Page 132 - AG 7-2011 Revised 2016

MID-SUMMER AND ALL SEASON WEED CONTROL IN ESTABLISHED LANDSCAPES
By: Gary King Ph.D.

In the heat of the summer, July through September, you may need follow-up applications of pre-emergent
herbicides. Weeds that have germinated must be stopped early on before they flower and seed or form resistant
structures. Considerable money can be saved with sprays to kill newly sprouted weeds and avoid hand weeding
which can be 2 to 10 times more expensive than other weed control methods.

Annual weed species may be easily controlled with one of the contact herbicides that will simply “burn” them
down to the ground. Perennial weeds that have grown enough to produce the resistant vegetative structures
(taproots, rhizomes, tubers, nutlets, and stolons) frequently re-sprout after the contact material is used. Contact
“burn-down” types of herbicides are insufficient on biennial or perennial weeds unless sprayed at an early, more
susceptible stage. Full control will normally require translocated types of post-emergent herbicides. Most profes-
sionals will simply choose the translocated post-emergent herbicides to be sure they get complete kill on nearly
all the weeds they encounter.

Tips and Techniques for Spraying to Kill Existing Weeds

Directed sprays of nonselective contact or translocated herbicides must be applied to the weeds without allowing
wind drift or other contact onto the landscape ornamentals. Coarse-droplet flat fan sprays will be safer at lower
pressure when working among complex plantings of ornamentals. Avoid spraying in any wind or using higher
pressure that produces a mist.

Lightweight plastic or cardboard shields can be used to protect ornamentals in close quarters, where a non-selec-
tive weed spray is required. Weed wicks or sponges wetted with herbicide solution can be wiped onto weeds for
very specific spot treatment.

Weed killing sprays should include suitable adjuvants, such as spreader and penetrants to increase the herbi-
cide activity. The adjuvants are normally much cheaper than the herbicide, but they help wet the leaves, spread
the spray droplets, increase penetration into the leaf, buffer the spray mix against alkaline water, and sometimes
reduce spray drift. When weed leaves are waxier and the cuticle is thicker, more adjuvant is needed for penetra-
tion. Too much adjuvant could cause ornamental damage when applying a selective herbicide, so follow label
directions carefully.

Temporary, biodegradable marker dyes are valuable aids to ensure that a complete spray job is done and that
double applications are avoided. Unintended drift or misapplication to ornamentals can be identified and the
chemical can be immediately washed off to avoid disaster.

Many of the herbicide sprays become more potent when the spray mix is acidified, especially for alkaline water in
California. This can be adjusted to around pH6.5 using a slight amount of acid or soluble fertilizer. Many soluble
fertilizers add buffering capacity that resists further changes in the acidity of the spray mix. They may decrease
the surface tension of water to facilitate leaf wetting. Fertilizer stimulates growth which aids the effectiveness of
the translocated herbicides.

When spraying weeds that have already formed flowers or seed, a liquid pre-emergent herbicide may be mixed
with the post-emergent product, if compatible, in order to spot spray the place where the dying weed will most
likely drop most of the seed. This may prevent much re-seeding where hand weeding cannot be done.

Depending upon the particular chemistry, some products may be applied to leaf, some others are applied to soil
to be taken up and translocated to kill the roots or growing points of the weeds. If the product is non-selective, it
may kill any plants that it enters in sufficient amounts.

Notes:
1. Use a committed sprayer for growth regulator or growth hormone herbicides.
2. Use caution with broadleaf herbicides for turf; some of these products can damage trees or shrubs if used
near their roots.
3. Withhold overhead irrigation for 24 hours, if possible, after spraying post-emergent herbicides.

ADDITIONAL WEED CONTROL SECTIONS: IDENTIFICATION- PAGE 100
MGMT BEFORE PLANTING- PAGE 112 EARLY SPRING- PAGE 114
AT TIME OF PLANTING- PAGE 118 LATE SPRING- PAGE 122
IN ESTABLISHED AREAS- PAGE 124 FALL- PAGE 134

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