WEEDS, WEED PREVENTION AND WEED CONTROL
A few weeds in the garden are unsightly but of no real consequence.
They are a problem in that they have the potential to take over and to
out-compete the garden plants for moisture, nutrients, sun, and space.
They are a particular problem in agriculture because weeds add so much
to the expense of the farming operation. The presence of weeds interferes
in the growing of a crop efficiently and the presence of weeds or weed
seed devalues the finished product. 8 out of every 10 pounds of pesticide
used by American farmers is a herbicide and the use of such huge
quantities has a major impact on the environment.
This session is about weeds, their characteristics, and the various ways
weeds can be prevented or controlled without the use of herbicides.
WEEDS
Gardeners know that the often heard definition of weeds as "plants
growing out of place" misses the point. There are some super- vigorous
plants that seem intent to take over and that once they become really
established that they are nearly impossible to remove. They simply
don't behave like other plants and it's more than just as issue of
location. This segment is about that quality that makes a plant to be
considered as a weed.
the nature of "weedness"
Mrs. Getty, one of the characters in Celia Thaxter's "An Island
Garden", challenges the definition of weeds as "plants growing
out of place" and says that the "essence of weed character is the
innate disposition to GET into the wrong place" - she then adds,
that this is also the character of men. " If you glance through
your botanical books you will see often added to certain names,
'a troublesome weed'. It is not being venomous or ugly, but its
being impertinent - thrusting itself where it has no business and
hindering other people's business [that makes a plant into a
weed]".
Hugh Johnson's "The Principles of Gardening" says, that
weeds are " a race of super-plants of unfair vigour and
tenacity".
The U.S. Bureau of Land Management, while not as poetic,
defines weeds as "non-native plants that have adapted to a new
area". Their thesis is that when these plants were growing in
their native area, they were well behaved because they were held
in check by the various indigenous organisms. Moving these plants
to a different location then separated them from their natural
control agents. In essence, saying that weeds are a problem
BECAUSE they are out of place.
The New York Times cited an incident involving a "sea weed",
native to the waters of the Pacific, that was accidently
introduced into the Mediterranean, "Caulterpa taxiflora now
occupies 8000 acres in the Mediterranean - and for the last three
years, it has tripled annually." They then quote Antoni Grau,
head of the Marine Resources in Mallorca, "Where this seaweed
grows, everything else disappears. There are no more sea
anemones, starfish, crabs, shrimp - and very few fish".
characteristics and control of annual weeds
Annual weeds are plants that grow from seed to adult plants that
are able to produce seed within a single season. The adult plant
dies after it has set the seed. Spotted spurge is an example of
an annual weed. It grows aggressively in hot weather, sets huge
quantities of seeds in Summer and early Fall, and then dies out
in the Winter. The seeds germinate as it starts to warm up in
early Spring.
This annual growth habit is the key to the control. Because
annual weeds are going to die at season's end and because they
only generate new plants from seed, you don't have to worry about
getting all the bits and pieces in the weeding operation, you
simply need to catch them before they have a chance to set seed.
characteristics and control of perennial weeds
Perennial weeds are plants that put on flowers and seeds from the
same root structure in consecutive seasons. They spread by wind-
blown seeds and by putting on new growth from buds on the roots.
Dandelion is an example of a perennial weed. It grows from a
deep, fleshy tap root, flowers in Spring and Fall, growth falls
off in the Winter and then moves back into aggressive growth the
following Spring. The cycle can go on for a number of years.
While catching perennial weeds before they have had a chance
to set seed is helpful in preventing the spread, major control of
perennials is by controlling the roots and this is a challenge.
Any remaining bit or piece of bulb, tuber, stolon, crown or
taproot of the perennial weeds carrying a single, growth bud is
capable of generating a whole new plant. Getting the bulk of the
perennial weed but leaving multiple pieces of the root system
still in the soil can create an even greater and/or stronger weed
presence.
Control perennial weeds in two stages:
- Cut off the top growth and dig up as much of the bulb, tuber,
rhizome, stolon, crown or tap root as possible
- Let the remnant root structure pieces send up new growth and
cut it off . . . the plant energy to generate new growth is
stored in the root, continually cutting off new top growth
eventually depletes the stored energy and eventually the root
system dies.
WEED PREVENTION
It is infinitely easier and considerably more labor efficient to work
at keeping weeds out of the garden than it is to work at their
removal. The use of a clean, generous layer of compost or composted
manure as a mulch is the single best way to keep weeds from happening.
Black plastic landscape fabric, gravel, decorative bark also work as
an effective mulch to keep weeds from happening but compost and
composted manure offer the advantage of also being a source of organic
material and nutrients for the garden. This particular segment is
about the ways compost or composted manures used as a mulch keep weeds
out of the garden, how these materials can be used, and the ways to
avoid the problems associated with the use of compost or composted
manure.
- the ways a mulch of compost and/or composted manure prevent
weeds: These materials prevent weeds by serving as a physical
barrier and by serving as a light barrier:
- physical barrier: The layer of compost and/or composted
manure simply keeps air-bourne weed seeds out of the soil. Should
weeds germinate in the mulch layer, they are easily removed. The
mulch layer also smothers weed seedlings that have germinated in the
soil level.
- light barrier: Some of the weed seeds need light to
germinate and some of the remnant bits and pieces of root structures
left over from weeding need light to put on new top growth. A thick,
undisturbed layer of compost or manure shades out that
light.
