Weeds are the bane of almost every grower. Much time, energy and resources are often devoted to their prevention or control. It is no wonder why. There are thousands of species of unwanted plants that can vie for the same sun, water, and nutrients as your valuable crops. This competition can cause stress on your plants resulting in weaker plants, with smaller stature, and lighter (in weight, not color) harvests.
Weeds can pop up in field rows, in between rows, outside greenhouses and grow rooms, and of course, inside them as well. Treatment, with the aim of eradication should be the goal. Not only can they rob your crops of what they need, weeds can host numerous insect and disease pests that can then easily transfer onto your plants. Treatment can be difficult, because unlike with insects or other pests, the same products that will kill weeds, can also kill your plants. Most herbicides are non-selective, meaning that they will kill any plant that they come in contact with in sufficient quantities. There are herbicides that just kill monocots (grasses and grass-like weeds), some that kill just broadleaf weeds,some that are just for aquatic weeds, and many that will kill indiscriminately.
A comprehensive strategy for weed prevention, and control needs to be determined before they become a problem for your crops. Like with other pests, the most effective remedies often are preventative. Read on for tips on how to control weeds in your growing areas, no matter where they are.
Weeds in the Greenhouse and Growrooms
While weeds are most often associated with field-grown crops, greenhouses and grow rooms are not immune from them and they can cause just as much damage to your crops inside as out. Weeds and weed seeds can be introduced to the greenhouse or grow room inadvertently through contaminated tools and equipment, infested plant material, animals and people. They can also blow in through vents, windows and doors. They can be introduced through irrigation water potentially as well.
Common weeds in greenhouses that can affect your crops include (but aren’t limited to) bittercress, bluegrass, chickweed, liverwort, mosses, pearlwort, sorrels, and spurge. If not caught before they go to seed, it can be difficult to control future generations. Management of these weeds is mostly in the form of cultural controls as weed killers in grow rooms have to be used with the utmost care, if at all, since these products will harm crop plants if they come in contact with them.
Cultural Controls of Greenhouse and Grow Room Weeds
Preventing weeds in your grow rooms is the most effective measure among all of the cultural controls that can be taken to tackle weeds. Prevention takes on many forms when it comes to weeds. First, it is imperative that any seed brought into the growing areas comes from a reliable source and that it is verified to be clean from any weed seeds or other debris. It pays to spend a little more on quality seed. Keeping a clean greenhouse is the next step to preventing weeds from calling your growing spaces “home”.
Every effort should be made to keep the tables, floors, walls, media, tools, and containers sanitized as often as possible. A clean environment keeps out all kinds of pests, weeds included. Covering growing media when not in use will keep air-borne weed seeds from landing into your welcoming soil or soilless mix. Applying a weed blocking fabric to the floors if they are not concrete is also necessary to help reduce the amount of weeds that can germinate inside. THe fabric should go over top of any other material, even gravel so that weed seeds cannot nestle down into the many crevices found amongst pea gravel or crushed stone.
Maintaining the floor cover is a task often overlooked. As spots tear or corners and edges come up, they should be tended to immediately. It does not take long for weed seeds to find a suitable spot to spring up from. Those that do pop up, from the floor or elsewhere should be pulled as soon as they are spotted. Any containers kept in a growing area for debris should be covered if they need to remain among crops before being disposed of so that weed seeds cannot accidently be stirred up and find their way into your crops or anywhere else.
For growing areas that do have solid or concrete floors, they should be swept and cleaned continually. Spilled soil or other media should never be allowed to linger on the floor as it can quickly become a suitable spot for weed seeds to take root. Other steps to take include steam sterilizing growing media if it has sat for a while and giving the entire growing area a thorough disinfecting between crops.
Performing all of these tasks will not guarantee that there won’t be weeds to contend with, but they will go a long way towards keeping most of them away from your plants. Once the inevitable weed does pop up, then other methods can be employed to control their spread.
Mechanical Controls of Greenhouse and Grow Room Weeds
Mechanical controls are probably among the least numerous options for controlling weeds in a greenhouse or grow room. The options for mechanical weed control with field-grown crops are numerous (more on that later), but inside the options are typically only trimming tools, fans, windows, and doors. It is impossible to completely exclude weeds seeds at some point, but trying to keep them from blowing into your growing area through a window, vent, or open door is a good start.
