During
the hot, dry months from July through September we really appreciate the
effects of our mediterranean climate.
Watering is foremost in every gardener’s mind and using water wisely is
becoming a necessity. Garden mulches can
play an important role in water conservation and much, much more! So grab a cup of coffee (or tea), sit back
and consider the many facets of garden mulches.
One
key element of water-wise gardening is insuring that water doesn’t evaporate
away before your plants can use it. Two
good ways to do this are: 1) to use water-efficient irrigation methods (more on
this in September) and 2) to use garden mulches. Mulches are particularly important in a
young garden like Mother Nature’s Backyard. Evaporation is a key issue in all Southern
California gardens, but young gardens are particularly vulnerable until plants
become established.
Introduction
Water
conservation is high on the list of reasons why gardeners use mulches. Mulch
serves as an insulating layer, helping maintain even soil moisture throughout
the garden and throughout the year. A good layer of mulch significantly prevents
water loss due to evaporation. Research suggests that evaporation rates
are decreased 50-70% when soils are covered by a layer of mulch, regardless of
whether the soil is sandy, clay or something in between. Mulches decrease soil water evaporation in
several ways: 1) by decreasing the soil temperature; 2) by protecting the soil
surface from the drying effects of wind; 3) by creating a layer of moist air
between the soil surface and the mulch. All
of these are important determinants of water evaporation rates.
Mulches
also prevent soil erosion due to wind and water runoff. They help to both increase water infiltration and
decrease water runoff. Mulches decrease
the speed and force of water droplets, allowing them to soak into the soil
rather than run off. They prevent the
formation of the soil surface crusts that can result when water droplets hit
and compact bare soil; this also helps to prevent water run-off. And mulches prevent
mud from splashing onto plants – which can be ugly and spread disease and
fungal spores. Mulch is important year-round
because the infiltration effects are the same for irrigation or rain water.
Mulches
are also used to suppress weed growth.
Many weed seeds need light to germinate; block the light with a layer of
mulch and they won’t germinate. Other
weed seedlings have a difficult time pushing up through a layer of mulch. Mulches even help when weed seeds blow in from
outside sources. It’s often easier to
remove weed seedlings from mulched areas – the roots are less well established
and weeds are noticeable while they’re still small. Preventing weeds from growing insures that
more water – and more soil nutrients – is available for your garden plants.
Mulches
can be used to change the temperature of a garden’s soil. We usually think of the cooling effects of
organic mulches like bark chips. A layer
of bark – or other organic mulch – does protect shallow roots from excessive
heat (or cold). But the soil warming
effects of a gravel mulch can be highly beneficial for winter/spring
wildflowers, native bulbs, desert plants and cool-season ornamental grasses. Similarly, you might use gravel or brick
pathways in a vegetable garden to warm the soil in spring and retain soil heat
in cooler coastal gardens. You might
even be able to grow watermelons – in some years – in foggy South Redondo!
Some
mulches actually change the composition of soil over time. For example, bark, straw and other organic mulches
will decompose, releasing their nutrients into the soil. The decomposition of organic mulch provides
food for beneficial soil organisms while adding organic matter (humus) to the
soil. Soil humus increases the
availability of certain nutrients (particularly important in sandy soils). But
it also increases air spaces in clay soils and enhances a sandy soil’s ability
to hold water. We’ll discuss more about
garden soils next month (August, 2012).
Another
common use of mulches is to provide access to the garden. Mulch materials can be used to create permeable
pathways throughout the garden. Mulched
walkways decrease soil compaction and help limit foot traffic. This is particularly important for gardens
with clay soils. On a practical note, mulches
will also keep the mud off your shoes!
Mulches
can be used to give your garden a tidy, finished appearance. This is especially important in a young
garden, where the open spaces between plants can appear enormous compared to
the new plants. By mulching between
plants you not only conserve water, you also indicate that your new garden is a
‘work in progress’. Renewing mulches
yearly will continue to provide a professional, finished look to your garden
over time.
