When most growers think of pests on their crops, usually insect pests or diseases come to mind. Animal pests, though, are another threat to crops grown both inside and outside in the field. It is equally important to try to mitigate this threat, as part of any grower’s planning and routine operations. Animal pests can cause damage not only to the crop itself, but to infrastructure as well. They can chew through structures, knock over fencing, and create tunnels which disrupt roots but can also be tripping hazards for humans.
Animal Pests in the Greenhouse and Growrooms
While animal pests are less commonly dealt with in most indoor grow rooms and greenhouses compared to those crops grown in the field, they nonetheless can still present a threat to the viability of your crops. This is usually a bigger concern in colder months when rodents and small animals are looking for food and warmth, but few animals turn down the chance for a meal when presented with them any time of year. Other animal pests are looking for shelter from predators, and the presence of a food source is a happy accident. Regardless of the reason for their presence, they need to be dealt with or your indoor crops will suffer damage.
In greenhouses and enclosed growing structures, the typical animal pests are mice, voles, and sometimes even chipmunks and rabbits. Rats occasionally are drawn in as well but mice and voles are by far the most prevalent animal pests in enclosed growing spaces. The damage they can cause is not only from chewing on plants. They might girdle stems, and uproot plants while burrowing for shelter. Though treatment methods to discourage, repel, or to kill them may be the same for most animal pests, it is good to try to identify the species so that future planning can be done to specifically keep those critters out.
There are several species of mice that can cause damage. Notably, the White Footed Mouse is a common greenhouse pest. They will find and eat seeds, nuts, berries and even recently germinated plants just out of the seed stage. Chipmunks are much larger than mice, but subsist on the same diet as mice. They are not typically active in the winter months though. Meadow voles, which look like short-tailed mice, will eat almost any kind of leaf, shoot, root, tuber, or grass. They tend to favor the younger, more tender shoots and leaves. They are also very adept at creating tunnels and runs among pots and flats on growing benches and tables.
Generally when damage is detected, or other evidence such as droppings, there is damage that is unseen and the true number of rodents in your space may be quite large. It is important to act fast when their damage is first detected and that plans are made and implemented to stop them as much as possible from both spreading as well as coming back. There are a range of practices and products that can be used to assist in dealing with these unwanted animal visitors.
Cultural Controls
The best way to keep animal pests from your grow rooms and greenhouses is to make sure they can’t get in to begin with. This is of course far easier said than done. Make sure that all entrances are sealed and that there are no gaps or holes, especially at ground level. This can be challenging for those greenhouses and grow rooms that have low to the ground ventilation, or those that rely on open doors to aid in air flow.
A good way to exclude animals is to install a mesh wire or hardware cloth barrier around the perimeter of the greenhouse or growing structure. The base of which should be buried and extended out at least 6 inches (15 cm) at a 90 degree angle from the building. More is better as it will prevent animals from burrowing up and under. This is true for the floor too. If concrete is not an option for the floor, then make sure it too is covered in a wire mesh that rodents cannot burrow up through.
Any cracks or holes in the structure should be filled as well. If the greenhouse is heated in the winter, then holes should be filled with steel wool, spray foam, or the like. If the growing area is warm enough year round, then mesh can be used to cover them.
Removing sources of food and shelter will also help to keep animals out of your growing area. Trimming or pulling all weeds and grasses around the perimeter will deny rodents cover and a snack. It will also reduce the chance of weed seeds getting into your growing area. Outside of the greenhouse or growing structure, keep all trash cans covered or better yet, inside a shed or the like until they can be emptied, but still away from your plants. Other items like lumber, tools, or equipment should also be moved away from the outside of the greenhouse as these too can be inviting shelter for many rodents.
Inside the greenhouse, keep plant debris cleaned up as well as any other decaying vegetation. This should be done regardless as good sanitation in the greenhouse improves plant health. In this case though it also removes another potential food source for animal pests. Anything stored that could attract animals such as seeds, bulbs, even bone meal and other fertilizers, should be stored in sealable containers, preferably up on a shelf or even in cool storage.
Mechanical Controls
Keeping the exterior area around beyond the immediate growing area mowed will be a further preventative from animals getting into your growing spaces. Unkempt lawn areas provide cover for rodents, not to mention food sources. Most wildlife, except for predators that don’t want your plants, will not cross large areas of wide open space, so the more the approach to your greenhouse is kept mowed, the less likely for animal pests to come near.
