Grower 101: Managing Weeds in Outdoor Cut Flowers

January 28, 2003 - 13:59

Don't let weeds take much-needed water, nutrients and light from your annuals, biennials and perennials. Find out how to stop them.

Weeds compete for water, nutrients and light, resulting in
reduced flower yield and increased threat of serious insect and disease
problems. A successful weed management program utilizes cultural practices such
as cultivation and mulching, or a combination of cultural and chemical
measures, taking into consideration labor costs and the cost and availability
of materials.

Weed management begins with a survey of the site. Weeds
should be identified, and the level of weed pressure noted. Weeds can be
classified according to their life cycles; knowing the weed life cycle is
important in determining the optimal timing for cultural management practices
or herbicide applications. Summer annuals emerge in the spring, flower
throughout the summer and set seed before the first frost. In cultivated
fields, summer annuals tend to predominate as the primary weed type. Winter
annuals germinate at the end of the summer or early fall, overwinter, then
flower and fruit in the summer. Biennials need at least two seasons to complete
their life cycles, and like annuals, biennials die after flowering. Perennials,
which survive more than two seasons, can propagate by seed or vegetatively.
Vegetative reproductive organs such as tubers, rhizomes (underground stems),
stolons (above-ground creeping stems), bulbs and corms are often resilient to
both cultural and chemical control measures and should be targeted for control
before planting the field. It is also important to scout weed populations
during and after the growing season to assess the success of the weed control
program.

Herbicides are available that may be safely used to control
weeds in cut flowers. However, in many situations, herbicides cannot be used or
are not effective in controlling all the weeds. Even if effective herbicides
are available, growers should utilize cultural practices that reduce weed
infestation and spread. These practices will be especially important where
herbicides cannot be used.

Cultural Practices

Field Preparation.
Minimize weeds by preparing the field the year prior to planting. Begin by
frequently tilling the soil 2-3 times early in the season until mid-July.
Frequent tillage reduces weed seed populations by bringing weed seeds to the
surface where they will germinate, grow a little and then be tilled under.
Allow weeds to germinate and grow from July until fall, then apply glyphosate
(Roundup-Pro) over the area. This will kill many perennial weeds that are best controlled
in the early fall when nutrients are being stored in the root system. The field
is now ready for planting.

Cultivation and Spacing.
Some growers have minimized weed competition with a combination of early
cultivation and narrow, between-row spacing. This can be effective if the crop
gets a head start on the competition. If the crop's growth is impeded in any
way, the weeds will take over. Regular cultivation can limit weed competition
between rows. However, cultivation can injure the roots of some cut flowers,
contribute to erosion and result in some water losses due to increased
evaporation. Also, deep cultivation tends to bring additional weed seeds to the
surface where they will germinate, grow and compete with crops. For best
results, cultivation should always be shallow and directed at young weeds to
limit the destruction of crop roots. In-row cultivation is particularly
difficult and typically requires hand-hoeing or hand-weeding.

Mulches. Mulches can
effectively control most annual weeds from seed. According to Dr. Andrew
Senesac of the Long Island Research Laboratory, black plastic or a geotextile
fabric such as Weedblock can control most annual weeds around field-grown
herbaceous perennials. In some studies, however, due to the physical restriction
of the spreading shoots, these mulches reduced Achillea and Stachys flower
production. Spreading perennials that do not produce adventitious roots did not
show any significant yield differences from the controls. Organic mulches such
as bark, straw and composted yard wastes effectively control many annual weeds.
Some growers use rotted sawdust, wood chips, spoiled hay and straw. If not
composted properly, sawdust and wood chips will rob the soil of nitrogen. Bark
mulches can be used but are often too costly. Hay generally contains too many
weed seeds and often increases the weed pressure. Clean, weed-free straw is
often the most cost-effective mulch available, but some growers may find other
economical alternatives. Organic mulches should be applied to weed-free, warm
soil soon after planting. To be most effective, they should be applied in a
layer 2-3 inches deep.

Chemical Control -- Herbicides

Herbicides can be classified into two general-use
categories: pre-emergent and post-emergent. Pre-emergence herbicides are
applied before the weeds germinate but after the crop has been planted.
Post-emergence treatments are applied after the weeds have emerged. Herbicides
may also be classified based on their selectivity. Non-selective herbicides
will control any herbaceous plants that they contact. Selective herbicides will
control or suppress only certain types of plants or weeds.

Herbicides are available in several formulations. Usually,
the sprayable formulations (emulsifiable concentrates, wettable powders, dry
flowables and water-dispersible granules) are less expensive than granular
formulations. But granular formulations are often safer on transplanted cut
flowers than are the sprays (due to reduced foliar absorption). Sprayable
formulations can be applied through a tractor-mounted sprayer or by hand-held
backpack sprayers equipped with a spray boom. With a backpack sprayer, maintain
a constant foot pace, even spray pressure and uniform nozzle sizes. Regardless
of the formulation or equipment used, it is important to apply herbicides as
uniformly as possible.

