Weed Control in Home Gardens.pdf

Agricultural Extension Service

The University of Tennessee

Vegetables

SP 291-I

Weed Control in Home Gardens

R. Allen Straw, Assistant Professor, Plant Sciences

Originally developed by David W. Sams, Professor Emeritus, Plant and Soil Science and

Craig H. Canaday, Associate Professor, Entomology and Plant Pathology

What is a weed?

A weed may be broadly deﬁ ned as any unwanted plant,

or as I have heard it said many times, “a weed is a plant out

of place.” For example, a corn plant is a weed if it is in a

strawberry planting. Most weeds are plants that are gener-

ally considered undesirable by gardeners. They are neither

eaten nor considered attractive. They are frequently inva-

sive and difﬁ cult to control.

used producing lush foliage, large seed or fruit. Therefore,

they often thrive where vegetables struggle to produce.

Many spread rapidly by vegetative structures as well as by

seed. The principle methods of weed control fall into three

categories: (1) mechanical, (2) cultural and (3) chemical.

Mechanical Weed Control

Mechanical weed control involves removing weeds

while they are small and preventing them from producing

mature seed. It consists principally of mowing, plowing,

rototilling, hoeing and hand pulling. These procedures

give immediate results and require little, if any, specialized

equipment. Their main disadvantage is that they work only

on growing weeds and may need to be repeated frequently.

Remove weeds while they are small because it is

quicker, easier and does less damage to desired vegetable

plants. Hoe or till shallowly (less than 2 inches deep) to

avoid damage to desirable plants and to minimize moisture

loss from the soil. The deeper the soil is disturbed, the

more weed seed will be brought to the surface where it will

grow. Most weed seed that germinate are in the upper 2

inches of soil. Weed seed may survive many, many years in

the soil. Each time it is worked, more seed is pulled to the

surface.

How do weeds harm gardens?

Weeds compete with crop plants for moisture, nutrients

and light. They may also harbor insects that harm ﬂ owers

or vegetables or transmit diseases to the crop. Weeds can

also serve as alternate hosts for diseases. They also pro-

mote diseases by increasing humidity, decrease vegetable

quality and make harvests difﬁ cult. Vegetable plants grown

under weedy conditions will have reduced yields, or they

may not survive to produce at all. A weedy garden or ﬂ ow-

erbed is also unattractive.

Why are weeds hard to control?

Weeds a re h ighly adapted to where they grow. They

have large, efﬁ cient root systems, grow rapidly and fre-

quently produce tremendous amounts of seed. Most weeds

tolerate drought and low fertility. Little of their energy is

Gardeners often quit weeding as a crop matures. Weeds

do not affect crop yields as much at this stage. However,

they will still produce seed. A single weed of some species

can produce hundreds of thousands of seed. Weed removal

should continue until the vegetable crop is completely har-

vested. While crops are not growing on the garden spot, the

soil may then be kept tilled, mowed or heavily mulched to

prevent weeds from propagating. Frequent tillage has the

additional advantages of turning organic material under

where it will add to soil organic matter and of mechanically

destroying any insects that are present.

increases of up to 25 percent have been documented. How-

ever, the results are not consistent enough to justify the

expense of the colored plastics.

Due to high temperatures and intense solar radiation,

the use of black plastic may cause damage to plants trans-

planted or seeded during the hottest part of the summer.

White plastic can be utilized during the summer months to

prevent root and stem damage to the plant. White plastic

will allow light to penetrate. Therefore, white plastic with

a black backing (known as white on black) is preferable.

White on black plastic is more expensive and may be dif-

ﬁ cult for home gardeners to locate in affordable quantities.

Another option is to whitewash or paint the black plastic

with white latex paint. The paint will eventually wear off,

but not until the plants have shaded the plastic around the

base of the plant, reducing the chance of damage.

Cultural Weed Control

Most cultural methods of weed control emphasize pre-

vention. These include mulching, solarization, and multiple

or wide-row planting.

Mulches

Any layer of material spread over the soil surface

may be considered to be a mulch. There are two classes of

mulch: organic and inorganic. Both reduce weed growth,

retain soil moisture and inﬂ uence soil temperature.

Hay, straw, sawdust, ground bark, leaves, compost and

even newspapers are among the many substances utilized

as organic mulches. All reduce weed emergence and make

it easier to pull those weeds that do emerge. Some weeds

such as nutsedge will grow through mulch, even through

plastic mulch. Extension factsheet SP 291-H, Mulching

Veget able Garden s , contains additional information re-

garding the use of mulches in home gardens.

