How to Mulch a Vegetable Garden

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Photo by Adobestock/zlikovec
Compost is one of the best vegetable garden mulches ever.

Mulch, as a verb, is the act of applying some covering to
the soil, usually for the purpose of controlling weeds. As
a noun, it is any material that will serve the purpose. If
that sounds like a very broad range of materials, it is. I
have used mulches that were living and dead and ranged from
dust to oil. What vegetable garden mulch accomplished—with any
luck—was: 

Controlling Weeds and Retaining Moisture

All of the mulches I’m
going to discuss retain moisture with the arguable
exception of a living mulch. A living mulch like clover
will draw moisture from the sod for its own growth and some
of the moisture drawn by the plants will be lost to the
atmosphere through the leaves. On the other hand, a carpet
of leaves on low-growing plants that catches morning dew
and holds a cushion of air beneath it may snake up for the
moisture it uses. By cushion of air I mean that low-growing
plants, such as clover grown between rows of corn, will
shelter the soil surface from drying breezes and from the
baking sun. Whether or not the beneficial influence offsets
the moisture used is something for scientists to
investigate.

All mulches, except dust, inhibit rainfall or irrigation
from above from reaching the soil. For this reason, it is
best to mulch a soil that is already moist. Here in New
England, the spring soil is usually moist. There has been a
time or two when I held back on mulching and waited for
rain. Once an impervious mulch like plastic is put down,
there is no chance of additional moisture reaching that
soil in any quantity. It amazes me when I check the
condition of soil under a plastic mulch in mid or late
summer and find it moist below the top half inch. Where are
the plants getting sufficient water to fill out eggplant
and tomatoes? The roots have probably extended beyond the
plastic, but still, the soil under the plastic has received
no additional moisture since the end of May.

Fertilizing

All organic mulches do this.
Grass clippings provide nutrients more quickly than other
organic mulches. Sawdust is at the other end of the scale,
as it will require nitrogen from a source outside itself to
break down into usable nutrients and humus. When we put
sawdust in contact with the soil, the sod organisms
recognize a big job ahead and they multiply rapidly. To
multiply, they need nitrogen. Since it is not available in
the sawdust, they use nitrogen that is already in the soil.
If the nitrogen supply in the soil is insufficient, the
sawdust is converted more slowly. Of greater importance,
the soil is deficient in nitrogen for the plants and they
will suffer.

The most important thing to keep in mind regarding soil
fertility and mulch is the carbon-nitrogen ratio. The
carbon-nitrogen ratio of sawdust is 400:1, for example,
while young sweet clover is 12:1. An average bale of hay
might be 80:1, while rotted manure might be 20:1. A
substance that has a C:N ratio below 17:1 will actually add
nitrogen to the soil while a ratio above 33:1 will take
nitrogen from the soil. Between those two figures the
result is neutral. Grass clippings might have a ratio of
16:1 but if the plant is allowed to grow to maturity, the
ratio might go to 30:1 or 40:1. If the plant starts to dry,
the ratio goes up even more. Once dried as hay the ratio
will be in the vicinity of 80:1.

There is one other factor to keep in mind regarding the C:N
ratio. If the material is just going to be used as a mulch,
the amount of interface with the soil is limited to the
soil surface. It is when a high C:N ratio material is
incorporated into the soil that the temporary loss of
nitrogen can become severe. You may use a sawdust mulch on
a section of the garden one year and have no problems. If
you turn it under the next year, you want to stake sure to
incorporate some additional nitrogen at the same time.

Materials from trees have the potential for lowering the pH
of the soil—making it more acidic. Leaves, sawdust,
and wood shavings are acidic. However, in the process of
becoming humus, they move toward a higher pH. Also, it
seems the more humus in the soil, the wider the band on the
pH scale that is acceptable to plants.

Warming/Cooling the Soil

Clear or black
plastic mulch will warm the soil more quickly in the spring
and hold the warmth over night. In my climate, that makes a
tremendous difference with eggplant and melons and usually
a noticeable difference with peppers and tomatoes. Organic
mulches insulate the soil. Mulching crops that prefer a
cool soil like lettuce, peas, and spinach can lengthen the
harvest period or improve the harvest. 

Keeping Plants off the Ground

A tomato
plant can be allowed to sprawl on a cushion of heavy straw,
grass, or hay mulch and the fruit will not be the worse for
the contact. If the same tomato is allowed to rest on the
bare ground, wire worms or some other soil critters may
damage the fruit. Melons will also benefit from this
technique.

