Weeds, Algae and Liverworts
Weed Management Inside the Greenhouse
Maintaining weed-free growing conditions is an essential part of producing high-quality greenhouse crops. Insects and diseases can be kept to a minimum only if proper weed control practices are carried out regularly, along with other appropriate controls.
Weeds may compete with crops for light, water, and nutrients; reduce the aesthetic value of crops; and create a poor impression. Weeds are also a primary source of aphids, whiteflies, leafminers, thrips, mites, slugs, and diseases. Low-growing weeds promote moist conditions, which favor fungus gnats and shore flies. Many common greenhouse weeds such as chickweed (Stellaria media), oxalis or woodsorrel (Oxalis spp.), bittercress (Cardamine hirsuta), jewelweed (Impatiens spp.), dandelion (Taraxacum officinale) and ground ivy (Glechoma hederacea) can host tospoviruses including impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) while showing few, if any, symptoms. Thrips can vector these viruses to susceptible greenhouse crops. Weeds can also carry other plant-damaging aphid-vectored viruses.
An integrated weed management program is achieved with a combination of (1) preventive measures, such as sanitation and physical barriers; and (2) control measures, such as hand weeding and selective postemergence herbicide use.
Prevention
Weed seeds are easily blown into the greenhouse through vents and other openings. Weeds and their seeds can enter a greenhouse on plants, re-used pots, tools and equipment. Soil, wind, irrigation water, animals and people can serve as transport mechanisms of weed seeds into the greenhouse. Creeping wood sorrel, (Oxalis corniculata), hairy bitter cress (Cardamine hirsuta), prostrate spurge (Euphorbia maculata), common chickweed (Stellaria media) and other weeds are persistent problems in greenhouses, reproducing primarily by seed, with several generations each year. Prevention is the grower's first line of defense.
Sanitation
Keep weed seeds, rhizomes and other propagules out of the greenhouse by using sterile media, new pots, "clean" plant materials, and by controlling weeds outside of the greenhouse. Clean up spilled growing media inside and outside of the greenhouse. When moist, it provides an ideal environment for the germination of weed seeds. Screen vents and other openings to help limit the entry of wind-blown seeds as well as insects.
When scouting, identify the type of weeds (broadleaf or grass), life cycle (annual, biennial or perennial), and location. It is critical to remove weeds from pots, benches and floors before they flower and produce seeds. For example, a single plant of bittercress can produce 5,000 seeds that can germinate in as little as 5 days and can propel the seeds over 9 feet from the plant. Creeping woodsorrel also expels seeds by force throughout a greenhouse.
Physical Barriers
Use a physical barrier like weed block fabric to limit weed establishment on greenhouse floors. Leave the fabric bare to facilitate sweeping. Covering the weed fabric with gravel makes it difficult to remove spilled potting media, providing an ideal environment for weed growth. Regularly pull escaped weeds before they go to seed. Repair tears in the weed block fabric.
Controlling Existing Weeds
Methods to control existing weeds include: 1) hand pulling, and 2) using a postemergence herbicide. These measures do not prevent reseeding of weeds.
Precautions on Herbicide Use
Few herbicides are labeled for greenhouse use because of the potential for severe crop injury or death to desirable plants. Crop injury may occur from 1) spray drift if fans are operating at the time of application, and 2) volatilization (changing from a liquid to a gas). Herbicide vapors are easily trapped in an enclosed greenhouse, and can injure crop foliage. Use only herbicides labeled for use in the greenhouse. Carefully follow all label instructions and precautions. It is the applicator's responsibility to read and follow all label directions. Dedicate a sprayer that is clearly marked for herbicide use only.
Symptoms of Herbicide Injury
Herbicide injury symptoms include discolored, thickened, or stunted leaves. Injury in growing point of young seedlings can result in severe stunting growth. Symptoms may be similar to those of nutrient imbalances, viral diseases, or air pollutants from a faulty heating system. Proper diagnosis is needed to determine the cause. Symptoms can be so severe that injured plants cannot be sold.
Types of Herbicides
Herbicides may work by contact or be translocated/systemic. They may be applied either before the emergence of the weed seedlings (preemergence) or after the weeds have emerged (postemergence).
