Perennials Section: Managing Weeds in Perennial Production

Broch Martindale

With profit margins tightening and labor costs continuing to rise, growers must efficiently manage weeds to maintain high-quality crops and reduce losses. Herbicides play a critical role in this effort, particularly preemergence herbicides, which form a chemical barrier in the soil to prevent weed seeds from germinating. For many greenhouse growers, however, the use of herbicides can be intimidating and may be something they haven’t experimented with at all.

When properly used, these tools can significantly reduce weed pressure throughout the growing season and even into winter, ensuring cleaner crops, fewer labor inputs and better plant performance.

Weed challenges in perennial production

Weeds not only compete with crops for water, nutrients and light, but can also serve as hosts for pests and diseases—negatively affecting the appearance and saleability of the final product.

Perennials, unlike annuals, are grown over extended periods—often through multiple seasons—making them particularly vulnerable to multiple flushes of weed growth. This extended production cycle requires long-term weed management strategies.

In container production, the compact spacing of perennials makes hand-weeding inefficient and labor-intensive. Left unchecked, even a small weed infestation can escalate quickly, compromising crop health and increasing production costs. This is where preemergence products—such as Snapshot, Gallery and Dimension specialty herbicides—come in as foundational tools.

Snapshot: Broad-spectrum and long-lasting
Snapshot (trifluralin + isoxaben) is one of the most widely used preemergence herbicides in ornamental horticulture. It offers broad-spectrum control of over 100 broadleaf and grassy weed species, including oxalis, spurge and bittercress. These weeds, while present throughout the production cycle, will really be challenging in the spring on overwintered material. This often takes labor away from producing and shipping plants, and moves it to labor-intensive hand-weeding.

What you don’t want to see—weeds like bittercress germinating in containers. A well-executed preemergence program can help prevent germination and make for an easier spring season.

Snapshot offers a range of benefits to perennial growers, including:

Broad crop safety: Snapshot is labeled for use on a wide range of perennials and ornamental plants, making it a go-to for mixed production nurseries.
Long-lasting control: Depending on rate and conditions, Snapshot can provide six to eight months of weed control.
Versatile timing: Snapshot can be applied in early spring or late summer to address both warm- and cool-season weeds.

Snapshot works by inhibiting root development in germinating seeds, which makes precise application timing essential—ideally before expected germination windows. The granular formulation is easy to apply in container yards, hoophouses and field settings.

Gallery: Specialist for broadleaf weed control
Gallery (isoxaben) is a preemergence herbicide with excellent control of broadleaf weeds and minimal activity on grasses. It’s a valuable addition when grassy weeds aren’t the primary concern, but broadleaf control is critical.

The key advantages of Gallery include:

Selective control: Gallery has excellent activity against chickweed, spurge, oxalis, henbit and other problem broadleaves.
Long residual: Gallery provides four to eight months of control, depending on rate and environmental conditions.
Tank-mix and rotation-friendly: Gallery can be used in rotation with grass-controlling preemergence herbicides, such as Dimension, to achieve broad-spectrum control.
Flexibility: Gallery can be used on the ground bed to prevent weeds between pots.

Because Gallery is gentle on a wide range of ornamentals, it’s especially useful in sensitive or high-value crops for which broader-spectrum herbicides might pose a risk.

Dimension: Pre- and early postemergence power
Dimension (dithiopyr) offers a unique advantage in that it provides both preemergence and early postemergence control of annual grasses, particularly crabgrass—a notoriously difficult weed in container production.

Dimension offers perennial growers:

Dual action: Dimension controls weeds before they emerge and can kill crabgrass in early stages of development.

Compatibility: Dimension can be used safely in many established perennials and ornamentals.
Flexibility: Dimension can be used on the ground bed in combination with Gallery to prevent weeds between pots.

The role of late-season preemergents
One of the most overlooked, yet essential, strategies in perennial weed management is the application of preemergence herbicides in late summer or early fall. This is the time when many cool-season weeds—such as henbit, chickweed and bittercress—begin to germinate. If left unchecked, these weeds can overwinter and explode in early spring, creating a mess just when growers are busiest preparing for sales.

Late-season applications of Snapshot, in particular, are highly effective at suppressing winter annuals. When activated with the right irrigation amount, they create a solid defense barrier during the off-season when hand-weeding is least desirable.

Best practices for preemergence herbicide use
To get the most from herbicides such as Snapshot, Gallery and Dimension, growers should adhere to best practices, including:

Apply to weed-free soil. Preemergence herbicides don’t kill existing weeds; they stop new ones from germinating. Clean beds or containers before application.
Watering in. Preemergence herbicides need to be activated by water—usually within 24 to 48 hours of application. This can be through irrigation or rainfall.
Reapply as needed. Depending on environmental conditions and herbicide longevity, reapplication every three to six months may be necessary.
Know your crop tolerances. Always check the label for crop safety, as not all herbicides are safe for all perennials.

Conclusion
Weed management in perennial production is a multilayered challenge, but preemergence herbicides, such as Snapshot, Gallery and Dimension, provide a powerful foundation. By strategically applying these products — especially during critical periods like early spring and late summer — growers can prevent most weed germination, reduce labor costs and produce higher-quality plants. GT

Broch Martindale is National Nursery & Greenhouse Strategic Account Manager for Corteva.

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