as a drought-tolerant plant
from Southern California. Get to know
the natural precipitation patterns for the plants you grow. The more you mimic them in your garden, the
happier a plant will be.

It is
possible to include plants from ‘summer dry’ and ‘summer monsoon’ regions
in the same garden. But it requires thoughtful
planning and grouping plants with similar needs together – then watering each
Water Zone appropriately. For more on
planning a garden based on Water Zones see: http://mother-natures-backyard.blogspot.com/2014/02/designing-your-new-california-garden-9.html.

The most successful water-wise gardens
are designed around the water needs of the plants they contain. They often
include some plants that need regular water, as well as those that are water
thrifty. And they do so while still
being water-efficient overall.

So, you can use plants that need
regular water and those that are drought-tolerant in the same garden – but you
need to plan and group plants. That’s
the bottom line!

4.
Not
all ‘water-wise succulents’ have the same water needs. Many California gardeners are attracted
to succulents; they are pretty and easy-care.
But not all succulents have the same water needs. The ‘succulents’ native to the LA Basin and
Santa Monica Mountains – the local Dudleya
species – are adapted to our Mediterranean climate. They grow (and need adequate water) in
winter/spring. And they must be dry in
summer. To water them in summer is
to doom them to an early death. This is
a good example of why it’s important to know where a plant grows
naturally – and what type of drought it needs.

On the other hand, succulents from the
higher mountain ranges of S. California (and from Northern California) need
occasional summer water to get them through the dry season. Native Sedum species like Sedum laxum (Northern California), Sedum niveum (San Bernardino Mountains),
Sedum spathulifolium (San Gabriel
& San Bernardino Mountains; Northern/Central California mountains) and Sedum stenopetalum (Sierras; Northern
California) are good examples. They
require a different drought regimen, and shouldn’t be inter-planted with the
Dudleyas.

The majority of succulents available
in local nurseries are not California natives at all. The ever-popular Echeverias, Crassulas, Aloes, Kalanchoes, Sempervivums and others are native to semi-dry regions that
get summer rains. They are adapted to
grow in the summer – the exact opposite pattern from our LA Basin natives. Contrary to popular myth, most non-native
succulents really do need some summer water to keep them alive and attractive.
And they will rot if they get too much winter water. Don’t
plant them close to Dudleyas or other summer-dry plants unless you truly enjoy
replacing plants!

5.
Southern
California native plants have unique roots that help them survive our variable
climate.   From roots that store water, to shallow
roots that utilize small precipitation events, to roots that mine water
resources deep in the ground - our local natives are uniquely suited to our
climate.

But as the climate changes, we need to
remember that local natives need adequate winter/spring rain (10-15 inches
per year) to survive the normal summer/fall dry season. The bottom line: water-wise natives must have
adequate winter soil moisture so they can store up water for the dry
season. If Mother Nature doesn’t supply,
then we must provide - if we want our gardens to survive and thrive.

And we must water correctly. Many California natives have deep roots requiring
‘deep irrigation’. This doesn’t mean 7
minute irrigation sessions, twice a week.
It means occasional water, provided more slowly (so it doesn’t run off)
and for longer times (hours rather than minutes). Can we save water by using such irrigation
practices? Certainly!!! Many of us have done so for years.

6.
It
takes time for most plants to develop their drought-tolerance. Since root adaptations play a key role in
drought tolerance, most S. California native plants must develop adequate root systems before they become truly drought
tolerant. Native perennials and grasses require a full year or two. Smaller shrubs need 2-3 years and large
shrubs/trees may take 4-5 years to become completely drought tolerant.

Any drought tolerant plant needs time
to develop roots and other desiccation avoidance mechanisms. Don’t presume that a drought tolerant plant
needs no water from the minute it’s planted.
Plants are like children – they need time to develop their full
potential. We’ll talk more about planting and watering
young plants in November and December (2015).

7.
Plants
with deep roots don’t like being moved. Many
drought-tolerant California natives have very deep roots. Even the perennial grasses can have roots
that are 4-6 feet long. Moving mature
plants can damage long roots, leading to plant death. So, think carefully before selecting a place
for deep rooted plants in your garden. They
may be living there a long time!

8.
Plant
roots develop best when soils are uniformly moist. That’s just one reason why drought-tolerant
plants are best planted during the rainy season. Cool
temperatures and moist soils promote strong, healthy root systems. By the summer drought season, winter-planted
plants are able to cope (with a little summer irrigation for the first summer
or two). So plant with the rains if at
all possible and your plants will get off to a good start.

9.
Water-wise
plants can be used in fire-prone areas, but with planning. Some water-wise plants are adapted to
burn; others become quite dry in summer and fall. If you live in a fire-prone area you don’t
want to plant these right next to your house.
But others, including some evergreen shrubs and plants with succulent
leaves are less flammable (they have actually been tested). For more on defensible landscaping see: http://www.readyforwildfire.org/landscaping

10.
Local
native plants have a variety of mechanisms to survive drought.
That’s one reason why our local plants are so hardy – they are used to
surviving our variable climate. When
choosing water-wise plants, consider
California native species in addition to those from Australia, South
Africa, Chile and the Mediterranean region.

You may have to search them out –
native plant nurseries are not as abundant as conventional nurseries.   But the search will pay off in the long
run. Remember, these plants have had
thousands of years to tailor themselves to our fickle climate. In addition to being drought tolerant, they
provide food, shelter, scents and beauty to the home garden.   They are our California legacy – something to
cherish and enjoy throughout the year.
California native plants ground us and give us a sense of place.   They are what California living is all
about!