Avoid Use of Preemergence Herbicides in the Greenhouse
Preemergence herbicides should be applied before the weeds emerge. They provide residual control of weed seedlings and can persist for many months and in some cases over a year. Preemergence herbicides can continue to vaporize, causing crop damage.
Only one preemergencece herbicide Marengo® (indaziflam), is labeled for greenhouse use on greenhouse floors in an empty greenhouse.
Selective Use of
Postemergence Herbicides
Postemergence herbicides are applied after the weeds have emerged. The postemergence herbicides labeled for use under greenhouse benches and on floors are listed in Table E1).
There are of two types of postemergence herbicides: contact and translocated/systemic. Contact herbicides kill only the portion of the plant that the herbicide contacts, so good spray coverage is needed. Contact herbicides, such as Axxe®, Fireworxx™, and Scythe® work best when applied to small-succulent weed seedlings. Reward® and Diquat SPC 2 L are also contact herbicides but have high mammalian toxicity. Translocated/systemic herbicides are absorbed and move through the plant. Target weeds must be actively growing for the herbicide to be effective. Translocated/systemic herbicides such as Roundup® (and other glyphosate products) and Finale® are best applied to actively growing weeds when the air temperature is above 50˚F. Note that Roundup® and other glyphosate products can only be applied in empty greenhouses. Finale® is also a non-selective, translocated/systemic herbicide, but Finale® produces symptoms faster (often in 2 to 4 days) than Roundup® (7 days or more on most perennial weeds). Translocated/systemic movement to deep roots and rhizomes is not as complete as with Roundup® so some deeply rooted perennials may regrow after application of Finale®. Only a limited number of herbicides can be used when crops are in the greenhouse: Axxe®, Fireworxx™, Scythe®, Envoy Plus®, Fusilade® II, Reward® & Diquat SPC 2L, and Finale® (see Table E-1 on page E.5). Axxe®, Fireworxx™, Scythe® and Finale® are non-selective herbicides.
Herbicides should not come in contact with desirable plant foliage. Irrigating crops too soon after applying an herbicide can wash it off from the target weeds under the benches and reduce its effectiveness.
Weed Management Outside the Greenhouse
Managing weeds outside the greenhouse is important to 1) prevent weed seeds from blowing into the greenhouse; 2) prevent perennial weeds such as bindweed, quackgrass, etc., from growing under the foundation of the greenhouse; and 3) help reduce the unwanted entry of winged insects into the greenhouse.
Prevention
Maintain a 10- to 20-foot weed free barrier around the greenhouse. Use geotextile fabric outside the greenhouse to prevent weed growth. Mow beyond this area before weeds set seed, to help limit entry of weed seeds, or maintain a boundary of grasses, such as a mix of chewings, hard and creeping fescues. Thrips tend to not reproduce well on these grasses.
Precautions on Herbicide Use
Labels should specifically state if application near greenhouses is allowed. Close greenhouse vents and openings while applying herbicides, to prevent drift inside and damage to sensitive crops. Soil residual and postemergence herbicides may be carefully used surrounding the greenhouse. Select effective herbicides with low volatility. SureGuard® (flumioxazin), Barricade® (prodiamine), Pendulum® (pendimethalin), Marengo® (indaziflam) and Surflan® (oryzalin) can be used to prevent weed emergence. They may be tank mixed with post emergence herbicides such as Roundup® (and other glyphosate products) or Finale® for controlling existing weeds. Do not use any auxin type herbicides, such as those labeled for broadleaf weed control in turf, near greenhouses. Their volatility and the extreme sensitivity of greenhouse crops to these herbicides can result in severe injury.
Algae Management
Algae are primitive organisms that lack many of the tissues that characterize plants. The greenhouse provides an ideal environment for the growth of algae.
Algae that grow on walkways, under benches, and in pots or plugs are a problem for many growers. Algae compete with crops for nutrients and form an impermeable layer on the media surface that can interfere with water penetration. During plug production, slow-growing plants can be especially sensitive to algae buildup. Algae are a food source for both shore flies and fungus gnats. Excessive growth on walkways can be a safety hazard to workers. Growth of algae on greenhouse coverings can also reduce light transmission in the greenhouse.
