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Trade Name: Fortress Common Name: isoxaben + dithiopyr Formulations: 0.75GR (0.5 + 0.25%)

Preemergence control of many annual broadleaf weeds and annual grasses in container or field grown nursery crops, and some field grown cut flower plantings. May be used on many woody and herbaceous ornamental species. Not for use in landscape plantings.

Amount of Active Ingredient Amount by Formulation*
Per Acre 1.125 lb (0.75 lb isoxaben + 0.375 dithiopyr) 150 lb
Per 1,000 sq ft 3.44 lb
* Note: do not exceed two applications per year

Major Weeds Controlled

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Broad spectrum control of annual broadleaf and grass weeds including bittercress, spurge, chickweed, henbit, groundsel, annual bluegrass, crabgrass, carpetweed, and many others.

Major Weeds Not Controlled

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Perennials and established weeds. Weak on dogfennel, eclipta, phyllanthus, doveweed, morningglory and rice flat sedge. Does not control nutsedge.

For Best Results

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Apply to dry foliage and shake plants to remove granules from growing points. In newly potted or planted crops, the soil or substrate should be settled by irrigation before herbicide application. In established container crops, hand weed before treatment and irrigate immediately after application.

Cautions and Precautions

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Do not apply to new transplants until soil has settled and there are no cracks. Do not apply to nursery seedbeds or transplant beds or to unrooted liners or cuttings. Delay applications to bare-root transplants for three weeks. Do not use in enclosed structures (such as greenhouses, poly-covered overwintering houses, or similar). Do not apply more than a total of 300 lb per acre per year. Use is prohibited on several species, check label for details and additional precautions.

Residual Activity

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Duration of weed control varies with management practices and prevailing weather conditions. In cool climates or seasons, longer residual is expected than in warm climates or seasons. In containers, residual weed control rarely lasts more than 60 days in the southeastern United States. Shorter residual is to be expected in mid-summer, longer residual in cooler weather.

Volatility and Leaching Potential

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Isoxaben is nonvolatile. Dithiopyr is somewhat volatile; therefore, the herbicide should be incorporated by irrigation or rainfall soon after application. Isoxaben leaches very little; dithiopyr leaching is negligible.

Symptoms and Mode of Action

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Isoxaben inhibits cell wall biosynthesis and appears to disrupt root and hypocotyl development. Mode of action group 29

Dithiopyr affects root growth by inhibiting cell division. Mode of action group 3

Additional Information

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Worker Protection Standards REI: 12 hours
PPE: applicators must wear long-sleeved shirt, long pants, waterproof gloves, socks, and closed shoes
Manufacturer: OHP EPA Reg. # 59807-19


Professor of Weed Science, Extension Specialist & Department Extension Leader
Horticultural Science

Find more information at the following NC State Extension websites:

Publication date: May 22, 2019
Revised: Feb. 13, 2024

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