MANAGEMENT ISSUES
• Marestail emerges very early in the spring (=Weed Group 0) and is present before crops are planted
(GDD17 <150 (base 48 °F).
• Marestail tolerates drought conditions well and continues to grow and produce seed under environmental
conditions too stressful for growth of many other plants,11 preferring undisturbed soil. Conversely,
marestail is not very shade-tolerant.(7)
• One mature plant produces an average of 200,000 wind-blown seeds that can be dispersed long
distances and are capable of traveling distances >¼ mile.4 One study found 125 seeds per sq. yd. at 400
feet away from the seed source.(7)
• Seeds do not require a period of dormancy to germinate, making it difficult to predict emergence. Mature
seed falling to the soil during late summer/fall can sprout as soon as adequate moisture becomes
available (usually with the first rains).(4)
• Marestail seedlings germinate earlier than other winter annuals – hence pre-emergent herbicides must be
applied by early August. Fall seedlings that overwinter can be more difficult to control than those that
emerge during early spring.(11)
• Removing mature plants (to prevent production of seed heads) helps considerably in long-term control,
particularly to prevent new weed introduction from neighboring fields.
• Herbicide effectiveness against horseweed significantly lessens after plants are >6” tall and the stem
begins elongating/bolting.(14)
• Marestail can be a host for the tarnished plant bug(11) and a host for aster yellows (a viral disease
transmitted by leafhoppers to several ornamental plants).
CONTROL
Cultural Control
• Use only well composted organic amendments from certified composting facilities that are considered to
be weed-free.
• Use weed barriers/Geotextile ground fabrics under containers as a weed free barrier in the greenhouse or
nursery container areas.
• Mulch landscape beds to control germinating marestail by depriving them of light. For bedding plant
areas that are to be transitioned two or three times a year, use a fine-textured organic mulch.
• Solarize the area to heat the soil in an effort to kill weed seeds.
• Rototill beds in the summer under long days and peak sunlight and temperatures, and then cover with
clear plastic for 6-8 weeks.
Mechanical Control
• Remove marestail seedlings when they are small, before they flower and go to seed. Hoeing, hand
weeding, or tilling can be effective, especially in container nursery crops, and is the only option for
removing large weeds.
• Remove larger weeds entirely, since marestail can re-sprout from broken stems.11 Cut the plant root
below the soil surface.
• Control weeds in adjacent fields/beds12 to prevent seed spread/new weed introductions.
• Tillage equipment can spread marestail in field nurseries. Clean equipment (tillers, tractors) before use
or between fields with high pressure water or compressed air, to prevent spreading cut vegetative pieces/
propagules.(1)
Biological Control
• A strain of the bacterium Xanthomonas campestris, was shown to be effective in controlling
glyphosate resistant marestail in greenhouse experiments.
• The USDA has a research project currently studying the effect of the fungal plant pathogen,
Myrothecium verrucaria on marestail in corn, co-applied with a surfactant as a directed spray.3
Chemical Control*
The timing of herbicide applications is extremely important in order to achieve acceptable control of marestail. Herbicides should be applied when weeds are small and most vulnerable to control (preferably in the seedling or rosette stage --- < 4” tall, and prior to flowering). Since marestail can germinate in the fall and in the spring, it may be necessary to split pre-emergent treatments to catch the different germination periods.
PRE-EMERGENT HERBICIDES: Follow label directions.
Pre-emergent herbicides are applied before weeds emerge. Because marestail seeds germinate before other winter annuals, a commercial pre-emergent treatment may not fit into other “scheduled” commercial herbicide application times. Fall pre-emergent treatments for marestail should be applied by early August to prevent seed germination in the fall. Spring pre-emergents should be applied by mid-to-late March.12
In a container nursery production cycle, herbicide application timing is important: 1) during liner propagation, 2) site preparation before setting containers on ground, 3) at potting, 4) approximately one month after potting.
SureGuard (flumioxazin) offers effective pre-emergent control of glyphosate resistant marestail.5 In the nursery, avoid over the top applications with most plants (check the label) It is safest to apply SureGuard to one year old and older spaced tree transplants with no green bark on the lower stem.
Annual landscape flower beds: Apply pre-emergent herbicides after transplanting to weed-free soil. Mulch the bed before herbicide application if possible, applying herbicide to moist mulch. Granular formulations of herbicides are safer to bedding plants than spray formulations. Bedding plant foliage should be dry at the time of application to prevent herbicide granules from sticking to the foliage. Irrigate after treatment to wash granules off of the plants and to “activate” the herbicide.
Greenhouse: Pre-emergent herbicides should be applied by early to mid August (or prior to applying the poly coverings to the overwintering houses).12
For nursery/greenhouse growers, herbicides labeled include Barricade, SureGuard (flumioxazin), and Snapshot. Freehand 1.75 G (1% pendimethalin and 0.75% dimethenamide) reportedly works well for summer pre-emergent applications.
POSTEMERGENT HERBICIDES: Follow label directions
Post-emergent herbicides are applied to germinated weeds. Marestail is most effectively controlled in the seedling/rosette stage. Residual herbicides applied at these stages can provide control through early June.11 Alternatively, burndown herbicides can be applied up to when plants are 4-6” tall (with < 21 leaves11; mid-to-late May), or before weeds reach the stem elongation (bolting) period. Postemergent herbicides applied to marestail should contain three to four sites (mechanisms) of action to manage glyphosate-resistant marestail.13 Do not rely solely on glyphosate to control emerged marestail due to resistance issues.
Pre-planting (for bed preparation): Options for fall or early spring herbicide applications include Paraquat, dicamba, 2,4-D LV ester, and glyphosate to control small newly germinated weeds. Use at least two different herbicide modes of action14 in burndown applications). Fall-applied (mid-October to mid November) postemergent herbicides may provide more effective control of emerged marestail than spring-applied herbicides16. Multiple burn-down applications consist of both fall and spring treatments. Fall applied herbicides may not adequately control spring-emerging plants;11 spring applications prior to early May should also include a residual herbicide to control later-emerging plants.
Roadsides/Rights-of-way:
Aminopyralid (Milestone). Do not allow drift to desirable vegetation. Many desirable broadleaf plants can be seriously injured or killed. Do not exceed 7 fl oz/A Milestone per year.
Clopyralid (Reclaim, Stinger, Transline, Confront, Lontrel, Curtail, Millenium Ultra); residues in grass clippings, straw, leaves, manure and bedding, and finished composts have been found to cause injury to other plants (http://ohioline.osu.edu/aex-act/0714.html)
Dicamba (Banvel, Banex, Trooper)
*Disclaimer: Complete reliance on any one herbicide may lead to resistance and population shifts. The listed chemicals exhibit activity against this weed; however, chemicals exhibiting activity will not always provide complete control. Tank-mixing more than one chemical often will improve efficacy, as will delivering sequential applications of a single chemical. Always use pesticides according to directions on the label.
The information given herein is supplied with the understanding that no discrimination is intended and no endorsement by University of Maryland Extension is implied. Read labels carefully before applying any pesticides.