Some
mulches are used simply because they are decorative. For example, you might use an attractive
pebble mulch around a fountain or sculpture to set it off from the rest of the
garden. A mulch of ‘glass pebbles’ or recycled
landscaping glass can be used to add color to pots, walkways, infiltration
‘streams’ and meditation gardens. The
landscaping industry continues to develop interesting and decorative materials
to make your garden more beautiful and functional. Some of these are even made from recycled
materials like glass, old tires and recycled plastic!
Finally,
some mulches are used because they are simply more functional than plants or
impervious hardscape in certain situations. For example, gravel or crushed rock may
provide the perfect ground cover for a utility area. These and other inorganic materials can be
used to create a low-maintenance, zero irrigation patio or seating area that
allows rain water to percolate into the ground.
In
summary, mulches have a number of potential benefits and uses in the home
garden. Understanding which type of
mulch is best for a given application is the key to reaping these benefits in
your own garden.
Types
of Mulch
There
are two main types of mulch: organic and inorganic. Organic mulches are made directly
from plant materials including wood, bark, leaves, nut hulls, etc. Inorganic mulches come from
non-living sources or from recycled products like rubber or paper/cardboard. There are a number of commonly used mulches
in both the organic and inorganic categories (see the table below).
There
is no one ‘universal mulch’ that’s superior for all situations. The trick to success with mulches –
particularly when you’re using them around plants - is matching the mulch to
the plants. A good basic rule is to mimic
the type and depth of mulch that occurs naturally around a plant. The table below gives some suggested ‘best
uses’ for different types of mulch. This
table is a good place to start. But you will
be a more successful gardener if you go beyond the table.
When
mulching around a specific tree or shrub, go on-line or consult a good
gardening book to learn about the natural setting in which it grows. For
example, a Magnolia (which grows on the edges of swamps in Southeastern U.S.)
will have a very different natural mulch than a Jacaranda tree (native to drier
open woodlands of Argentina). The more you know about the natural conditions
under which a plant grows, the better you can mimic these conditions in your
garden. And the best way to keep your trees/shrubs healthy is to follow nature’s cues –
there simply is no better gardener than Mother Nature herself!
Class
|
Type
|
Best Uses (mulch/other) / Characteristics
|
None
|
No mulch/ bare ground
|
Best uses: for annual wildflowers; local
native grasses; native Salvias, Penstemons & Buckwheats
Benefits: no cost; the only healthy choice
for the native plants listed above; allows CA natives to re-seed; fire
retardant
Drawbacks: has none of the
benefits of mulch
|
Organic
|
Chipped bark
|
Best uses: around many native trees &
shrubs; pine & fir trees; many other trees, shrubs; informal paths
Benefits: long-lasting; attractive; readily
available (bag or bulk); moderate cost
Drawbacks: moderate cost; the finer the grade,
the quicker it breaks down; may
encourage root fungal disease in some S. CA native shrubs; flammable
|
Shredded Bark (‘gorilla hair’
mulch)
|
Best uses: around many native and non-native
trees & shrubs; best choice for slopes, hillsides
Benefits: attractive; stays in place on
slopes
Drawbacks: decomposes quickly; not as
available as chipped bark; may encourage root fungi; flammable
|
|
Wood chips
|
Best uses: around many native and water-wise
trees & shrubs; under most non-native trees/shrubs; informal pathways
Benefits: inexpensive (may be free from
arborists/ tree trimmers); long-lasting
Drawbacks: may have weed seeds - be sure to get fresh from an arborist,
then let it compost several months;
non-uniform appearance; may attract termites; flammable
|
|
Dyed/colored wood chips
|
Best uses: non-planted areas (walks; sitting
areas)
Benefits: range of colors; made from recycled
wood
Drawbacks: dye materials may be toxic;
decomposes quickly, particularly when watered; very flammable