Prevention measures include sanitation, environmental modification, and frequent use of disinfectants.
For more specific information on water quality issues including water treatment technologies for control of algae, see the Clean WateR3 website: cleanwater3.org and the Purdue Extension Factsheet HO-247-W, Controlling Algae in Irrigation Ponds, by D.M. Camberato and R.G. Lopez, found on the web at: www.extension.purdue.edu/extmedia/HO/HO-247-W.pdf
Prevention
Sanitation
Keep all surfaces free of plant debris and weeds that can be a nutrient source for the growth of algae. A physical weed mat barrier helps to prevent both weed and algae growth.
Environmental Modification
Proper ventilation reduces the amount of moisture in the greenhouse. Horizontal airflow fans help regulate greenhouse temperatures and reduce excess condensation. Retractable roof or open roof greenhouses provide superior ventilation benefits.
Proper Watering Practices
Train employees on proper watering practices. Overwatering crops frequently leads to algae and liverwort buildup on the surface of the growing media. Select a growing media with the proper drainage for your crops. Water the growing containers only as needed, to prevent excess puddling on the floor. Avoid overwatering crops, especially early in the crop cycle, to allow the upper surface of media to dry out between every irrigation.
Prevent algae growth by avoiding excessive fertilization, runoff, and puddling on floors, benches, and greenhouse surfaces. The use of porous concrete floors limits the development of excessive moisture in the greenhouse. The greenhouse floor should be level and drain properly to prevent the pooling of water.
Disinfectants and Algaecides
The disinfestants and algaecides that are currently registered for algae control in greenhouses are listed in Table E-3.
Use disinfectants on a routine basis as part of a precrop clean-up program and during the cropping cycle. Disinfectants like quaternary ammonium compounds, hydrogen peroxide, activated peroxygens, or sodium carbonate peroxyhydrate may be used.
Controlling Existing Algae
Quaternary Ammonium Compounds
Quaternary ammonium compounds include Physan 20® and Green-Shield®11. They can be applied to floors, walls, benches, tools, pots, and flats as disinfestants. Before using these quaternary ammonium compounds, remove debris from all surfaces. Contact with any type of organic matter inactivates these compounds. Surfaces should remain thoroughly wet for at least 10 minutes. Prepare fresh solutions daily or when the solution becomes visibly dirty.
A fourth generation quaternary ammonium product (KleenGrow™) has higher organic tolerances and longer residual activity on hard surfaces. It can also be applied through drip irrigation or flood systems. However, if applied too frequently, KleenGrow™ can be phytotoxic to plant material and cause stunting.
Hydrogen Dioxide & Peroxyacetic Acid Products
Hydrogen dioxide and peroxyacetic acid (ZeroTol® 2.0 and PERpose Plus™) are labeled as a disinfectant for use on greenhouse surfaces, equipment, benches, pots and trays. All surfaces should be wetted thoroughly before treatment.
Their labels indicates several precautions for safe use of these products. Hydrogen dioxide is a strong oxidizing agent and should not be mixed with any other pesticides or fertilizers. When applied directly to plants, phytotoxicity may be of concern for some crops, especially when the applications are above labeled rates or if plants are under stress.
Hydrogen peroxide and peroxyacetic acid (Sanidate® 12.0) is also labeled as a disinfectant for use on greenhouse surfaces and equipment, pots, trays and walkways and can also be used in greenhouse irrigation systems. It is also a strong oxidizing agent.
Hydrogen Peroxide, Peroxyacetic Acid and Octanoic Acid
X3® (hydrogen peroxide, peroxyacetic acid & octanoic acid) is a strong oxidizing agent. It may be used as an algaecide on greenhouse structures, floors, etc. For best results, use with water with a neutral pH and low levels of organic materials. Do not use rates higher than recommended on the label or it may result in leaf burn. It is advisable to test X3® on a few plants before treating large numbers. Take care when applying X3® as a foliar spray following applications of metal based products. It is also labeled for use in chemigation.