Any grow room or greenhouse that relies on traditional windows or doors as part of its ventilation, should make sure to screen them. At no time in a greenhouse should there be no barrier between outside pressures and your crops. Even roof and side vents should have a screen to prevent insects and weed seeds from entering. Fans should ultimately be directed towards another fan or exhaust port that prevents weeds seeds from blowing in either by blowing outwards or because it is closed up when not in use.
Trimming tools are other options for mechanical weed control inside the greenhouse. When hand-pulling of weeds growing on the floor is not an option, pruners, shears, or even string trimmers are an option. In the case of the latter, it should be an electric or battery powered one as the exhaust from a gas-powered trimmer can be detrimental to indoor plants and to the operator.
Controlling Greenhouse and Grow Room Weeds with Herbicides
As mentioned above, herbicide usage inside a greenhouse or grow room is not the best option for their control. Once the weeds are there though, they need to be dealt with. There are a range of chemical as well as organic options. Hand-pulling of the weeds is always best when and where practical, but of course this is not always the case. Herbicides are formulated to be pre-emergent, meaning that they control weeds by not allowing them to germinate, or post-emergent meaning that they kill the weed after it has germinated. Most weed killers on the market are post-emergents. Generally speaking, the younger the weed, the easier to kill.
When applications of any type of herbicide must be made inside the greenhouse, the best time is of course when there are no crops or desirable plants present. This scenario however, does not always present itself. Weeds tend to develop when the conditions are favorable for plant growth when plants are being intentionally grown.
Applications of herbicides when there are other plants present must be done with the utmost care. Controlling drift is essential. Turn off all fans and ventilation when applying herbicide inside to reduce the chance of non-targeted plants getting hit. Where possible, use a cone. Place the cone over the targeted weeds and then spray into the cone. This will eliminate the chance for most of the product to go off-target. Another application strategy is to apply a small, but highly concentrated amount of herbicide onto the weed plants by using a paint brush. This allows for control of where the product is applied and also greatly reduces the chance of harming the “good guys”.
Keep in mind that there is no herbicide that will give you 100 percent control of all weeds at all times. There is also no method of weed control, even prevention, that will stop 100 percent of weeds or weed seeds from getting into your interior grow spaces. When using an herbicide is necessary, consider the options that follow.
Organic Control Options for Greenhouse and Grow Room Weeds
When selecting herbicides for use inside the growing areas, the options do not have to be chemical or toxic. This is not to say that they should be assumed to be safe or that no preventative measures should be taken. Many organic pesticides can cause just as much damage if misused as chemical pesticides. There are many on the market though that are formulated to use less caustic ingredients.
For growers looking to control their weed problems using organic herbicides, look for the OMRI label. The Organic Materials Review Institute is a third party reviewer that assesses pesticides for their suitability to be incorporated into organic gardening and growing practices. Any herbicide bearing their stamp, can be used without fear of jeopardizing any organic statuses that a grower may have. Most herbicides that get their approval have salts, oils, or vinegar as their main ingredients.
Some growers can make their own non-chemical herbicide by using several common items. Highly concentrated vinegar will burn plant foliage. This will not result in a permanent kill of weeds in all cases. It generally needs to be reapplied, but it causes the weeds to use up their stored reserves to the point where eventually they die after repeat applications. Salt will kill plant material as will some botanical oils like clove and citrus. When they are young and susceptible, weeds can also be killed with boiling water, but this exposes the applicator to the risk of getting burned and is not a recommended method in most cases. As with the application of any pesticide, protective equipment and coverings should be worn when applying non-chemical herbicides.
Chemical Treatments for Greenhouse and Grow Room Weeds
While there are numerous herbicides available on the market, not all of them can be used for weeds growing inside greenhouses and grow rooms. It is important to check the label carefully to know what can and can’t be used. Many herbicide vapors, when used inside, are toxic to crop plants. Even though a particular weed killer may do great in a garden, they may be deadly in your greenhouse.
This is especially true for common and effective herbicides such as those containing 2,4D dicamba. Vapors from this herbicide buildup quickly in a greenhouse setting and affected crops can show signs of damage such as cupping or strapping of the foliage. This is true for use of these products immediately surrounding the exterior of growing areas if they can potentially work their way in through ventilation systems.