Inorganic mulches include black and other opaque plas-

tic as well as landscape fabric. Landscape fabric is much

more expensive than plastic mulch. However, it has the

advantage of allowing water to pass through into the soil.

Black plastic (polyethylene) is the most commonly used

inorganic mulch. Clear or translucent plastic should not

be used as mulch. These plastics allow light to penetrate,

which permits germinated weed seed to grow under the

plastic mulch. Black plastic eliminates growth of most, but

not all, weeds. For example, yellow nutsedge will grow

through plastic mulch. Also, weeds will grow in the holes

cut in the plastic for the vegetables. More information re-

garding the proper time and method of application, as well

as the advantages and disadvantages of plastic mulches, can

be found in SP 291-H .

Opaque plastic in colors other than black is sometimes

used as mulch and will also control weeds. However, many

of these plastics are quite expensive. Some data suggests

that tomatoes grown on red plastic or that muskmelon

grown on blue plastic will produce higher yields. Yield

Solarization

Solarization is the use of clear polyethylene plastic

sheeting (2 to 6 mils thick) to capture the radiant energy of

the sun, thereby raising the soil temperature to levels lethal

to many weed seeds. The plastic sheeting is placed over

bare, moist soil during a summer fallow period. In Tennes-

see, best results are obtained by using two layers of plastic

sheeting and separating the layers with boards, bricks or

small squares (ca. 3” X 3”) of polystyrene insulation called

spacers. Spacers are placed every 1.5 to 2 feet on top of the

ﬁ rst layer of plastic. The second layer of plastic sheeting is

then placed on top. This creates an air gap between the two

layers, which provides a reservoir of warm air that mini-

mizes nighttime cooling. The two layers of plastic sheeting

are then sealed together by placing soil along the outer

edges of the top layer.

The top layer of plastic should be kept clean to maxi-

mize solarization efﬁ ciency. An occasional dusting with a

dust mop or rinsing with a garden hose should be all that

is necessary. The plastic is left in place for 4 – 5 weeks.

During this period, soil temperatures are frequently raised

to 120 – 125 degrees F. This practically eliminates viable

weed seed in the top 2 to 3 inches of soil. After the plastic

is removed, care must be taken not to mix the deeper layers

of unsolarized soil with the nearly weed-free solarized soil.

Rototilling no deeper than 1 – 2 inches is recommended.

As long as the soil is not rototilled deeper than this, weed

control by soil solarization can signiﬁ cantly reduce weed

infestations for 12 months or more. Many soil-borne plant

pathogens will also be reduced during the solarization pro-

cess. This improves plant stands and vigor and may double

yields.

The best time to solarize soil in Tennessee is from May

through early August. Two o r t h r e e p e r io d s , each consisting

of several days in a row of warm weather and bright sun-

shine, are necessary. Late summer and fall vegetables may

be planted the day after removing the plastic.

centrates (EC), ﬂ owables (FL), wettable powders (WP)

and dry ﬂ owables (DF). All are designed to be mixed

with water and sprayed on the area to be treated. Wet-

table powders and dry ﬂ owables may settle out unless

the sprayer is shaken periodically. Herbicides may also

be formulated as granules (G). These are to be spread

evenly over the soil surface.

Multiple or wide row planting

Closely spaced double or triple rows may also assist in

weed control. Simply plant two or three rows of a vegetable

close enough so that the leaves will cover the area between

them rapidly as the plants grow. Very small vegetables such

as radishes may be broadcast in a long row a foot or so

wide. These techniques allow growing vegetables to shade

the soil, which reduces weed growth. They are discussed

in more detail in the publication PB 901, Growing Veg-

etables in Home Gardens .

3. Plan the garden in detail. Plan to locate all the crops for

which a speciﬁ c herbicide may be used near each other.

This allows treatment of larger areas with less effort.

4. Follow all instructions on the label. This is extremely

important. Failure to follow the label instructions

precisely may result in harm to the applicator, the envi-

ronment or the crop. Preemergence herbicides require

weed-free soils without lumps or clods. Most are best

applied to moist soil and shallowly incorporated by till-

ing, irrigating or natural rainfall. They must be applied

uniformly and at the proper rate to be safe and effective.

Postemergence herbicides may be applied over the top

of the growing crop and weeds. Again, it is essential

that all aspects of the label directions be understood and

followed. If you have any questions, consult your county

Agricultural Extension agent.

Chemical Weed Control

Chemicals (herbicides) are only occasionally used by

home gardeners to control weeds because suitable herbi-

cides are seldom legally available in small, economical

amounts.

There are also several other difﬁ culties with the use of

herbicides by home gardeners. Many herbicides are non-

selective and will kill the vegetables, as well as the weeds.