Sodium Carbonate Peroxyhydrate
Sodium carbonate peroxyhydrate (GreenClean® Pro) is labeled for algae control in greenhouses. Treat when growth first begins to appear. Effects of treatment are immediately apparent (bubbling and bleaching/discoloration of algae).
GreenClean® Pro is water-activated. Upon contact with water, sodium carbonate peroxyhydrate breaks down into sodium carbonate and hydrogen peroxide. Non-target plants suffer contact burn if undiluted granules are accidentally spilled on them. It is an also strong oxidizing agent.
Liverwort Management
Liverworts (Marchantia and closely related genera) are branching, ribbon-like plants that lack distinct roots, stems, and leaves. They reproduce vegetatively via spores. Stalked, umbrella-like structures release spores (Fig E1). Small, bud-like branches produced in cup-like structures on the surface of the plant also help spread liverwort from pot to pot in water droplets during irrigation. Liverworts thrive in conditions of high fertility, moisture, and humidity.
®
Figure E-18: Liverwort
Inspect incoming plants for signs of liverworts and isolate infested plants. Remove contaminated plants, pots, or growing media. If the growing media stays moist, small infestations of liverwort can quickly spread through an entire greenhouse. Liverworts growing on the media surface will prevent the movement of water into the growing container to the target plant roots. Clean and disinfest empty greenhouses to remove spores. Store growing media properly to prevent contamination by spores.
Avoid overwatering crops. Water according to plant need. Use parboiled rice hulls (3/4 to 1 inch depth) to reduce surface moisture levels. Topdressing with a slow release fertilizer contributes to increased fertility levels on the media surface and to the growth of liverworts. Proper plant spacing helps to reduce humidity levels. Liverworts lack true roots, so allowing the media to dry between watering helps reduce their vigor.
There are no preemergence herbicides labeled for use against liverworts in greenhouses or enclosed structures. However, a few non-selective postemergence herbicides such as Axxe® and Scythe® are labeled for careful use against liverworts.
Table E-1: Herbicides labeled for use within a greenhouse
If any information in this table is inconsistent with the label, follow the label. By law, a pesticide product must be registered in the state in which it is used. The products registered by a company change from year to year. The information in this table is accurate as of publication but is subject to change.
|
Common Name |
Trade Name |
Tox.* |
EPA Reg. No. |
Manufacturer |
Mode of Action |
REI (hrs.)** |
|
ammonium nonanoate |
Axxe® |
W |
70299-23 |
BioSafe Systems |
Non-selective. Works by contact. Cell membrane disruptor. |
4 |
|
caprylic and capric acids |
FireWorxx™ |
C |
67702-54-59807 |
OHP |
Non-selective. Works by contact. Cell membrane disruptor. |
12 |
|
clethodim |
Envoy Plus™ |
C |
59639-132 |
Valent USA |
Selective, postemergence herbicide. Works by contact. Inhibits lipid synthesis. No residual activity. |
24 |
|
diquat dibromide |
Diquat SPC 2L |
C |
228-675 |
NuFarm |
Non-selective. Works by contact. Cell membrane disruptor. |
24 |
|
diquat dibromide |
Reward® |
C |
100-1091 |
Syngenta |
Non-selective. Works by contact. Cell membrane disruptor. |
24 |
|
fluazifop-P-butyl |
Fusilade® II |
C |
100-1084 |
Syngenta |
Selective, postemergence herbicide. Translocated/systemic. Inhibits cell division by blocking an enzyme involved in lipid biosynthesis. |
12 |
|
glufosinate-ammonium |
Finale® |
W |
432-1229 |
Bayer |
Nonselective, postemergence herbicide. Translocated/systemic. Works by disrupting cell membranes. |
12 |
|
glyphosate |
Razor® Pro |
C |
228-366 |
NuFarm |
Non-selective, postemergence herbicide. Translocated/systemic. Inhibits plant enzyme essential to form specific amino acids. No residual soil activity. |
4 |
|
Roundup Pro® |
C |
524-475 |
Bayer |
Non-selective, postemergence herbicide. Translocated/systemic. Inhibits plant enzyme essential to form specific amino acids. No residual soil activity. |
4 |
|
|
Roundup Pro® Concentrate |
C |
524-529 |
Bayer |
Non-selective, postemergence herbicide. Translocated/systemic. Inhibits plant enzyme essential to form specific amino acids. No residual soil activity. |
4 |
|
|
indaziflam |
Marengo® |
432-1518- |
Bayer |
Selective, pre-emergent herbicide. Inhibitor of cellulose biosynthesis. |
12 |
|
|
pelargonic acid & related fatty acids |
Scythe® |
W |
10163-325 |
Gowan |
Non-selective, postemergence herbicide. Works by contact. |
12 |
|
* Tox = Toxicity ** REI = Re-entry Interval |
Table E-2: Herbicides and their characteristics
If any information in this table is inconsistent with the label, follow the label. By law, a pesticide product must be registered in the state in which it is used. The products registered by a company change from year to year. The information in this table is accurate as of publication but is subject to change.