Mechanical traps are an option to control animal pests. This is not a long term solution for large populations but can be helpful to control smaller populations. Baited traps should be placed near known entrances or at least in places that rodents are thought to cross. They tend to travel along walls or along other objects. Obviously make note of where they are placed so as not to find them the “hard” way. Both glue traps and snap traps are effective. Make sure to bait them with something at least as, if not more appealing than what they have been eating. Use seeds, peanut butter, or a piece of apple. Wear gloves while baiting and setting the traps to try and avoid inadvertently leaving a human scent that they may try to avoid.
Controlling Animal Pests with Pesticides
When all other methods have failed, it may be time to consider pesticides. There are several options, both organic and chemical. Each grower needs to determine what are the potential risks to the crop by using each. Most organic options are designed to repel animals either by making crops taste bad or making the smell unpleasant to them. Chemical options can also be designed to repel, but many are designed to kill. In either case, often frequent applications are needed as repellants wear away over time.
Organic Control Options
“Organic” controls can be certified organic products or just those that do not require the use of chemical pesticides. There are many products on the market that are approved for use in organic growing systems, but these are not the only options for non-chemical animal control in greenhouses.
One such control option is the use of high-frequency sound devices. These are typically used to keep out rodents such as mice, voles, and rats. The batteries in these need to be kept fresh to work around the clock. They are useful at keeping out rodents, but do not do much to expel those that may already have nests in your growing areas.
Some growers have luck keeping out rodents by soaking cotton balls, or other absorbent material with peppermint oil or other strong smelling repellants. These need to be changed or refreshed often, but when used in large quantities, they do a pretty good job in keeping rodents out of indoor growing areas. They can be placed on the floor level and on shelves or even in between plants.
Though unorthodox, and not appropriate for all grow rooms and greenhouses, cats have been used for millennia to keep out rodents. If this is the only place they live and it does not impede the operation, it works for many growers. If the cat also goes outside, then it is probably not worth the risk that they might bring in unwanted insects into the greenhouse.
If traps and non-chemical options are not getting the job done, then it is time to consider chemical solutions to get rid of animals in your indoor growing areas.
Chemical Treatments
Once the decision has been made to resort to chemical pesticides, it is important to consider options. Most of the time, poison bait is used. If there are pets or children that come into the greenhouse or grow area it is imperative to make sure that they cannot get to this bait. They are however very effective in controlling rodents. Often the bait is taken back to a nest where other rodents feed on it as well, killing multiple pests at once.
Poison bait usually comes in pellets or embedded onto grain. There are single dose toxicants, or there are anticoagulants. If the goal is to kill more than one pest, then the anticoagulants are generally better as they take longer to work, meaning each pest can bring it to other pests. Single-dose chemical rodenticides often work fast, whereas anticoagulants can take upwards of two weeks to kill a rodent. They often require more than one feeding to be effective.
As always, an ounce of prevention equals a pound of cure. Make sure to try and make your greenhouse and grow rooms rodent proof from the outset and then act quickly upon discovery of any animals that shouldn’t be where your plants are.
Outdoor Animal Pests
The number and range of outdoor animals that can bother your field-grown crops far exceeds those that find their way into greenhouses and enclosed growing areas. Fortunately, the number of possible remedies also far exceeds those that can be used inside. It is also unlikely that a grower will ever be able to entirely eliminate the risk of animals doing damage to outside crops, but there are many steps that can be taken to reduce the chance that they can do much damage.
Crop damage from wildlife is not a new phenomenon. Many of the strategies used to mitigate or control this nuisance have been around for centuries. Choosing when to employ any particular method is where dealing with animal pests is as much art as science. Some methods work at first, and then animals get used to them and ignore them. Then other methods must be tried.
Complete exclusion of animals into your growing areas is probably not possible, but it is important to keep them out. In addition to the commercial loss, there is the chance that they are spreading diseases onto your crops such as Salmonella, or E. coli.
Some of the same animals that bother indoor crops can be found in the field bothering plants there too. Mice, voles, rats, and chipmunks can just as easily be found in the field. It is estimated that for every one rodent seen, there are at least 25 more that you have not.
The population of other potential animal pests found outside is significantly larger though. You can add moles, rabbits, birds, squirrels, raccoons, gophers (groundhogs), and deer to name some of the more common ones. If all precautions have been taken and there is still animal damage to your crops, then successfully dealing with these animal pests starts by correctly identifying which ones are causing damage.
