Weed management strategies in greenhouses – Part 2: Chemical weed control strategies - Floriculture & Greenhouse Crop Production

Weed management strategies in greenhouses – Part 2: Chemical weed control strategies

Debalina Saha<sahadeb2@msu.edu>, Michigan State University Department of Horticulture, and Erin Hill<hiller12@msu.edu>, Michigan State University Department of Plant, Soil and Microbial Sciences - April 22, 2020

Chemical weed control methods inside greenhouses and enclosed structures for ornamental crop production.

Photo 1. Weeds growing on the landscape fabric surrounding overhead irrigation sprinkler inside greenhouse. Photo by Debalina Saha, MSU Horticulture

Maintaining a clean and weed-free greenhouse is required for producing good quality and high market-value ornamental crops. Weeds and their seeds are brought into greenhouses via infested plant materials, media, tools, equipment and humans. Weed seeds can disperse into greenhouses by wind (dandelion, horseweed, groundsel) moving them through vents and side walls, irrigation water (chickweed) and by natural propulsion (woodsorrel and bittercress can propel seeds up to 12 feet). Once weeds become established in or near greenhouses, they can produce an enormous amount of seed, which can then be distributed throughout the greenhouse to perpetuate problems with crop competition and pests (Photo 1.).

In Part 1 of this series, we discussed the importance of scouting and options for non-chemical weed control inside greenhouses and other enclosed structures. However, non-chemical weed control strategies can be laborious, time consuming and expensive, therefore this last article in the series will discuss chemical control strategies as well as currently available products for effective weed control in these environments.

There are very few herbicides labeled for use in greenhouses due to the potential for crop injury or death. According to Smith, 2019, injury can occur from spray drift if fans are operating at the time of herbicide application or from volatilization (vapors can easily buildup within the enclosed greenhouse and injure the crop plants). Some organic herbicides (naturally derived) such as ammonium nonanoate (Axxe) and pelargonic acid (Scythe) can be used inside greenhouses when crops are present (Table 1).

All of these products are non-selective, meaning they will injure or kill any plant material with which they come into contact, therefore these products must be used as directed sprays to avoid crop injury. The organic herbicides only kill tissue with which they contact (not systemic), therefore they are more effective on smaller weeds and less effective on those that are well-established and/or perennials with extensive root or rhizome systems. Finally, they are only effective on weeds postemergence (after germination).

Synthetic herbicides labeled for use inside greenhouses (Table 2) are still limited but allow for additional types of activity and in some cases selectivity of particular groups of weeds. Some of the synthetic herbicides have contact activity, similar to the organic products, while others are systemic, meaning the herbicide will move within the weed often resulting in more complete control, depending on the lifecycle and stage of growth. Most of the synthetic herbicides are labeled for postemergence use, though there are some preemergence options as well (targeting the germinating seedling) that have residual (long-lasting) activity.

The ability to utilize the synthetic herbicides while a crop is present is variable. As an example, flumioxazin (SureGuard) is a preemergent product, however there can be no crop plants present at the time of application. Plants can be moved back inside the closed structure after 24 hours and after the site has been irrigated, according to Marble and Pickens, 2015. Indaziflam (Marengo) is a newer preemergence herbicide that has been labeled for use inside greenhouses and enclosed structures. According to Marble and Pickens, 2015, this particular herbicide can only be applied to greenhouse floors in absence of crops.

Regardless of whether the herbicide is naturally or synthetically derived, it is always recommended to carefully read and follow the manufacturer’s label before application to achieve the best weed control and to avoid crop, environmental or personal harm.

The integration of strategies for weed control, both non-chemical and chemical, is recommended to ensure successful control and prevent crop injury and the development of herbicide resistance in weeds.

Table 1. Organic products for effective weed control that are labeled for use inside greenhouses and enclosed structures.
Trade name	Active ingredient	Activity	Weeds controlled	Use with crop	REI¹ (hrs)
Axxe	ammonium nonanoate	Contact (postemergent)	Non-selective	Yes	24
GreenMatch Ex	lemon grass oil	Contact (postemergent)	Non-selective	Yes	0
Scythe	pelargonic acid	Contact (postemergent)	Non-selective	Yes	12
WeedPharm, other vinegar products	acetic acid	Contact (postemergent)	Non-selective	Yes	48
¹REI= Restricted entry interval. It is the time period after a pesticide is applied when employees may not enter the pesticide treated area without required personal protective equipment.

Table 2. Synthetic herbicides for effective weed control that are labeled for use inside greenhouses and enclosed structures (adapted from Marble and Pickens, 2015; Neal, 2015).
Trade name	Active ingredient	Activity	Weeds controlled	Use with crop	REI¹ (hrs)
Marengo	indaziflam	Residual (preemergent)	Annual and some perennial weeds	No	12
Fusilade II	fluazifop-butyl	Systemic (postemergent)	Grasses	Yes	12
Finale	glufosinate	Systemic² (postemergent)	Non-selective	Yes	12
Envoy Plus	clethodim	Systemic (postemergent)	Grasses	Yes	24
SureGuard	flumioxazin	Residual (preemergent with some postemergent activity)	Broadleaf and grasses	No	12
TerraCyte	sodium carbonate peroxyhydrate	Contact (postemergent)	Moss, algae, liverwort	Yes	0
Reward	diquat	Contact (postemergent)	Non-selective	Yes	24
RoundUp Pro	glyphosate	Systemic (postemergent)	Non-selective	No	4
BareSpot Monobor Chlorate	sodium chlorate and sodium metaborate	Contact/residual (postemergent)	Non-selective	Yes	12
¹REI= Restricted entry interval. It is the time period after a pesticide is applied when employees may not enter the pesticide treated area without required personal protective equipment. ²Glufosinate is minimally translocated and can act as a contact herbicide. Thorough coverage is needed.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).