|
|
Nut hulls
|
Best uses: ‘woodland’ type plantings; around
strawberries, fruit trees; as decorative mulch
Benefits: attractive; natural-looking
Drawbacks: cost; may be toxic to pets; light
materials may blow or wash away; may be flammable
|
|
Pine needles
|
Best uses: under forest-type trees/plants
(pines, etc); on slopes; around ‘acid loving’ plants
Benefits: may be cheap, readily available;
increases soil acidity (beneficial to some plants)
Drawbacks: may not be available; slippery to
walk on; very flammable until decomposed
|
|
Straw
|
Best uses: vegetable garden
Benefits: cheap; lots of air spaces/good for
temp. control; decomposes well
Drawbacks: may have weed seeds – get
certified weed-free if available (water & pre-sprout if not); slippery to
walk on; may blow around when dry; very flammable until it decomposes/compacts
|
|
Leaves (just left in place or
shredded)
|
Best uses: under native trees/shrubs (oaks,
pines, etc); under local native shrubs (Salvias; Buckwheats); potentially
under disease-free non-native trees/shrubs
Benefits: cheap; readily available; don’t
need to rake or move leaves; recycles nutrients naturally; leaves may confer
protection against soil fungi & other pests
Drawbacks: leaves may contain pesticides;
leaves may contain leaf fungal spores (roses; berry vines) or other disease/pests
- check carefully; flammable until decomposed (will decompose quickly in
winter)
|
|
Grass clippings (dried)
|
Best uses: home composting; vegetable garden
Benefits: cheap; available; decomposes
quickly
Drawbacks: often has weed seeds; may have pesticides/herbicides;
flammable; may compact to a dense mat and prevent water infiltration when dry
|
|
Home composted ‘green waste’
(composted garden and kitchen
vegetable waste)
|
Best uses: as soil amendment in vegetable garden; as mulch for vegetable garden
& conventional (non-native) flower beds
Benefits: cheap; readily available; recycles
green waste locally; homeowner can control content (keep out weed seeds and
other undesirable materials)
Drawbacks: too nutrient-rich for most native
plants; takes time and a little space to compost; most compost slightly
alkali (pH ~ 8.0)
|
|
Municipal composted ‘green waste’
|
Best uses: use with care – see below; vegetable
garden & conventional (non-native) flower beds
Benefits: inexpensive (may be free)
Drawbacks:
often has weed seeds, rhizomes and other pest/disease problems – USE WITH CARE; must transport from
source to garden; too rich for many CA native plants
|
|
Aged/composted manure
|
Best uses: best used as a soil amendment in compost or in
vegetable gardens; as a top dressing (fertilizer) for roses, conventional sod
lawns
Benefits: cheap; readily available (bag or
bulk)
Drawbacks: must be composted; increases soil
salts; nutrient content too high for most native plants
|
|
Commercial ‘topper’ or ‘top
dressing’ products
|
Best uses: as top-dressing for conventional
(non-native) flower beds; roses; conventional sod lawns
Benefits: readily available (bag or bulk);
less flammable than other organic mulches
Drawbacks:
cost; nutrient content too high for most native plants; decomposes
quickly
|
|
Inorganic
|
Fine-grade crushed rock/Crusher
fines (small size crushed particles);
Decomposed granite
|
Best uses: Annual wildflowers; bulbs; desert
plants; native Salvias, Penstemons, Buckwheats; pots & planters; medium/high
use walkways & seating areas
Benefits: surface dries quickly; doesn’t
decompose; interesting colors, textures; natural-looking; fire-resistant;
moderate cost; easy to walk on; increases soil temperature
Drawbacks:
particles migrate (particularly on slopes); particles tracked indoors;
heavy to work with; increases soil
temperature
|
Crushed rock / lava rock (larger
size)
|
Best uses: decorative uses; in rain gardens
& infiltration swales; pots/planter mulch; over bulbs
Benefits: interesting colors, textures;
natural-looking; fire-resistant; moderate cost; doesn’t break down;
appropriate for desert-themed gardens; increases soil temperature
Drawbacks: heavy to work with; difficult to
walk on; may look too coarse or too ‘desert-like’ ; increases soil
temperature
|
|
Pea Gravel
|
Best uses: non-planted areas other than
walkways; utility areas; formal gardens (Japanese; European)
