All plant descriptions and control methods courtesy University of Nevada Cooperative Extension
Hoary cress, or whitetop, is a member of the mustard family. This perennial weed commonly grows in dry, disturbed, alkaline soils, but can grow in a variety of soils and moisture conditions. Hoary cress is highly competitive; it reduces forage production and displaces native vegetation because it starts growing early in the spring and uses the available soil moisture other plants depend upon. In Nevada’s desert, its expansive root system collects any available springtime surface water. Hoary cress grows from both seed and root fragments. Its deep root system, vegetative reproduction, and exceptional seed production make this plant difficult to control. Like many members of the mustard family, hoary cress produces a significant percentage of hard seed with delayed germination, which ensures seeds are available for germination for many years. Glucosinolates in hoary cress can be toxic to cattle.
Distinguishing features:
CONTROL MEASURES
Most Effective Strategy:
Hoary cress is difficult to control once established.
Research and experience have shown that the
most effective treatment is a combination of herbicides and cultural controls.
Cleaning equipment and using weed-free hay
limits the number of infestations. Finding and controlling hoary cress during
its first year is the best management strategy.
Metsulfuron or chlorsulfuron are effective at killing hoary cress, but these
chemicals remain active in alkaline soils for many
years, making establishment and maintenance of competitive vegetation difficult
after treatment and limiting the effectiveness
of these herbicides over the long term. If the infestation is on highly alkaline
soil, use repeated applications of 2,4-D coupled
with efforts to grow competitive vegetation. This must include seeding with an
appropriate species for maximum production
of competitive vegetation. A long-term effort over many years is required to
have any effect on an infestation. Missing even
1 year of control severely reduces the effectiveness of years of control
efforts.
Mechanical Control:
Small infestations can be controlled by digging and
completely removing the plant, including the roots,
within a few days after emergence. This must be continued until no new seedlings
emerge, at least 4 to 6 years or more. Do
not till or mow an established plant stand. This stimulates the rhizomes to grow
new plants.
Cultural Control: Moderate grazing by sheep or goats in the rosette stage provides some control by lowering hoary cress density and preventing flowering. Do not graze later in the season, since this will stimulate vegetative growth. Hoary cress may be toxic to cattle. Planting competitive legumes such as alfalfa or clover can reduce an infestation. Flooding is effective on heavy soils, but hoary cress must be completely submerged for weeks to kill it, which may also kill desirable vegetation. Flooding also reduces land fertility due to the leaching or erosion of nutrients from the soil.
Biological Control:
There are currently no biological controls available for
hoary cress.
Read and follow the label. It is the law!
Chemical Control:
2,4-D amine or ester
Rate: Use 2 pounds a.e. per acre.
Time: Apply 2,4-D in the spring prior to flower bud formation. Repeated
herbicide applications may be necessary during
the growing season whenever green leaf material is present (every 4 to 6 weeks).
When hoary cress is present in
cereal crops, apply 2,4-D before the cereal crop reaches the boot stage.
Remarks: When possible, use 2,4-D on hoary cress populations before cultivating
fields in the spring. Reapply 2,4-D in the fall
if new weed growth occurs.
Caution: It is not lawful to apply more than 2 pounds a.i. per acre per year.
Avoid drift to crops, especially with a volatile
ester formulation. Follow the label directions. Do not contaminate water unless
using a product specifically
formulated for use in water.
MCPA
Rate: Use 6 pounds a.i. per acre.
Time: Apply from the bud to early bloom stages of growth and again in the fall.
Remarks: Apply when weeds are young and continue treatment for 2 to 3 years.
Apply in 20 to 100 gallons water carrier per
acre.
Caution: Do not plant crops for 3 months following treatment. Use according to
label instructions.
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