Several animals make tunnels. Moles, while not looking to eat your crops, can damage plants by burrowing around them and disturbing roots. Look for small mounds of soil and raised tunnels just below the surface of the soil. Ground squirrels can tunnel under plants too. Though they do eat seeds, they are not looking to eat your plants but they don’t care if they are killing your plants when they burrow under them. They may also dig up plants when hiding food stashes. Gophers and groundhogs (often used interchangeably), will not only tunnel under your field plants, but they will eat their roots, shoots, and leaves. They cause a lot of damage to field plants. Foxes can also dig holes in your field area. They don’t want your plants but they might dig them up looking for the source of your fertilizer, depending on what is used. They also dig holes for their dens.
Like Groundhogs, rabbits will eat the roots and leaves of plants. They will also eat the lower bark on larger plants, even trees which will ultimately kill the plant. Raccoons do eat plants as part of their diet, but they are more likely to rip your plants out of the ground in search of one of their favorite foods, worms.
Deer can cause damage not only by eating your crops, but due to their size they can also trample your crops. A few of the smaller rodents mentioned above won’t typically affect your crop as a whole, but a few deer can do an immense amount of damage. Birds are another animal pest- sometimes.
Birds are good in a garden area because they eat bugs and other pest insects. They do however love seeds and can potentially harm your crops in their pursuit of them. They can also dig small holes near plants to create a stash of food and in doing so disrupt some of your plants’ roots.
To control any of these outdoor animal pests will likely take a whole range of approaches.
Cultural Controls
A major piece of cultural control of animals in the field is diligence. Growers need to walk their fields and rows frequently to look for examples of animal browsing. This could be in the form of eaten plants, but also animal wastes, and broken plants. It also involves the management of the non-crop portions of your field operation, such as where garbage cans, dumpsters, compost piles, and even water is accessed. Paying attention to seasons and cycles to know when hibernating animals might be emerging or when migratory birds will be around is useful when planning out a strategy to deal with animal pests.
Anything that can potentially attract animals to your property should not be placed near field crops if at all possible. Bird feeders should also not be placed near crops as they often attract other types of pest animals that may find your crops to be a better meal than the seed. If birds are one of the animal species that you are trying to keep away from your crops, it is possible that hanging bird feeders near but not right next to your crops can deter them from feeding on your plants. This strategy can backfire if too many birds are then attracted to the property.
Placing physical barriers can also be an effective step to keeping animals out. This is of course not always possible depending on the size of the field area and the resources of the grower, but any type of fencing that can prevent animals from entering the field area will often pay for itself in the form of plants not lost to animals. This is also difficult because many rodents can wriggle through holes as small as ¼ inch (6 mm).
Many animals can also jump over low fencing. The most effective fencing would be solid, 8- 10 feet (2.5 - 3 M) tall, and have wire mesh buried up to a foot (30 cm) below ground and angled away from the field. It would also have mesh over top to prevent birds from grazing on the crops as well. This is of course not practical for most growers, so other methods usually need to be deployed. Electric fencing in conjunction with lower mesh fencing can help to exclude many types of pest animals such as rabbits and groundhogs. If your crop area is small enough, erecting cages or small fences around each plant can be an effective deterrent.
Signs of animal habitat in your field areas should be addressed. Fox or groundhog holes should be filled as soon as discovered. Check back often to see which are still in use. If animals have re-burrowed holes that you have filled, it may be necessary to bury heavy rocks or even fill the holes with mortar.
The choice of nutrients in your field crops can also affect the presence of pest animals. Blood meal and bone meal in particular can attract predators to your crops. They might dislodge plants by looking for the “animal” that they smell.
In barn or storage areas near the field crops, it is important to make sure that feed bags, seed, or anything else that might attract animals is tightly closed up. Loosely piled materials can become homes to many rodents so maintaining order and as much as possible keeping things off of the floor should be a priority. Water sources should be maintained so that water does not leak and form pools which may attract animal pests.
Mechanical Controls
If preventative measures have failed to keep pest animals out of your field crops, then it is time to take some additional steps. Noise can be an effective deterrent to keep animals away from your crops. These could be high pitch, ultrasound devices that humans can’t hear, or so-called noise cannons. Just make sure that they are legal to use where you live or that any permitting is not required. There are solar powered noise makers that can be placed in animal tunnels. They mimic the sound of animal footprints walking overhead and are fueled by solar energy.
Trapping is also useful when populations of pest animals are low. These can be lethal traps or those designed to capture the animal alive. As with anything regarding animal control, check your local laws to determine if any permitting is needed to set traps. In most jurisdictions, it is not legal to trap a wild animal and then release it elsewhere, unless it is still within the boundaries of your property. Glue boards are effective for controlling small rodents, but they do not work well if they get dirty.