Many herbicides can be used in a greenhouse when there are no crops present and when their use will not immediately precede the introduction of new plants. Allow sufficient time for any secondary material and vapors to fully dissipate before starting to use the space again. This goes for commonly used herbicides containing glyphosate and other non-selective, post-emergent weed killers. Pre-emergent herbicides containing the ingredient indaziflam, are safe (if directions are followed) to be used on greenhouse floors and under benches and tables even with crop plants present.
Some other active ingredients allowed for use when plants are present include:
- Ammonium nonanoate
- Clethodim
- Glufosinate-ammonium
- Fluazifop-P-butyl
- Pelargonic acid
The above list is not exhaustive, but rather represents common active ingredients found in a wide range of commercially-available herbicides.
Weed Control Outside the Greenhouse
Outside of the greenhouse or grow room is an important area not to overlook. Of course if your growing area is inside of another building or in an urban area devoid of nature outside the door, this may not apply. If however, your growing area or structure is in a stand-alone building, it is worth making sure that weeds that may be growing outside don’t get to the point where they can set seed.
Many growers can keep the incidences of airborne weed seeds from right outside down by mowing frequently immediately around the growing area. This scenario exists if there is turf growing right up to the outer walls. Ideally an area of 4 to 6 feet (1.3 to 2 M) has been stripped of the sod and fabric and gravel or equivalent measures have been taken. Assuming that is not the case though, then mowing is the easiest way to keep weed pressure down.
Herbicides can be used outside as well, so long as the label allows for its use near structures. It is important to use them on a calm day to reduce the risk of overspray or particulates to travel into the greenhouse through ventilation. An application of a granular herbicide, or controlled applications of liquid herbicide will help to reduce this chance. As a precaution, greenhouse vents should be closed during any herbicide application in the general vicinity of the structure.
Herbicides containing sodium chlorate and sodium metaborate have shown high efficacy in reducing the amount of weeds outside of greenhouse structures and have some residual activity which helps to prevent them from growing back. Avoid fumigants as they will inevitably enter the structure, potentially harming your plants inside.
Outdoor and Field Weeds
Weeds in outdoor crops are far more common and span a much wider range of species than greenhouse or indoor weeds. Fortunately, there are more options for dealing with weeds in the field than weeds in a grow room. Some of the same control methods exist for outside weeds as for indoor ones, but others could only be performed within field rows or on an entire field. A wide range of cultural and mechanical options exist, as well as a much broader product selection of herbicides.
Cultural Controls
The options for controlling field weeds culturally differ quite a bit from those employed in a greenhouse or grow room. It is still necessary to start with clean seeds or certified stock. Any tolls used including tractors should be cleaned thoroughly between uses to avoid inadvertently transferring weed seeds. Irrigation water, especially if the source is a pond, stream, or river should be screened and filtered to prevent the unintentional distribution of waterborne weed seeds into your field and crop areas.
One of the more commonly utilized methods of cultural weed control is to use cover crops. Cover crops are intentionally planted grasses, broadleaf plants, or vines whose purpose is to outcompete weeds. Many growers select plants in the legume family as they will add nitrogen to the soil, improving it for future planting. Whatever crop is selected, it needs to be sown in sufficient quantity to suppress the germination and development of weed species. This means that the timing must be considered carefully. The cover crop must also be killed off if it is not an annual crop, lest the cover crop comes back the next year where you don’t want it and it becomes a weed itself.
Cover Crops are not the end-all, be-all in cultural weed prevention and control, but they do a lot of good in regards to weed suppression. They do so much more though. They help prevent erosion and runoff, while adding organic matter to the soil. They also aid in the sequestration of carbon. If properly timed, cover crops are a great tool for fighting field weeds.
If the idea of smothering field weeds appeals to you but are not sure about cover crops, then mulching around the crop plants and in the rows can help to suppress field weeds. Many growers roll out plastic over long, mounded rows and plant directly into pre-punched holes or make them as they go along. Hay should not be used as mulch because of the seeds in it that will germinate. Straw is much better for mulching around crop plants, but weed seeds can still be in straw bales. Heavy kraft paper can be used if the sides can be buried. This will not last all season, but may last long enough for plants to get a head start over weeds. The benefit is that it will not have to be collected at the end of the season like most plastics because they will decompose.