- the ways compost and/or composted manure should be used as a
mulch: Put down a two inch (5 cm) layer of compost or composted
manure over the entire surface of the beds. The goal is to provide
the physical barrier that will minimize problems with weeds and to
conserve soil moisture. Do not incorporate the organic material into
the soil. The soil organisms will do that job for you. It is best to
leave some air space between the stems or stalks of the various
plants and the mulch layer to prevent possible rotting.
If the weed problem is particularly difficult, an additional layer
of newspaper or cardboard can be used along with the mulch for
better control. Weed the bed as best you can, rake it smooth, and
put down a layer of black-and-white newspaper 5 to 10 sheets thick
or a single layer of cardboard and then bury under a two inch (5 cm)
layer of compost or composted manure. The newspaper or cardboard
layer does a particularly good job of blocking out the light. The
top coating of compost or composted manure holds the newspaper or
cardboard in place and gives the garden a better look. The layer of
newspaper or cardboard eventually bio-degrades.
- the ways to avoid problems associated with the use of compost
and/or composted manure as a mulch: Ruth Stout advocated putting
down layers of raw or semi-raw organic material as a way of feeding
the garden and to also gain the benefits of a mulch. There is also
current interest in doing the same thing but under the title of
"sheet composting". This involves some risk as the organic material
may well contain pests, pathogens and weed seed. It is much safer to
only use compost or composted manure that has been "hot composted".
The OCIA's International Certification Standard (See
rec.gardens.ecosystem's FAQ; Appendix A) requires compost piles to
reach a temperature of 120 to 140 degrees F for a period of about 6
weeks.
Another problem is to keep in mind that compost and composted
manure have the capacity to hold a lot of water. Putting on a very
thick mulch layer with these materials can interfere with the
watering. If the beds are watered artificially, enough water needs
to added to saturate the mulch and, in excess of that, to irrigate
the garden as well. Learn how much is actually required to penetrate
the soil to the right depth and water accordingly.
WEED CONTROL
Gardens Alive markets an organic herbicide that is derived from corn.
(I have no personal experience with this material nor have I seen any
reports on it's effectiveness) All of the commercial herbicides,
including the product derived from corn, are prohibited for use on
certified organic crops by the majority of the various certifying
agencies. Herbicides, as a group, have a very poor environmental
record and should be avoided.
- hand weeding:"The Principles of Gardening" by Hugh
Johnson says, "Whether weeding is a pain or a pleasure depends more
than anything else on the condition of your soil. To weed in baked,
dry ground is utterly frustrating - in fact, virtually impossible:
the fork won't go in, and the roots won't come out. On very wet
ground it is just a muddy mess. Your hands and your tools become
clogged, slippery and unusable. The time to weed, therefore, is
after, but not immediately after, a good downpour - the interval
depending on how quickly your soil drains"
. . . . "Take a small "border" fork, a hand-fork, a bucket for the
spoil, something to knell on . . . and prepare for a gently
pleasurable afternoon".
Keep your weeder "knife sharp", weed thoroughly - one section at a
time, and keep it at "gently pleasurable". It's a time to really be
with your garden.
- use of flame or heat: Propane weed flammers have been in
use by home gardeners in Europe for a number of years and with good
success. They are said to be relatively effective, convenient, and
easy to use. The down side is that the units cost in the vicinity of
$200. Plow and Hearth @ (703) 672-1712 markets these units in the
U.S.
"Common-Sense Pest Control" by Olkowski, Daar & Olkowski
says, "There is an art and a science to flaming weeds. The mistake
people often make is to assume that they must hold the flame to the
weed until they see the plant burn up or at least look sizzled and
scorched. Nothing could be further from the truth. The whole point
of flaming is to heat the cell sap, which is accomplished by slowly
passing the flame over the plant. You will not see any evidence of
wilting, let alone plant death, for several hours or even until the
next day."
Flaming works best on young seedlings and the base requirement is
that there is sufficient room between the weeds and the garden
plants to use flammers safely.
- soil solarization: Soil solarization uses trapped solar
energy to raise the soil temperature high enough to kill weeds and
their seed. The thermal "death point" for weeds by soil solarization
is 140 F which is also hot enough to destroy most of the soil
organisms. It is not a recommended practice for most situations in
the home garden but it is a better option than the use of toxic
herbicides.
- Cultivate the soil, removing the larger rocks and clods, rake the
soil surface smooth, water the soil throughly, wait two weeks for
the weed seed present to germinate. The germinated seedlings are
allowed to remain in place. They are particularly sensitive to
heat.
- Water the soil thoroughly again and cover with a clear, 4 mil
plastic tarp. Tuck the edges of the tarp into the soil, weigh down
the edges with rocks or bricks, and let the soil cook for at least
four weeks. (The black plastic sheeting doesn't trap enough heat -
the throw-away clear plastic sheeting used as a painter's drop cloth
is too flimsy.)
- The soil can be planted as soon as the soil has cooled.
- use of biological agents: Biological agents would include
the use of insects, pathogens, "myco-herbicides", fish, geese, and
an assortment of grazing animals. Take these options seriously. They
do work. There is an entrepreneur in San Francisco who has started a
"rent-a-goat" program for weed control and he is said to have a
thriving business. He comes out, looks at the weed situation,
decides how many goats would be required, puts up a temporary chain
link fence enclosure, and then charges by the number of "goat days"
involved.
Olkowski et al. have a good section on the use of weeder geese in
the home garden.
ASSIGNED READING
- SECTION I; ; FAQ; "Weeds, Weed Prevention and Weed Control"
- Olkowski, Daar & Olkowski: "Common-Sense Pest Control"
Chapter 27: "Meet the Weeds"
Chapter 28: "Safe and Sane Weed Management"