Benefits: natural-looking; fire-resistant; moderate
cost; doesn’t break down; increases soil temperature
Drawbacks: heavy to work with; difficult to
walk on (compared to fine-grade crushed rock); usually limited colors (gray
or tan); particles can migrate; increases soil temperature
|
|
Pebbles (rounded rocks)
|
Best uses: decorative uses; around ponds; in
rain gardens & infiltration swales; pots/planter mulch
Benefits: interesting colors, textures;
natural-looking; fire-resistant; moderate cost; doesn’t break down; may
increase soil temperature
Drawbacks: heavy to work with; difficult to
walk on; may increase soil temperature
|
|
Shredded/Ground rubber; ‘Crumb
Rubber’
|
Best uses: play areas; walkways; utility
areas
Benefits: heavy duty; breaks down slowly;
Drawbacks: increases soil temperature; ? toxic
breakdown compounds; high cost; flammable
|
|
Glass & other decorative
inorganic mulches
|
Best uses: decorative uses; around ponds/fountains;
in rain gardens & infiltration swales; pots/planters
Benefits: moderate cost (some); attractive;
range of colors; inflammable; don’t break down; ; may increase soil
temperature
Drawbacks: some are expensive; availability;
may heat soil; children & birds may find them irresistible – move them
around, throw, etc.; may increase soil temperature
|
|
Landscape fabric/ Weed-block
|
Best uses: under pathways, seating areas
Benefits: warms soil; blocks weeds ; fairly air/water
permeable; relatively cheap
Drawbacks: unattractive – need to cover with
another mulch; breaks down/blows around; difficult to weed; may overheat roots; ?? effects on soil
organisms; weeds that grow from seed on top of it are hard to remove; need to
remove it & cover mulch to replace it
|
|
Plastic sheeting (black or clear)
|
Best uses:
as temporary mulch (when clearing weeds/grass or solarizing soil)
Benefits: warms soil; blocks/kills weeds ; cheap
Drawbacks: unattractive; breaks down/blows
around; prevents normal air/water/soil relations; may overheat roots; ??
effects on soil organisms
|
|
Newspaper/ cardboard
|
Best uses: as temporary mulch when clearing
an area of weeds/grass; under
pathways; in vegetable garden
Benefits: cheap; readily available;
decomposes in 1-2 years
Drawbacks: needs to be weighted down; toxin
in some colored inks; may be impervious to water
|
Applying Mulch
How to apply mulch: mulches need
to be thick enough to effectively prevent evaporation and weeds, but not so
thick that they promote disease. A good
rule of thumb is 2-4 inches deep for organic mulches and 1-2 inches deep for
inorganic mulches. If your soil drains slowly, use the minimal suggested depth; if you have a well-drained soil, then you can
use up to the maximum. And remember,
take your cues from nature as to the correct amount of mulch. See the section on Mulches and S. CA Native Plants
for recommendations specific to local native plants.
Before applying mulch, the area should be completely
weeded. Trust us – it’s easier to remove
the weeds before you mulch rather
than after. This is also the time to
check your irrigation system and make any needed repairs/changes. We suggest
that you water the soil deeply just prior to mulching to insure that plants are
well-watered during the transition period while the mulch is settling.
When applying mulch, be sure to leave 6-8 inches of
bare ground around the trunk or stem of each plant. This helps prevent fungal
diseases and discourage gophers and other rodents that might chew on the trunk. In most cases, mulches should be applied in a
flat layer (rake to smooth) and not mounded; never mound up mulch around tree trunks. Avoid packing organic mulches – this diminishes
their benefits and makes it difficult for water to penetrate. Water the mulch thoroughly after application
to start the settling process.
Monitoring soil moisture is important after you
mulch. You’ll probably need to water
less - and may actually run the risk of over-watering your plants. You’ll have to dig down into the soil (beneath
the mulch) to see if it’s dry – or use a soil moisture sensor. Remember, more garden plants are killed by
over-watering than under-watering. For
more on watering see the post on Water Zone/ Hydrozone Gardening,
April 2012.
