Experiments With Plastic Organic Mulch in the Garden

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Mulching, one of nature's oldest processes, has long been used by natural gardeners to control weeds and conserve moisture around fruit, vegetable and flower plants. Traditionally, only hay, grass clippings, leaves and other easily biodegradable matter has been used for this job. Recently, though, a number of gardeners have begun spreading sheets of black poly (a seemingly unlikely mulch) across their gardens with rather startling results. 

Well, folks, we’re still not convinced it’s a good idea
but there’s definitely a growing trend (among some otherwise conservative organic
gardeners, too).

Great bunches of onions, carrots and beets hang from the
joists in our basement this fall, the freezer is full to
the top with peas, beans and corn and Evelyn is beginning
to tire of canning tomatoes and making relish. In the
garden, several rows of cabbages are waiting to be cut just
before first frost and a bountiful harvest of cauliflower
will be ready just after. We shan’t be buying vegetables in
the supermarket for some time.

Although we’ve gardened for years, this is the first time
we reaped such a superabundance of produce. Needless to
say, there is a gimmick . . . but a gimmick anyone
can use. We’ve simply learned about black polyethylene

Mulching, one of nature’s oldest processes, has long been
used by natural gardeners to control weeds and conserve
moisture around fruit, vegetable and flower plants.
Traditionally, only hay, grass clippings, leaves and other
easily biodegradable matter has been used for this job.
Recently, though, a number of gardeners have begun
spreading sheets of black poly (a seemingly unlikely mulch) as an organic mulch across their gardens with rather startling results.

We discovered black plastic mulch two years ago in the Canadian
Department of Agriculture’s book, Growing Vegetable
The publication said, “Black polyethylene
film, usually 1-1/2 mils thick, is spread over the ground
and the edges are covered with soil. Slits or holes are
made in the film to allow planting through the film. The
black film over the soil tends to warm the soil, reduce
evaporation and prevent weed growth. Each of these effects
is beneficial to crop plants.”

That sounded good to us so we checked around and found that
black poly is available almost anywhere transparent
polyethylene is sold (hardware stores, farmers’ supply
houses, lumber yards, etc etc.) The black, 1-1/2-mil sheets
come in rolls 100 feet long and 36 inches wide and
currently costs about $2.50 a roll.

May I draw a veil over our first year’s mistakes?

This year, with a little experience behind us, we started
our garden just after the snow left the ground. First we
cultivated the soil lightly, then put 10-foot-long strips
of polyethylene down side by side with about a tenth of an
inch between the edges. We planted early peas in this
narrow “slot” and finished off that section of the garden
by spreading about an inch of loose soil over the plastic
to hold it down out of the wind.

One sunny but cold day shortly thereafter, we pushed a
thermometer into the soil under the poly. It registered
nearly fifty degrees. In unprotected soil nearby, the
temperature was just over freezing. No wonder the peas
germinated early!

Later we rolled out more of the poly and set onions through
it. Since the onions were bigger and we wanted them farther
apart than we had planted the peas, we simply made
two-inch-long slits through the plastic for each onion and
pushed the sets through the slits.

Don’t try sowing small seeds — even seeds as big as
peas — through slits this way. If the poly sheet
moves ever so slightly as it will when you walk on it (yes,
we walk on ours . . . but only in soft, light shoes), the
small seeds will not find their way up to the sunshine and
the air. Instead, they suffer the fate of weeds and smother
under the plastic. We lost a lot of our 1969 crop this way.

For parsnips, carrots and other small-seeded plants, we
slit the black poly lengthwise into strips ten inches wide
and ten feet long. We placed these strips side by side with
their edges an inch apart and, as we did with the peas, we
then planted the tiny seeds in the narrow slot of earth
that was left.

By the way, there’s nothing particularly magic about the
fact that we cut all our plastic sheets ten feet long. Our
garden just happens to be that distance across. If your
vegetable patch is twelve feet wide, cut your strips to
reach. If the garden has exceptionally long rows, however,
you may want to lay down a series of shorter pieces instead
of one long strip of poly. Long strips are too liable to
blow about in the wind . . . which always seems to get up
just when the plastic is in the most awkward position. Your
strips should simply be as long as you find convenient.