Mole traps are useful over active mole tunnels. To determine which tunnels are currently being used, tamp down all discovered mole tunnels. The ones that pop back up are in active use and where the mole trap should go. It should be noted that moles do not eat your plants. The damage they do is by tunneling around plants, potentially dislodging them. Moles near your field are good as they eat grubs which can pupate into moths that will eat your plants. Moles in your field need to be discouraged.
Another option to deal with moles and other burrowing animals is to create an open-ended steel or mesh cylinder that is wide enough for your plants roots to grow unimpeded, but that animals cannot penetrate. This is not a practical option for large amounts of plants, but if there are a limited number of valuable plants, this could be a good option to keep their roots from being tunneled around.
Decoys are another option. There are many life-like looking fox, coyote, and wolf decoys which move in the wind and can deter some amount of animal pests. These and other decoys should be moved around often or else pest animals will not be threatened by them over time if they never move.
Failing all of these, and if pesticides are not desired to be used, it is legal in most areas (check first with your state or province!) to kill many animals that may be harming your crops. Some may require a permit or can only be killed during certain months, but this is an option that indoor growers don’t (for good reason) have. Laws vary regionally and among jurisdictions regarding the reporting of animal kills as well. It is also required to dispose of the dead animal according to your laws and ordinances. Generally speaking, the smaller the animal, the fewer the restrictions on using lethal force.
Even if you do not want to personally kill the animal, you may be able to encourage other hunters to use your property to reduce the populations of animal pests that are bothering your crops. Experienced hunters, doing so legally and in-season will then be responsible for proper field dressing of the killed animals. In any case if the decision to rid your crops of animal pests is through direct killing, then make sure that the offending animal is legal to kill. Many bird species are protected as are animals considered endangered. Most pest species that bother crops however, are legal to kill with no risk of going extinct.
Pesticidal Controls
Just as there are both chemical and non-chemical or organic pesticide options for indoor crops, there are similar options for outdoor crops. In any case, it is again important to weigh your control options with the desired results. You can broadcast poison all over and kill the offending animal pests, but that may render your crop unsafe to consume. In most cases, alternatives to pesticides are more effective, but one should not rule out pesticide uses, even chemical pesticide use if it is used judiciously and minimally.
Organic treatments
There are some options to keep pest animals away from your crops that don’t involve killing or trapping them, or the use of chemicals. There are of course repellants labeled for organic crops, just like with indoor-grown crops, but there are additional options too.
Predator scents can scare pest animals away from an area. It is possible to buy wolf, coyote, or fox urine to spray around the perimeter of your fields to deter animals like groundhogs, rabbits, squirrels, or chipmunks from getting any closer. Other organic repellents are available too.
If you don’t want to use predator bodily fluids, you can spray castor oil and garlic around. Most animals will stay away from those smells. Putrefied egg solids is another main ingredient in some organic repellents. These can be sprayed around perimeters or right down tunnels and hoes that are found to drive the pest animals away from your crop areas. These work well for keeping raccoons away too. Any repellent will need to be reapplied regularly as it will dissipate over time and rain will reduce its effectiveness.
Another option is to plant species that have natural repellent qualities. Caper Spurge for example, secretes a substance from its roots that repels tunneling animals like moles. Other animals do not like the smell of some herbs and they can be interplanted among your other crops. Lavender is another natural repellant and is good to plant along the perimeter of your field area.
Chemical Treatment
Acute (single-dose) and anticoagulant chemical pesticides are available to poison animal pests that are affecting outdoor crops. Their use should be weighed against the potential for non-pest animals finding and eating them. These can be formulated as bar baits, hanging baits, pellets, granules, powders, or concentrated liquids. Baits can be bought ready to use, or those that have to be mixed.
Like with indoor rodenticides, the anticoagulant pesticides often require multiple doses, though newer ones on the market work much faster than their “ancestors”. Typical active ingredients include chlorophacinone, diphacinone, pindone, and warfarin, among others. Acute pesticides include zinc phosphide, bromethalin, and cholecalciferol. These work by interfering with the animals’ bodily functions.
Baits should be placed at least 3 feet apart (1 meter) for smaller rodents and up to 30 feet (10-11 meters) apart for larger ones. It should go without saying that the use of any pesticide carries inherent risks and that their use and application needs to be done exactly as prescribed on the label. When possible, tamper-proof bait stations should be used. All recommended personal protective equipment should be used. All recommended clean-up procedures and re-entry intervals should be observed to the letter.