When to apply mulch:
organic mulches break down over time, so you’ll need to re-apply fresh
mulch. Inorganic mulches may need
occasional renewal as they settle. It’s
a good practice to check the depth of mulch yearly, to see if new mulch is
needed. If you have a well-drained soil,
a good time for mulching around native plants is in the fall, after you’ve
completed your fall pruning and before the winter rains begin. Another good time to apply mulch is in the
spring, as you prepare the garden for the dry season. If you have a heavy clay soil, we suggest
you apply new mulch in the spring.
That’s what we do here in Mother Nature’s Backyard.
How much will I
need? Bulk mulch is often sold by the cubic
yard; some mulches are also available bagged.
Here’s the easiest way to calculate your needs:
1. First, figure out the size of the area you want to
cover in square feet (multiply the length times the width).
2.
Next, decide how deep you want your mulch to be, in inches.
3.
Now multiply the size of your garden in square feet (#1) by the depth of your
mulch in inches (#2).
4.
Divide the number you get in #3 by 324. This is the number of cubic yards of mulch you will need.
5.
If you are purchasing bags of mulch,
you will need to divide the cubic yards of mulch (#4) by the number of cubic
yards per bag. Most bagged garden
products are listed by the cubic foot rather than by the cubic yard. So first convert cubic feet to cubic yards
with the following conversion factor: 1
Cubic Foot = 0.037 Cubic Yards. Then divide the number of cubic yards needed
(#4) by the number of cubic yards per bag.
This will give you the number of
bags of mulch you will need.
Let’s take an example. You need to mulch an area that measures 15
ft. by 10 ft.
1. 15 ft x 10 ft = 150 square feet
2. You want your mulch to be 3 inches deep
3. 150 sq. ft x 3 inches = 450
4. 450/324 = 1.39 cubic yards of
mulch (you might want to round up to 1.5)
5. If you’re purchasing bagged mulch in 2 cubic foot bags, you’ll need to
convert the 2 cubic ft. to cubic yards: 2 X 0.037 = 0.074 cubic yards per bag. To calculate the number of bags you’ll need:
1.39 cubic
yards/0.074 cubic yards per bag = 18.78 or 19
bags of mulch
So now you can calculate the amount of mulch you
need like a pro!
Special Topics
Mulches
and S. California Native Plants: We
are blessed with a wonderful gardening climate in western Los Angeles County. The fact that we can – and do - grow plants
from several California plant communities actually makes choosing appropriate mulch(s) a little more complex. Beginning native plant gardeners sometimes
assume that all California native plants need a 2-4 inch layer of bark mulch.
This idea probably got started in northern California, where many of their
native plants do produce a good layer
of organic mulch. In contrast, some of
our local native plant communities actually produce very little ‘natural mulch’,
while others (like local Chaparral & Oak Woodland) are indeed characterized
by a thick layer of organic mulch.
Using
the correct mulch can mean the difference between success and failure with
native plants. For example, many
gardeners are surprised at the number of local wildflower seedlings that
germinate in their decomposed granite pathways.
This phenomenon really shouldn’t surprise us – after all, these plants
evolved to reseed on bare ground or in an inorganic mulch. Using a gravel/inorganic mulch makes sense if
you want to grow the wildflowers and bulbs native to our coastal areas.
When
using mulches around California native (and other water-wise) plants, it’s particularly
important to consider both the type and thickness of ‘natural
mulch’ that surrounds them in the wilds. In general, plant communities that include
trees and large shrubs produce more mulch than communities composed of smaller
shrubs and grasses. But there are some
nuances depending on the Plant Community.
To help you, we’ve given some mulching recommendations for the Plant
Communities most used by local gardeners (see table below). We
hope this clears up some confusion about mulching common native plants.