When applying an herbicide, the square footage of the area
to be treated and the calibrated sprayer/spreader output (amount per area)
should be known. Misapplication of the chemical can result in poor weed control
or injury to the crop. Read and follow all label directions before applying any
chemical. A sprayer that is to be used for herbicides should be labeled as such
and used only for that purpose.

The following is a partial listing of herbicides that can be
used in cut flower production. Because of the wide variety of cut flower
species grown, it is difficult to recommend any one herbicide that will cover
all crops. Due to labeling restrictions, possibility of crop injury, limited
market and difficulty in obtaining new labels, many chemical companies do not
actively pursue cut flower labels. It is the user's responsibility to follow
label instructions, especially prior to purchasing a product. Some labels
provide scientific names and common names; some list common names; and some
provide a combination.

Nonselective, post-emergence herbicides

Nonselective herbicides can be used to control weeds in a
field prior to planting or to spot-spray weeds growing between crop rows. Care
should be taken in selecting an herbicide to ensure that there will be no
residual chemical present that could damage the crop. Chemicals that would be
used for this purpose include glyphosate (Roundup-Pro), glufosinate (Finale),
paraquat (Gramoxone), diquat (Reward) and pelargonic acid (Scythe). Do not
apply these herbicides over the top of cut flowers; plants will be injured or
killed.

Glyphosate (Roundup-Pro).  Glyphosate is absorbed by
green tissue and translocated to the root system of the plant. Since there is
no residual soil activity, a crop can be seeded or transplanted into the field
soon after application. Actively growing weeds are much more susceptible to the
herbicide. Many perennial weeds are best controlled in the early fall when
nutrients are being stored in the root system. Glyphosate is effective for
field preparation to control perennial, broadleaf and grass weeds.

Glufosinate (Finale).
Glufosinate is also a translocated herbicide, but not as well-translocated as glyphosate
in perennial weeds. Like glyphosate, it has no soil residual activity and can
be used as a site preparation treatment or as a spot spray to control escaped
weeds. Since glufosinate is not as well-translocated as glyphosate, complete
spray coverage is essential to obtain maximum control, and good control of
perennial weeds may not be achieved.

Diquat (Reward), Paraquat (Gramoxone) and pelargonic acid
(Scythe).
These are contact herbicides
(i.e, they kill foliage on contact but are not translocated in the plant) and
have no residual soil activity. They will suppress many species of annual
grasses and some broadleaf weeds. Repeated applications may be needed to weaken
and suppress perennial weeds. Complete coverage is essential.

Selective, post-emergence herbicides

Sethoxydim (Vantage), clethodim (Envoy) and fluazifop-p
(Fusilade II) control most annual and perennial grasses, while
fenoxaprop-p-ethyl (Acclaim Extra) controls summer annual grasses. They can be
applied over the top of many broadleaf crops when grasses are actively growing
and before they reach maximum size. When applied to a labeled cut flower crop,
all open flowers should be harvested before application to avoid injury. Do not
use spray adjuvants with Vantage. With Envoy and Fusilade II, use only the
spray adjuvants specified on the labels. Use of non-labeled spray adjuvants may
result in contact burn on cut-flower foliage and buds. Additionally, to avoid
over-dosing and associated crop damage, these herbicides should be applied on
an area basis, not a spray-to-wet basis.

Some flowers on the Vantage label include antirrhinum,
centaurea, chrysanthemum, dahlia, Dianthus barbatus, Dianthus deltoides,
gladiolus, gypsophila, iris, physostegia, rudbeckia and tagetes; plus some
varieties of aster, celosia, coleus, gerbera, lavandula, limonium, salvia and
zinnia.

Some flowers on the Fusilade II label include achillea,
ageratum, Alcea rosa, antirrhinum, calendula, campanula, coleus, Coreopsis
verticillata, Dianthus barbatus, iris, heuchera, liatris, sedum, Statice
sinuata, tagetes and zinnia.

Envoy is labeled for over-the-top applications to achillea,
ageratum, antirrhinum, chrysanthemum, coleus, dahlia, dianthus, gazania,
heuchera, iris, pansy, phlox, salvia, tagetes and some varieties of zinnia. Envoy
is the only post-emergence selective grass herbicide that controls annual
bluegrass (Poa annua).

Acclaim is more effective on young, actively growing grassy
weeds than on large grassy weeds. Some flowers on the Acclaim label include
achillea, antirrhinum, aquilegia, astilbe, campanula, centaurea, chrysanthemum,
coreopsis, cosmos, delphinium, dianthus, dicentra, doronicum, echinacea,
gazania, gypsophila, iberis, iris, liatris, oenothera, paeonia, papaver,
rudbeckia, Statice sinuata and zinnia.