Following along with the idea of crop plants outcompeting weeds, the timing and spacing of crop rows can create an environment that is not conducive for weeds. The canopies of the crop plants act as a protective barrier of sorts. This can be accomplished by starting out with seedlings already started in a greenhouse and then transplanted as opposed to direct seeding for some crops. Ideally the crop plant is at least 4 weeks ahead of the field weed. However it is timed, the goal is to have the crop plant develop first to get a “leg up” on the weeds, rendering their potential for damage to be minimal. Determining this sweet spot will depend on the maturation rate of crop plants, the population of weeds in the field’s seed bank, the cropping practices of the grower, and the specifics of the unique environment where the crop plants are growing.
Employing crop rotation, while not the most effective approach at weed control, is another tool to consider. Alternating growing areas can disrupt the life cycle of some weed species. Because different weeds thrive under different conditions, by constantly changing what is growing where, the weed population can change as well, encouraging competition among the weeds. Like using cover crops, crop rotation offers many additional benefits. As different crops plants draw out nutrients from the soil in different amounts, one area is less likely to be depleted of a particular nutrient, or at least will not be spent so soon.
The orientation of the planting rows can also help deter weed growth. Rows that are spaced closer together give weeds less opportunity to thrive. Crop plants are also better able to outcompete field weeds when they are planted in more of a square, grid layout as opposed to long rows. Rows that are oriented on the north-south axis instead of east-west, make better use of the sun, tend to grow bigger and faster, resulting in more shading out of weed species.
Nutrients, and how they are applied can also promote or deter field weeds. Carefully applying fertilizers only into the rows where crops are or even more strategically delivered to each plant individually will help the crop plants thrive, helping them to outcompete the unwanted weeds. Organic fertilizers or other slow release fertilizers will also favor crop development as opposed to water-soluble, fast-acting fertilizers that can actually end up giving weeds the advantage if an entire field area is fertilized, weeds and all.
If a combination of any of the above does not reduce the weed pressure to an acceptable amount, then it is time to look at adding mechanical control options to the fight.
Mechanical Weed Controls for the Field
Once all of the preventative measures have been taken, there are mechanical options to control weeds before choosing to use herbicides. Some of these are in direct response to weed pressure, others are alternative measures to prevent future weeds.
Tilling is a time-honored mechanical method of weed control. As fields are prepped for initial planting, it is good to allow dormant weed seeds to germinate before planting. Then, the field can be tilled again, turning over those newly-grown weed seeds and allowing them to dry out and die. This can be done as many times as there is time between prep and planting. The more it is done, the less likely there will be weeds competing with your crops.
After planting, tilling is still an option. It is however limited to between rows. If there is no cover crop, weeds will appear in between rows. To prevent them from developing seeds and guaranteeing future weed problems, they can be tilled. To effectively use tilling as a weed control method, rows must be wide enough to get your tiller through, without encroaching into the root zone of your crops. So long as the spacing is correct, this method can be used all season long to combat emerging weeds in rows.
Mowing those same weeds is another option for between rows. Some growers will opt to allow for weeds to develop, but then to keep mowing them down before they can develop seeds. This is a good option for growers with narrow rows where a tiller might not be able to get to. While it does allow weeds to live, it acts somewhat like a cover crop to hold the soil in place, though it is not an ideal management system as it still allows weeds to compete to some degree for water and nutrients with crop plants.
For in-row weeds amongst crop plants there are still a couple of mechanical options. Hoes have been used to chop at and turn over weeds for centuries, if not millenia. Careful hoeing can be done around crop plants to control weed growth without disturbing the crop roots. This will need to be done on a regular basis to prevent weeds from taking over, but for those that do not want to resort to using any herbicides, this is a time-honored method.
A “newer” approach is to use fire. Utilizing a flame weeder, weeds can be directly burned out, even among crop plants. Care needs to be taken and it should be done on a calm day, but the above ground foliage of weeds can be burned with a propane torch in controlled measures. Much like many organic herbicide options, this will not kill the weed’s roots, but if done repeatedly, it will cause the weeds to exhaust their stores of energy and nutrients and eventually die under the constant stress of having to regrow.
Other options for mechanical control include reduced tillage and even no-till systems. In reduced till systems, only the part of the soil that is going to be planted will be tilled and prepped. The rest of the area can be alternatively controlled with any of the other measures listed or explained. This system works best when used in concert with cover crops.