Keeping Plants Clean

Lettuce and celery
are especially prone to getting soil splashed all over
them. Not so when they are mulched with something other
than a dust mulch.

Controlling Erosion

While we are
picturing the rain drops splashing the sod up on our clean
plants just before we harvest them, let’s think about the
soil. Those rain drops are falling hard on the soil and if
enough of them fall and there is any slope to the garden,
they will start to flow. That will cause erosion. Mulch can
stop erosion on any slope.

Making the Garden Attractive

Beauty is
clearly in the eye of the beholder. I think a mulched
garden is beautiful. We need some spaces in the garden for
ourselves, and I like to have my walkways covered with
mulch. There are also spaces into which plants haven’t
grown yet which simply look better with a little covering.
Call me a prude, but I think the winter squash patch looks
better covered with mulch than showing bare exposed soil.

Controlling Insects

Have I ever
mentioned that I think the best way to control of insects
in a garden is to grow healthy plants in a healthy soil?
Organic mulch goes does much for the soil by providing
plants with humus. The plants are also healthier with a
steady supply of moisture available as needed. I have seen
claims that mulch interrupts the life-cycle of some
insects. That may be true, but I have not seen any evidence
of it. To be fair, I have not looked. I’m convinced that
anything good that happens is simply the result of a
healthy growing environment.

Preparing Soil for Planting

An organic
mulch can also be used to prepare sod for planting the next
year. Spreading it over the sod will smother the plants
underneath, making it easier to turn the soil a year later.
You’ll be pleased next planting season by sod that is soft,
friable, and easy to work.

Which Mulch is Best?

I’m going to start with the dead organic mulches first. This
is the biggest category, the most common, the best for most
things, and the most likely to be the least expensive.

I have heard several stories of people who have been able
to get the municipal road crew to deliver leaves and/or
grass clippings. If you live in a place where the
municipality collects leaves and your garden is as close or
closer than the place they are planning to dump them, there
is a good chance you can get in on this free delivery
service.

Lawn-care people may also deliver for you, but there are
some things to watch out for. Sometimes the leaves or grass
clippings come with unwanted materials.

I have several bags of leaves next to the garden right now
that have enough gravel in them to make a path. I could
actually use the gravel, but separating the two is a task
that I have not found the time to do yet. I’m fond of
saying that time is a function of desire.

The other cautionary note involves grass clippings. Lawns
receive more chemicals than agricultural
fields—wonderful (he says sarcastically) mixtures of
all-purpose concoctions that will make the lawn green, kill
certain kinds of “unwanted” grasses, kill broad-leaf
plants, and kill insects. I’m not going to go into why that
is a bad idea. It would take another article. Besides, if
everyone agreed with me on lawn care there wouldn’t be any
grass clippings for gardens. What we are concerned with is
keeping those chemicals out of the garden, off our hands
and clothes, and out of our house. If you are dealing with
reasonable lawn care people, they may be happy to tell you
when the lawn was last treated. After all, there are
broad-leaf plants in the garden and you wouldn’t want to
unknowingly introduce a herbicide designed to kill such
plants into the garden. They would understand that
reasoning better than telling them, “I hate chemicals.”

If you can’t get free home delivery, you can probably find
a good neighborhood for collecting bagged leaves and
clippings. I always ask before collecting. Even if bags are
at curbside, it is possible they are there waiting for
another gardener. Besides, talking to someone who is
bagging mulch for you may be the beginning of a treasure
trove. They may be willing to call you when they have some
good stuff and they are likely to keep the bags more pure.
You might want to heft the bags just to check the weight
before asking for them. If I had an opportunity to do that
with the leaves and gravel I have now, I would have avoided
a job.

Most organic mulch is easy to use. The finer materials,
such as clippings and leaves, are the easiest to use. Just
spread or place them with your hands where you want them,
between four and eight inches thick. The thickness depends
on the coarseness of the material. Sawdust will smother
weeds at a couple of inches, leaves three to four, grass
clippings four to six, hay or straw eight inches. If the
materials are too coarse (such as corn stalks or sugar
cane) they should be shredded to make a good mulch or mixed
with other mulch materials. Don’t obsess over the thickness
of the mulch. You are unlikely to use too much, and if you
use too little you will know when the weeds poke through
it. If that happens, just drop some more on top of the
impertinent weeds.