Others are selective and can be used only with certain

vegetables or control only certain weeds. Home gardens

generally contain many vegetable and weed species, which

makes using selective herbicides difﬁ cult. Some herbicides

may also damage nearby vegetables or remain in the soil

and damage future plantings.

Even if available and effective, herbicides may not be

legal for use on a speciﬁ c vegetable or at the time when

they are needed. The herbicide application rate may be very

low and extreme accuracy in application may be absolutely

essential. Overlapping applications may kill vegetable crops

and, if areas are skipped, weeds will not be controlled.

Required pre-harvest intervals (PHI's) or waiting periods

between application and harvest can be lengthy and must

be observed. Herbicides may also be effective only for a

short period of time or produce results slowly.

Despite all these problems, there are occasions when

herbicides may be successfully used in home gardens. The

following suggestions will assist home gardeners in effec-

tively using herbicides.

5. Rinse spray equipment. Residual herbicide in sprayers

may damage crops. Many gardeners who use herbicides

purchase spray equipment for herbicides only and keep

it separate from equipment used for insecticides and

fungicides.

6. Calibrate application equipment accurately. Inaccurately

applied herbicides may be ineffective or dangerous. If

there is a question on how to apply a speciﬁ c herbicide,

contact your local county Agricultural Extension ofﬁ ce.

The two most suitable herbicides for use on home gar-

dens are Triﬂ uralin (Treﬂ an ™ ) and Sethoxydin (Poast ™ ).

Triﬂ uralin is sold in many formulations, each designed

for speciﬁ c uses. Triﬂ uralin prevents the germination of

most grasses and some broad-leaved weeds (for several

weeks) and must be applied before these weeds germinate.

Sethoxydin kills growing grasses. It requires only a short

waiting period between application and harvest and may be

applied to a wide range of vegetables.

Proper use of herbicides may involve grouping vegeta-

bles according to the herbicide that may be applied to them,

as well as paying attention to rate, timing, uniformity and

method of application. Remember, label directions must

always be read and followed.

1. Understand the difference between preemergence and

postemergence herbicides. Preemergence herbicides are

effective only before weeds germinate. Postemergence

herbicides work on weeds that are actively growing.

2. Understand the different formulations of herbicides

available and how the formulation affects use. Some of

the more common formulations are emulsiﬁ able con-

The Stale Seedbed Technique

The stale seedbed technique is a system that controls

weeds prior to the planting of the crop. The theory is that

most weed seed that germinate are found in the top 2 to 3

inches of soil. With soil temperatures of 70 degrees F or

higher and moist soil, most of the weed seed in the top 2

inches of the soil will germinate in 2 to 4 weeks after a till-

age operation. Therefore, a generalized procedure follows.

3. Once most of the weeds have germinated, kill the

emerged weeds. This can be done by using a non-

selective herbicide such as glyphosate (Roundup) or by

performing a shallow tillage (less than 2 inches).

4. Then seed or transplant the crops with minimal soil

disturbance.

Since warm soils are required for this technique to

be effective, it generally is not used for spring-planted

gardens. It can be very effective when utilized prior to the

planting of a fall garden.

1. Broadcast fertilizer and incorporate with a ﬁ nal tillage.

2. Allow the weed seed to germinate for 2 to 4 weeks.

Precautionary Statement

To p r ot e ct p e ople and the environment, pesticides should be used safely. This is everyone's responsibility, espe-

cially the user. Read and follow label directions carefully before you buy, mix, apply, store or dispose of a pesticide.

According to laws regulating pesticides, they must be used only as directed by the label. Persons who do not obey

the law will be subject to penalties.

Disclaimer Statement

Pesticides recommended in this publication were registered for the prescribed uses when printed. Pesticide reg-

istrations are continuously reviewed. Should registration of a recommended pesticide be canceled, it would no longer

be recommended by the University of Tennessee. Use of trade or brand names in this publication is for clarity and

information; it does not imply approval of the product to the exclusion of others which may be of similar, suitable

composition, nor does it guarantee or warrant the standard of the product.

SP291I-5M- 6/03(Rev) E12 -5115 - 00 - 021- 03

The Agricultural Extension Service offers its programs to all eligible persons regardless of race,

color, national origin, sex, age, disability, religion or veteran status and is an Equal Opportunity Employer.

COOPERATIVE EXTENSION WORK IN AGRICULTURE AND HOME ECONOMICS

The University of Tennessee Institute of Agriculture, U.S. Department of Agriculture,

and county governments cooperating in furtherance of Acts of May 8 and June 30, 1914.

Agricultural Extension Service Charles L. Norman, Dean

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