|
Common Name |
Trade Name |
Target weeds |
Comments |
|
ammonium nonanoate |
Axxe® |
Annual and perennial broadleaves and grasses, mosses and liverworts |
Thorough coverage is important for good activity. Avoid contact with desirable vegetation. Does not provide any residual weed control. Organic. |
|
caprylic and capric acids |
FireWorxx ™ |
Annual and perennial (suppression) broadleaves, grasses, mosses |
Avoid contact with desirable vegetation. Apply to young, succulent weeds. Cool or cloudy weather may slow down activity. |
|
clethodim |
Envoy Plus™ |
Annual and perennial grasses |
Apply to actively growing, non-environmentally stressed grasses beneath greenhouse benches. Does not control sedges or broadleaf weeds. |
|
diquat dibromide |
Diquat SPC 2 L Reward® |
Annual and perennial broadleaves and grasses |
General weed control in/around greenhouses and under greenhouse benches. Most effective on small, actively growing weeds. Avoid contact with desirable vegetation. Do not use in greenhouses containing edible crops. |
|
fluazifop-P-butyl |
Fusilade® II |
Annual and perennial grasses |
Does not control broadleaf weeds or sedges (such as nutgrass). Thorough coverage is important for good activity. Optimum weed control is achieved when young, actively growing weeds are treated. |
|
glufosinate-ammonium |
Finale® |
Annual and perennial grasses and broadleaves |
Apply to actively growing weeds under greenhouse benches. Air circulation fans must be turned off during application. Apply as a directed spray, using a large droplet, low-pressure nozzle to reduce drift. Avoid drift and direct contact with desirable vegetation. Do not use in greenhouses containing edible crops. No soil activity. |
|
glyphosate |
Razor® Pro, Roundup Pro®, Roundup Pro® Concentrate |
Annuals and perennials grasses and broadleaves. |
Use to control weeds in and around greenhouses, under greenhouse benches and in walkways. Desirable vegetation must not be present during application. Must turn off air circulation fans to reduce drift. No residual soil activity. |
|
indaziflam |
Marengo® |
Annual grasses, broadleaves |
General weed control under greenhouse benches, in empty greenhouses prior to plant production. Water in after application and allow applied surface to dry prior to introducing plants. Do not use around bedding plants or other plants not specified or listed as tolerant on the label. |
|
pelargonic acid and related fatty acids |
Scythe® |
Annual and perennial broadleaves and grasses, mosses and liverworts |
Avoid contact with desirable vegetation. Apply to young, succulent weeds. Cool or cloudy weather may slow down activity. Provides no residual weed control but leaves a strong odor. Use under benches and in walkways. |
Table E-3: Algaecides labeled for use within a greenhouse
If any information in this table is inconsistent with the label, follow the label. By law, a pesticide product must be registered in the state in which it is used. The products registered by a company change from year to year. The information in this table is accurate as of publication but is subject to change.