Plant Community(s)
|
Details
|
Coastal Strand
Coastal Bluffs
Coastal Prairie
Coastal Shrubland
|
Occurs: non-wetland areas from coast
through western Los Angeles Basin – elevations < 800 ft; includes Gardena
Typical
plants: mostly
short plants, including: native grasses, annual wildflowers, native bulbs, smaller
shrubs (Buckwheats; Goldenbushes; Deer Vetch; CA Encelia)
Natural
mulch(s): no
mulch or inorganic rock mulches
Garden
mulch(s): no
mulch or inorganic rock mulches, especially for wildflowers &
bunchgrasses; thin (1 inch) layer of organic mulch ( leaf mulch, wood chips
or bark) is OK around shrubs & sub-shrubs. You can let leaf litter build up as a
natural mulch under shrubs.
|
Coastal Sage Scrub
|
Occurs: Palos Verdes Peninsula; south and
west-facing hillsides from 1000-3000 ft elevation in much of Los Angeles
County. Usually drier climate than
Chaparral.
Typical
plants: Many medium-size shrubs ( < 6 ft tall) and
a few larger shrubs/trees commonly thought of as typical CA natives: Sages (Salvias); Lemonadeberry; Bush
Monkeyflowers (Diplacus species);
CA Sagebrush (Artemisia californica;
native currants & gooseberries (Ribes
species); Baccharis species; ); CA
Fuschia ; native Goldenrods; local Penstemon species.
Natural
mulch(s):
moderate layer of organic mulches
Garden
mulch(s): 2
inch layer of organic mulch
|
Chaparral
Southern Oak Woodland
|
Occurs: mostly western, northern &
eastern slopes of the southern California mountains. Elevation generally
above that of coastal sage scrub and with more rainfall. Santa Monica
Mountains and foothills of San Gabriel Mountains.
Typical
plants:
mostly larger shrubs/small trees (8-12 ft), including many used in gardens:
S. CA Ceanothus species; Rhamnus species (Coffeeberry;
Redberry); S. CA Manzanita species (Arctostaphylos);
Sugarbush; Bush Mallows (Malacothamnus
species); Bush Poppies (Dendromecon);
scrub oaks (Quercus); tree oaks (Quercus
agrifolia locally)
Natural
mulch(s):
thick layer of organic mulches
Garden
mulch(s): 2-4
inch layer of organic mulch (2-3 inch for clay soils) ; oaks best with natural
oak leaf mulch
|
Desert Plant Communities
|
Occurs: Mojave and Sonoran Deserts of S. CA
Typical
plants:
Desert plants: cacti; annual wildflowers; desert perennials; desert shrubs; desert trees including Desert
Willow (Chilopsis), Palo Verde, Desert
Olive, etc.
Natural
mulch(s):
inorganic rock mulches; very little organic matter
Garden
mulch(s): 1-2
inch layer of inorganic mulch
|
Wetland & Riparian Communities
|
Occurs: around lakes, ponds, wetlands or
stream sides
Typical
plants:
wetland species including trees (Willows, Cottonwoods, CA Sycamore, Alder),
Goldenrods, wild rose (Rosa
californica), Mulefat, rushes (Juncus species), sedges (Carex) species, wild grape (Vitis girdiana)
Natural
mulch(s): 3-4”
organic mulches like leaf mulch; often covered by natural groundcover vines,
grasses and grass-like plants
Garden
mulch(s): 2-4
inch layer of organic mulch; leaf mulch is excellent – others are fine.
|
North & Central CA Coastal
Plant Communities
Northern Maritime Chaparral
|
Occurs: along the CA coast from Ventura
county north
Typical
plants: many
plants used as native shrubs and woody groundcovers: northern Ceanothus and Manzanita (Arctostaphyllos) species; Madrone (Arbutus); perennials from central & northern coast
Natural
mulch(s): 2-4
inch layer of organic mulch
Garden
mulch(s): 2-4
inch layer of organic mulch (2-3 inch in clay soils)
|
Mulches as the garden
matures: A
garden’s mulch requirements change as the garden matures. In fact, the most important period for mulch
is actually the first few years after planting.
There are a number of reasons why mulch requirements change over time:
· The young garden looks empty – mulch makes it
look less so.
· Soil water evaporates most readily from un-shaded
soil. As more soil is shaded, evaporation decreases significantly, so your
garden actually needs less mulch as areas become shady.