I generally have a variety of materials and I use them in
different places. The coarsest materials are easiest to
spread in large areas like the squash patch, between the
corn rows, and on the walkways. My favorite material is
freshly cut grass clippings that will go right where you
put them and stay there.

Grass clippings are so easy to use that when I started
market gardening, the only motorized vehicle I bought was a
riding mower and grass collector. Instead of driving
equipment around in the garden, I drove around on sod and
collected grass clippings. I had twenty rows 100 feet long
of carrots, all mulched with grass clippings. I doubt that
anyone has done that before. It really wasn’t much work.
They were all mulched before the radishes that were planted
with them got their first true leaves. When I harvested the
radishes I did a little in-row weeding, and that was the
end of the care of those carrots until harvest. If I ever
have another market garden, I will do it the same way. Some
enterprising soul could develop a machine that cuts the
grass and deposits it as mulch in adjacent rows.

Bark and wood chips are popular mulches around ornamentals
and shrubs. These plantings do well in an acidic soil so
there is no concern about the wood products making the soil
too acid. Wood products break down more slowly and thus
provide less nutritive value to the soil. As a barefoot
gardener, I prefer a mulch that feels good underfoot. This
is my particular reason for not using them.

Wood shavings and sawdust are sometimes used as bedding for
animals and come mixed with manure. The manure has nitrogen
that improves the carbon-nitrogen ratio. Manure by itself
or in combination with other materials is generally not
used as a mulch. If you put it on thickly enough to smother
weeds, it will likely be too rich for the soil. The
combination of manure and straw, may be a good growing
medium for plants and while smothering seeds in the soil,
seeds in the manure mix will germinate.

The list of possible dead organic materials is long: potato
peelings, buckwheat hulls, peanut shells, egg shells,
lobster shells, feathers, hair, grapefruit skins, leather
scraps, pea pods, pine needles, prune pits, and on and on.
While some of these are exotic to most of us, they are an
easily obtained waste product for others.

There is one other that is readily available to just about
anyone. It is different from the those mentioned so far
because it is a processed organic material. Newspaper is a
wood product. It performs well as a mulch in all aspects of
other organic mulches except that it has limited nutrient
value and it doesn’t look, in my opinion, as nice as
natural organic materials. I have read nothing that leads
me to believe it should not be used. The paper is OK and
the ink will not create problems in the soil or be taken up
by the plants. The best way to use newspapers is probably
to put them down on the sod first, several pages thick, and
cover them with a light natural mulch to hide them. This
will make it possible to stretch your natural mulch farther
if it is in short supply.

I have been making a distinction for dead organic mulch
because a living mulch is also providing organic material
for the soil. Living mulches work best, I think, for crops
like corn that will grow tall and not have to compete with
the mulch for sunlight. Vine crops will also do well as
they will grow above the living mulch. I have two favorite
living mulches. Clover is a legume and will actually add
nitrogen to the soil. It is also low-growing and spreads
out over the surface of the soil, blanketing it easily. It
seems like a natural for growing in corn as the corn is a
heavy nitrogen feeder.

The other favorite living mulch is weeds. Nice
strong-stemmed weeds like lambs quarters will support
tomato and squash vines, keeping the fruit off the ground.
As they grow along with the crop, they keep the ground
covered from early in the season. They grow the structure
to support my plants. After my experience last summer,
which I’ll get into in a minute, I am not sure I will use
this method again.

A dust mulch is the simplest but it also provides the
fewest positive results. Dust mulch will help the soil
retain moisture. control weeds and, perhaps, insulate the
soil slightly from temperature fluctuations. Dust is not
applied. It is created by cultivating the surface
shallowly. By loosening the soil on the surface, air spaces
are created that inhibit the capillary action that draws
moisture from below. Without the dust, the moisture on the
surface evaporates and more moisture is sucked to the
surface to be last to the air.

The “oil” I use as a mulch is in the form of plastic that
is a petroleum product. I prefer black plastic, though I
have been assured that clear plastic works just as well.
Although plastic will do just about everything an organic
mulch will do except add fertility, I use it for only one
reason: to warn the soil. Even then, I only use it for
crops that will not do well otherwise in my climate. I
could get cucumber and squash earlier if I used plastic,
but I am satisfied with the date and production of these
crops. I usually mulch them with hay that can have a
cooling effect but I don’t mulch them until the plants are
up and growing, and I mulch at the end of the day when the
soil is warm.

Plastic gives me considerably better yields of eggplant,
melons, and tomatoes. I think it gives me better yields of
peppers, and so I usually plant them in plastic but pepper
yields seem to operate on some cosmic scale that baffles
me.