|
Common Name |
Trade Name |
EPA Reg. No. |
Tox* |
Manufacturer |
REI |
|
hydrogen dioxide & peroxyacetic acid |
Jet-Ag® |
81803-6 |
Jet Harvest Solutions/Marrone BioInnovations |
4 (spray or fog) |
|
|
hydrogen dioxide & peroxyacetic acid |
ZeroTol® 2.0 |
70299-12 |
D |
BioSafe Systems LLC |
1 (spray or fog) |
|
hydrogen peroxide & hydrogen dioxide |
Perpose Plus™ |
68539-15 |
D |
Bioworks |
1 (spray or fog) |
|
hydrogen peroxide & peroxyacetic acid |
SaniDate 12.0 |
70299-18 |
D |
BioSafeSystems LLC |
1 (spray) |
|
hydrogen peroxide & peroxyacetic acid & octanoic acid |
X3™ |
49538-4 |
D |
Phyton Corporation |
2 (fog) |
|
quaternary ammonium |
Green-Shield® 11 |
10324-94-7969 |
D |
BASF |
12 (drip irrigation) 0 (hard surfaces) |
|
quaternary ammonium |
KleenGrow™ |
81820-2 |
D |
Pace 49 Inc. |
48 |
|
quaternary ammonium |
Physan 20® |
55364-5 |
D |
Maril Products, Inc. |
12 (plants) 0 (hard surfaces) |
|
sodium carbonate peroxyhydrate |
GreenClean® Pro |
70299-15 |
D |
BioSafe Systems LLC |
0 (non-spray and non-fog applications) 1 (spray or fog) |
|
* Tox = Toxicity |
Table E-4: Algaecides and their characteristics
If any information in this table is inconsistent with the label, follow the label. By law, a pesticide product must be registered in the state in which it is used. The products registered by a company change from year to year. The information in this table is accurate as of publication but is subject to change.
|
Common Name |
Trade Name |
Target Application |
Comments |
|
|
hydrogen |
Jet -Ag ® |
Use on greenhouse surfaces, equipment and greenhouse irrigation systems. |
Strong oxidizing agent. Works by surface contact. Use on greenhouse structures, benches, floors. Do not use at higher than recommended rates as leaf burn may result. See label for specific plant applications. Spot test before treating large numbers of plants. Organic. |
|
|
hydrogen |
ZeroTol® 2.0 |
Use on greenhouse structures, benches, walkways. Chemigation. |
Strong oxidizing agent. Works by surface contact. All surfaces must be wet before treatment. Use on greenhouse structures, benches, pots, floors. See label for rate if areas are not precleaned to remove organic deposits. Heavy algal growth may have to be scrubbed off after application. Do not use at higher than recommended rates as leaf burn may result. See label for specific plant applications. Organic. |
|
|
hydrogen peroxide & hydrogen dioxide |
PERPose Plus™ |
Use on greenhouse structures, benches, walkways. Chemigation. |
Strong oxidizing agent. Thoroughly wet all surfaces. Organic |
|
|
hydrogen pero-xide & peroxy-acetic acid |
SaniDate®12.0 |
Use on greenhouse surfaces, equipment. |
Strong oxidizing agent. Clean surfaces before treatment. Thoroughly wet all surfaces. Organic. |
|
|
hydrogen peroxide & peroxyacetic acid & octanoic acid |
X 3™ |
Use on greenhouse structures and walkways. Chemigation. |
Strong oxidizing agent. Clean surfaces before treatment. Allow solution to remain wet for 10 minutes. Reapply as needed for control. Not for use in greenhouses where food crops are grown. |
|
|
quaternary |
Green-Shield® 11 |
Use on greenhouse glass or walkways. |
Pre-clean heavily soiled areas. Treated surfaces must remain wet for 10 minutes. |
|
|
quaternary |
Physan 20® |
Use on greenhouse walkways and benches. |
Preclean all surfaces. Thoroughly wet all surfaces for at least 10 minutes. Not for use in greenhouses where food crops are grown. See label for rates and specific plant applications. |
|
|
quaternary ammonium compound |
KleenGrow™ |
Use on greenhouse surfaces and walkways. |
Thoroughly clean all surfaces, and saturate for 10 minutes. Ventilate closed spaces. Reapply every 14 days to keep algae from regrowing. |
|
|
sodium carbonate peroxyhydrate |
Green Clean® Pro |
Use on walkways, and weed control mats under benches. |
Strong oxidizing agent. Treat when algal growth first appears. Non-target plants suffer contact burn if undiluted granules are accidentally spilled on them. Organic. |