· Young plants have less developed root systems; this
makes them more vulnerable to dry soils.
Mulch plays a key role in keeping the soil moist and cool for young,
shallow roots. As a plant matures, its
roots generally penetrate the soil more deeply and widely. A mature plant is tapping deeper sources of
water and can withstand more surface dryness. This is why many older trees and
shrubs – even non-native ones - are quite drought tolerant.
· Young gardens are more vulnerable to wind and
water erosion. Until roots bind the soil,
fall winds and winter rains can cause significant erosion. Erosion decreases as plants mature – even on
slopes.
· A young garden has less natural mulch. As trees/shrubs mature they drop leaves and
twigs, providing natural mulch beneath them.
Thus gardens may need less purchased mulch as they mature.
· Weed problems often decrease as a garden matures
· Many mature California native trees and shrubs
actually need the surface layers of the
soil to dry out between deep waterings.
If the upper layer of the soil remains too wet, root fungal diseases can
thrive, decreasing the lifespan of native – and other water-wise - plants.
As your
garden matures, take the time to re-evaluate your mulching strategy each
year. This allows you to spend your
time, effort and money wisely – and insure the health and longevity of your
plants.
Is there such a thing as
too much mulch? Unfortunately,
yes – particularly when you garden with water-wise plants. The key is to balance the good aspects of
mulch (deep enough to reap the benefits) with the potential drawbacks of too
deep a layer of mulch (water-logged soils; soil fungal diseases/pests; water not reaching the soil). We’ve discussed several factors that
influence mulch requirements including soil drainage, the maturity of the
garden and the optimal mulch needs of individual plants. The
other important factor is the amount of water the garden receives – both from
irrigation and from rain.
Mulching
too thickly can cause problems, particularly if you also water regularly or if
your soil drains slowly. In fact, you need to monitor your soil
moisture to be sure that you aren’t over-watering your garden. The key to healthy mulching is: 1) knowing
the optimal mulch requirements for your plants; 2) not over-watering mulched
areas, particularly those with water-wise plants; 3) checking occasionally to
be sure that the bottom layer of mulch does not remain soggy, have a sour smell
or show signs of fungal growth (mushrooms; thread-like white or gray hyphae).
If your mulch seems too wet, cut back on
watering and monitor soil moisture. If
your mulch has a sour odor or you see signs of fungi, also consider aerating
the mulch by raking/turning it or removing the affected areas and replacing
with new mulch.
Can I use several
different kinds of mulch in a garden? Yes – in fact that’s often a good idea. For example, you may want to use a bark or
wood chip mulch around your shrubs and a gravel mulch for your native
wildflowers or in a native bulb area. We
use both organic (chipped wood) and inorganic (gravel; sandstone chunk) mulches
in Mother Nature’s Backyard.
Mulches in fire-prone
areas: Most
organic mulches - as well as rubber, plastic and paper mulches - will burn
under hot, dry conditions. If you live
in a fire-prone area, the UC Division of Agriculture and Natural Resources
makes the following observations and recommendations based on a recent study:
‘With the exception of the composted
wood chips, all of the mulch treatments readily began flaming. Composted wood
chips smoldered, but produced very little flame. Fire spread the fastest in
shredded rubber, pine needles and shredded western red cedar.’
‘Quarles recommends not using any organic mulch within 5 feet of a house located in wildfire prone areas. Instead, place non-combustibles such as rock, pavers, brick chips or well-irrigated, low-combustible plants such as lawn or flowers closest to the house. The more combustible mulches should be used more than 30 feet from the home.’
For
more details on the research on which these recommendations are based see: http://www.unce.unr.edu/publications/files/nr/2011/sp1104.pdf
On-line
Resources about Mulch
The
following on-line resources provide more information about mulches. Remember that not all of the specific recommendations
may be appropriate for our area or for mulching California native plants.
Please
feel free to comment on your own experiences with garden mulches. You can also e-mail your mulch
questions to: mothernaturesbackyard10@gmail.com