Convenience is worth looking for. When I was planting those
onions I crouched down to cut the slit, then sat up
straight, found an onion in the sack, planted that and
— finally — moved on to the next position.
Evelyn — attracted by my mutterings and groans
— looked thoughtful for a moment, then brought out an
empty five gallon drum from behind the barn.

“Here,” she said, “try this. Turn the drum on its side so
it’ll roll and sit on it.”

I did as she said and found the small barrel to be a
comfortable moving seat, easy to roll back along the poly
as I continued to plant onions every six inches . . . until
the school bus brought our daughter home. When she saw all
the fun I was having-rolling along backward, sowing onions
through the slits -she took over the task. I was
demoted to scattering earth on top of the plastic strips to
keep them down.

After the peas had been harvested, we rolled up the black
poly to take a look at the weeds that had tried to grow
under it. All of them were white from lack of light (a
botanist would say “etiolated”) and most were lifeless.
Without air and light, they could not survive. The weeds
that had sprung up in the inch or two of soil covering the
polyethylene had also quickly died . . . of thirst, as that
shallow seedbed dried out.

A few weeds, of course, did grow between the edges of the
polyethylene strips where the soil was left bare for the
peas. Weeding here, however, was a small chore compared to
the endless hacking with the hoe that would have been
needed if we hadn’t used the plastic mulch. One or two
stray volunteers sprouted up through the slits made for the
onions, too, but they were easily pulled by hand.

Be that as it may, there was one weed that surprised us no
end. As the summer wore on we came to ignore the volunteer
plants that sprang up across the polyethylene because we
knew they were growing in only an inch of soil and would
soon die. One especially large and healthy volunteer did
attract Evelyn’s attention, though, and she gave it a heave
. . . only to find that the plant had sprouted through a
hole in the plastic. We didn’t think much about it until,
later, we found another weed . . . and another . . . and
another . . . always the same kind of grass and always
growing through a hole in the poly.

At last it dawned on us that this grass — later
identified as “quack” or “twitch” grass — can
actually push its way up through the plastic. We’ve since
found that it will also creep out from under the edges of
the poly. The survival mechanisms built into this grass are

Actually, the quack grass — although definitely hardy
— is not the overwhelming danger it might seem and we
only had our major troubles with it when we were breaking
new soil. In our old plot, where we’ve gardened for years,
the twitch grass has long been weeded out, so to speak.

We’ve also found that quack grass cannot grow through loose
plastic and, after some time underneath, it dies. So now,
in previously unworked ground where the grass may be
lurking, we leave the plastic loose and keep it in place by
burying the edges of each sheet a few inches deep. The
strips billow up in a fresh breeze, but they stay put.
Evelyn likes to lay a two-by-four on each loose piece of
plastic when there’s a high wind but I don’t think it’s
necessary. After a season with the poly buried only around
the edges to kill off the twitch grass, we go back to the
neater method of securing the plastic with an inch of soil
across its surface.

What happens to the rain that falls on the poly? With the
exception of a little that stays on top where the plastic
is dished by irregularities in the ground, it seems that
most of the water runs off the edges of the strips and
soaks into the soil. To be sure, the small amount of
rainfall trapped on top of each sheet evaporates . . . on
the other hand, the plastic prevents a much larger
evaporation from the soil and smother all the weeds that
would otherwise drink the moisture in the earth. On
balance, I don’t believe there’s any loss of water from the
ground. The poly, in fact, seems to help the earth
hold water, if anything.

Our success with black poly mulch was noteworthy enough
this season to make us want to experiment further next
year. We’re going to try to adapt the plastic to the flower
beds for one thing, and it should be quite a challenge to
get all the blooms in the correct position and the correct

We’d also like to try “sterilizing” an area by covering it
with a 10-foot by 50-foot roll of poly. This should allow
us to start a lawn free of dandelions, plantain and all the
other weeds that compete for the space. This seems to us to
be a better idea than poisoning the ground with
weed killers.

Yep. We’re going to keep working with our newfound mulch.
Plastic or no, it seems to give us good results. So good,
in fact, that we just may run out of hanging, canning and
freezing room if we aren’t careful!