The no-till system still involves machinery. Essentially at the end of a season, all of that year's growth, be it weeds or crops. are knocked down with a machine that has a wheel or roller in front with a chevron design. The knocked down debris over-winters and crops are then “surgically” planted into the prior year’s residue which should in effect work like a mulch to prevent new weeds from popping up. This is not a perfect system and will often require successive years of managing it this way to achieve optimal weed control.
The added benefits to both the reduced and no-till systems are that they do not disturb the microbial ecosystem that exists below the soil. These microorganisms serve as a benefit to crop production by aiding in the breaking down of nutrients into more usable forms by plants and to act as a force multiplier to pull nutrients from further away towards crop plants. Any form of tilling will disrupt this system or prevent it from forming in any meaningful way.
Once all preventative, cultural, and mechanical weed control options have been utilized, it may be time to consider bringing in herbicides.
Herbicidal Controls for Field Weeds
If despite your best efforts, weeds still appear (and they will), it may be the time to use herbicides. Herbicides for field use come in pre-emergent and post-emergent formulations with chemical, or organic options. The timing of their appearance, the risk to the crop, and the limitations on the product label will all affect the decision on which product to use and when.
There are hundreds of different options for weed killers. As with herbicides for greenhouse applications, make sure to follow all precautions on the label. The label should be considered a legal document outlining the parameters for use as well as the required personal protective equipment needed for safe handling. Any prescribed actions listed on the label are not optional. The label is the law and should be adhered to accordingly.
Organic Treatments for Field Weeds
There are no additional organic herbicide options for field-grown weeds that aren’t the same as for greenhouse or grow rooms. Additionally though, there are some non-spray options that are conducive for organic growers to use. Obviously hand pulling of weeds is the ideal method, but not typically a useful solution for rows of weeds or for a field full of them. Biological weed control is an expanding area for organic weed control.
This can include incorporating animals into your weed control regimen. This has to be timed of course because those same animals that are great at eating your weeds won’t distinguish your crop plants and in fact may prefer them. Sheep and goats can provide a great opportunity to graze on your weeds prior to planting. Goats are especially useful for mowing down an area that you might want to consider expanding into in the future. Chickens can also be useful in eating some of your pre-season weeds, but they are easily distracted and tend to wander if they opt to give chase to an insect flying or scurrying by.
Some intentional releases of insects can be done to control specific types of weeds. There is likely to be more research on this particular method in the future, but there are two good examples of this method currently. The Cinnabar moth and the Tansy flea beetle can be introduced to control tansy ragwort in fields. Also, the Chrysolira beetle can be released to consume St. John’s Wort where it may be more common.
Even if animals do not graze your weeds, that doesn’t mean you still can’t feed them with pulled weeds. This should not be done haphazardly though. If you are unsure of what types of weeds you have it may be too risky to offer them up for animal consumption, but most common field weeds can safely be given to a wide range of farm animals to eat. When in doubt, make a positive identification of your weeds first.
Chemical Treatment of Field Weeds
Options for chemical treatment of weeds in the field are numerous. Like ones discussed above, there are pre-emergent and post-emergent herbicides, along with target-specific and non-selective types. The same active ingredients like 2,4-DB, EPTC, bromoxynil, and paraquat to name a few are found in herbicides for field weeds. The same care in application must be exercised when spraying herbicides near crop plants. Spraying on a calm day and setting up physical barriers to reduce drift onto desirable plants is highly recommended.
Herbicides that don’t have lingering soil activity can be used in planting areas prior to seeding if the label on the herbicide lists this possibility. It is also possible to use pre-emergent weed killers after your crop plants have been seeded and germinated, or have been transplanted. Pre-emergent weed killers prevent seeds from germinating. Once your plants have survived past that stage, there are many pre-emergents that can be used around your crop plants.
It is possible for herbicides to lose their efficacy over time when the same product is used time and again. Some weeds can develop resistance to the effects of an overused herbicide. It is important to use products with different active ingredients and modes of action in rotation to ensure that when you do use chemicals for weed control they are effective and you don’t need to overuse them.
Whichever product is selected to kill or prevent the proliferation of weeds, make sure that action is taken before they go to seed, so that the problem will not get worse. For best results, employ as many of the above referenced weed control methods as your time and budget allow for.