Using plastic is fairly easy. I bought a roll years ago for
the market garden and I’m still using it. You want to he
sure to store plastic out of the sun, as sunlight
deteriorates it. I start by hoeing a trench eight or so
inches deep down the center of the row I plan to plant. I
half fill the trench with compost and then fill it in and
smooth the bed. My plastic is three feet wide but most sold
now seems to be four. I dig a six or so inch deep trench on
either side of the first trench about eight inches in from
the edges of the plastic. Then the plastic is rolled out
and the edges are tucked into the trenches and covered to
keep the plastic from blowing away. Seedlings are
transplanted through holes poked in the plastic. Be sure to
remove the plastic as soon as the plants die in the fall.
If left over winter, it is much more likely to deteriorate
and become a problem. The best method I have found for
removing it is to cut the plant sterns close to the soil
line rather than trying to pull the plants or tear the
plastic around them. Leaving the roots in the soil is a
good idea anyway.

Solving Problems

If I were as fortunate as some and had no slugs or moles or
mice, I would garden with a permanent mulch. I did for a
couple of years and loved it. It wasn’t quite as simple as
that, because each year the garden got larger and I would
have someone come to till the new ground. While they were
here, I would have them till part of the old ground. There
was always some tilled ground to be dealt with each spring
it seems.

There were also the witch grass rhizomes that were in
abundance then. Tilling was necessary to get that plant under control. By mid-summer most of the garden had been
mulched.

Slugs were a problem. There were whole families living in
lettuce and cabbage plants. I could pull back mulch just
about anywhere and find slugs. They did considerable damage
to the pea vines one year and another year they wiped out
my late planting of lettuce when it reached an inch in
height. I used pans of beer, which do attract slugs, and
various other control methods. However, I really didn’t
want to spend my life feeding slugs mulch so their
populations could explode while also trying to kill them.
The solution seemed to be less mulch.

I removed the dead organic mulch from the lettuce and
cabbage. Lettuce hates to be crowded, so a living mulch
would not work. A living mulch actually provides shade and
moisture at ground level which slugs need. I wanted to
create a desert between the slugs and my precious lettuce
and cabbage. A dust mulch was the answer. That meant,
though, that the lettuce would get dirty in a hard rain and
that it wouldn’t have the benefit of the cool stable soil
temperature of mulch. It’s a trade-off.

Potatoes can be planted right on top of the soil and
covered with eight inches or so of hay. It works very
well—most years. It is the other years, when the
rodents find them, that have caused me to change my method.
Now I plant potatoes a couple of inches deep and mulch up
around them as they grow rather than hilling. It doesn’t
matter if it is soil or mulch that protects them from the
sun just so the sun doesn’t touch the tubers.

Rats and mice have occasionally gotten into my carrots and
potatoes in the fall. This usually happens after a section
has been under mulch for a couple of years. When it becomes
a problem, I usually till a major section of the garden
the following spring to disrupt their tunnels, nests, and
to let them know this is not a good place to call home.

Two years ago, moles became a serious problem. They are
insectivores so I wasn’t worried about the plants this time
but rather about the earthworms which are at the core of my
fertilization plan. I put up a vibrating windmill that was
supposed to drive them away. It didn’t. I put out traps,
also to no avail.

Last spring, virtually any place in the garden where I
pushed my hand into the soil, it would drop through a mole
run. With all that air in the soil, it was quite possible
roots would be affected. Worse, where a shovel would
usually turn up a couple dozen worms, now there might be
shovels-full of soil with no worms. I had two-thirds of the
garden tilled. The mole runs that we see in our lawns and
gardens are where they eat. Their homes are dug deeper,
below the tines of the tiller.

I took up mulch in some places. I used a living mulch in
the corn instead of hay, which I prefer. The moles left in
early summer to aerate the front lawn. I don’t think they
left because of anything I did. I think they ate most of
the worms and moved on in search of more food.

By fall the earthworm population was up to a respectable
level again. The moles, however, have circumnavigated the
house and are headed back toward the garden. I don’t have
the answer to this little problem yet. I’m thinking about a
moat.

Using less dead organic mulch left more of the garden open
for weeds to grow. I am going to be more careful about
letting weeds go to seed in the garden. That is why I am
probably not going to use a living mulch of weeds to
support squash or tomatoes. Of course, if weeds get away
from me to the squash or tomato patch, you can count on me
to claim I planned it that way.