Pre-emergence herbicides

To prevent seedling weeds from emerging in a crop, a
pre-emergence herbicide may often be used. Several pre-emergence herbicides are
available for controlling annual grasses and small-seeded broadleaf weeds, but
large-seeded broadleaf weeds are not as easily controlled. Careful weed
scouting can identify hard-to-control species and facilitate the selection of
the most effective herbicide for the crop. If pre-emergence herbicides are to
be used, be sure they are labeled for use on the crop to be grown. Also, in a
mixed field of cut flowers, all species being grown should be listed on the
herbicide label.

Cut flowers are usually started from transplants, divisions
or tubers, but sometimes they are grown in the field from seed. Generally, pre-emergence
herbicides should be applied after transplanting. Most direct-seeded flowers
are more susceptible to damage from pre-emergence herbicides than transplanted
seedlings. To achieve the same level of safety, the herbicide usually Á
should not be applied until after plants emerge and are established. Each of
the herbicides described below should be watered-in to "activate," or
move, the herbicide into the soil where it can be absorbed by germinating weed
seeds.

Bensulide (Bensumec 4LF). This controls crabgrass, annual bluegrass, other annual grasses and a
few broadleaf weeds for 3-4 months. Ornamentals need to be well-established
before the application of bensulide. Some labeled flowers include aster,
bachelor's button, calendula, campanula, coral bells, daffodil, dahlia, daisy,
freesia, gazania, gladiolus, marigold, pansy, primrose, ranunculus, stock,
sedum, sweet pea, tulip, wallflower and zinnia.

Dithiopyr (Dimension).
It is primarily used to control crabgrass in turf but is labeled for annual
grass and small-seeded broadleaf weed control in several flowers, including
achillea, antirrhinum, aquilegia, celosia, centaurea, coleus, coreopsis,
delphinium, dianthus, iberis, iris, monarda, narcissus, osteospermum, pansy,
rudbeckia, salvia, sedum, tagetes, tulip, zinnia and others.

Napropamide (Devrinol).
This will control certain annual grasses and annual broadleaf weeds. Flowers on
the label include African daisy, aster, chrysanthemum, dahlia, daisy,
gladiolus, narcissus and zinnia. In field trials, high rates caused yield
reductions in zinnia and marigold. For effective control, the chemical must be
watered-in after application.

Oryzalin (Surflan A.S. Specialty). style='font-weight:normal'> This chemical controls most annual grasses and many
annual broadleaf weeds and should be applied only to established plants.
One-half-inch of rainfall or irrigation is needed to activate oryzalin. Flowers
on the label include aster, Callistepheus chinensis, campanula, digitalis,
doronicum, gaillardia, heuchera, iberis, limonium, osteospermum, sedum, achillea,
antirrhinum, coreopsis, dianthus, dicentra, echinacea, gladiolus, gypsophila,
hyacinth, iris, liatris, narcissus, ranunculus, rudbeckia, salvia, tagetes,
tulip and zinnia. However, severe injury has been observed on transplanted
celosia, begonias, gomphrena, salvia, phlox and several other species. Surflan
XL is a granular formulation containing oryzalin plus benefin that, in research
trials, has been safer for transplanted herbaceous ornamentals than
spray-applied Surflan.

Trifluralin (Treflan 5G). This herbicide controls annual grasses and a few broadleaf weeds for
about 6-8 weeks. It is volatile and must be incorporated by irrigation
immediately after application. The granular formulation is more often used to
reduce vapor losses. Treflan is probably the safest herbicide on transplanted
cut flowers discussed herein; however, it is the weakest on broadleaf weeds.
Flowers on the Treflan label include: achillea, ageratum, antirrhinum, aster,
calendula, centaurea, chrysanthemum, coreopsis, dahlia, dianthus, dicentra,
digitalis, echinacea, gaillardia, gypsophila, heuchera, iris, lavandula,
liatris, limonium, matthiola, monarda, oenothera, papaver, phlox, rudbeckia,
salvia, scabiosa, sedum, tagetes, zinnia and more.

Metolachlor (Pennant MAGNUM). This is another pre-emergence herbicide that controls annual grasses,
but its main use is for pre-emergent control of yellow nutsedge (Cyperus
esculentus) from tubers. Pennant does not control purple nutsedge and is
currently only available as an emulsifiable concentrate formulation that can
burn tender foliage. Pennant is labeled for use on achillea, ageratum, allium,
antirrhinum, aquilegia, artemesia, aster, campanula, chrysanthemum, coreopsis,
delphinium, dianthus, gaillardia, gladiolus, hyacinthus, iris, lilium, lythrum,
oenothera, ornithogalum, phlox, physostegia, pedum, Statice sinnata, tagetes,
tulips, zinnia and a few other species. Injury to gladiolus and zinnia has been
reported.

Herbicide labels can be obtained from the Web sites
www.cdms.net, www.bluebooktor.com/index.html and www.greenbook.net.

 

This article was reprinted with permission from Floral
Notes.

 

The author would like to thank Joseph C. Neal, North
Carolina State University, and Richard Bonanno and Randall G. Prostak,
University of Massachusetts Extension.

About The Author

Tina Smith is an outreach educator in the Extension Floriculture Program at the University of Massachusetts, Amherst, Mass.

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