Cannabis Science and Technology, January/February 2019, Volume 2, Issue 1
What are the typical sources of pathogen and pest contamination in grow facilities? Find out and receive practical advice on how to reduce the associated risks by using some best practices.
This tutorial article addresses some typical sources of pathogen and pest contamination in grow facilities and provides practical advice on how to reduce the associated risks by using some best practices. It discusses preventive measures directed at minimizing the breeding grounds for fungi and bacteria inside the grow facility, limiting the spread of existing contaminations, and implementing sanitary measures at and around the entrances. The latter include controlling the presence and proliferation of other plant species growing nearby, some of which may be common garden vegetables.
There are certain tips and tricks a grower can use to prevent or limit the spreading of contamination with pests or pathogens in the grow facility. Some of those are simply about avoiding common mistakes, while others require some financial investment and labor to implement. The latter group may be particularly appropriate for larger facilities, where a widespread contamination can be particularly devastating.
The following tips and tricks are divided into three groups, based on the place or time of their use. The first group is about the area surrounding the grow facility and involves preparations to be done prior to starting the grow inside, as well as ongoing measures to be carried out throughout operation. The second group focuses on preventing contamination inside the facility, while the third one is about limiting one that has already been found. A short rationale is also provided for each case.
Tips and Tricks for Outside the Grow Facility
Regularly cut grass and minimize other vegetation (such as shrubs) around the facility. This practice limits the breeding grounds for insects and pathogens in places from where they can get inside most easily. It is particularly important to follow in rural and suburban areas.
Do not plant cucurbits (pumpkin, squash, cucumbers, zucchini) anywhere near the facility. This family of plants is particularly known to harbor certain insects and pathogens that can easily jump hosts to cannabis, hence the recommendation.
Monitor any plants growing nearby for pests and disease. Many other plant species can act as a reservoir of infestation as well, so it is important to check those out on a regular basis for any signs of insect proliferation or disease that may be transferable to cannabis.
Also thoroughly inspect and monitor any plants growing inside your home for pests and disease that may be transferable to your grow area. Do not assume that your indoor house plants are free of disease or insects, especially if recently brought in from the outside. Signs of infestation may be very subtle and hard to detect without close inspection (which would typically require the use of a strong magnifying glass), while the associated risk of it spreading to the grow facility on the surface of your clothing or skin can be quite high.
Create plant-free walkways covered with gravel, stone, or concrete and use only them to walk around and enter the grow facility. Keep these walkways clean and regularly swept. Any organic matter covering the ground (such as fallen tree leaves and even mulch) is an effective “shelter” for both fungus and insects, including through the dead of winter. Walking on cleaned pathways made of inorganic materials reduces the risk of contaminating the grow facility with anything sticking on shoes and getting brought inside.
Do not take inside the facility any gardening tools used on other plants. This is to prevent any microscopic “hitchhikers” from getting an easy ride in.
Make sure no house pets (dogs, cats, and so on) follow you inside. Pets can carry all kinds of microorganisms on their paws or legs and spread them everywhere they go.
Be careful with your grow medium (for example, bringing in outside compost can bring in outside disease). Same rationale as for any organic matter above, with the added danger of putting infected material in direct contact with the plants inside.
Do not bring in outside plants or cuttings unless reasonably certain that they are pest- and disease-free. This is important to follow if you are using any external source of green plant stock. Using multiple such sources also multiplies the related risk.
Wash your hands and use a bacterial disinfectant before you enter your grow rooms. Place a disinfectant on the wall by your door to use before entering. This is a matter of basic hygiene-think of it as a necessary precaution before visiting your “babies.”
Do not let any unfiltered outside air flow into the facility. It is simply not enough to put a fly screen on the entrance of an air duct (or a window for that matter) and allow air to enter the facility though it. Insect screens would still let all spores and bacteria through. To stop those, it is absolutely necessary to use a high efficiency particulate air (HEPA) grade filter for the incoming air-preferably in tandem with a full heating, ventilation, and air conditioning (HVAC) system.
Tips and Tricks Inside the Grow Facility
Place disinfecting mats right after each door leading into the facility and make sure that everyone coming in uses them. This is to kill any pathogens stuck on shoe soles.
Use a dedicated set of tools that should not be used on any other plants. This way no contamination gets transferred in from other plants that may carry pests or pathogens affecting cannabis as well.
Install a plastic curtain right inside each entrance. This precaution aims to prevent insects from flying in while the entrances are being used.
If possible, create a positive air pressure inside the grow area (such that air flows from the inside out). This tip reduces the risk of airborne contaminants getting in from the outside.
Install an air shower at the entrance and have everyone going in pass through it. In this case, contaminants on people entering the grow area are blown away.
Ensure that all floor surfaces can be wet-cleaned and scrubbed with disinfectant as necessary. (This also means no carpeting anywhere.) This measure is necessary to enable the next tip.
Swipe and scrub the floors clean regularly. Use mops or rags soaked in clean water or disinfectant solution.
Do not dry sweep the floor. Ever. Dry sweeping puts back into the air whatever spores and bacteria have settled on the floor. Because of this, it is potentially harmful to the health of both people and plants. The right way to sweep is after sprinkling water (or disinfecting solution) onto the floor, such that the dust on it is wetted and doesn’t rise into the air. All sweepings should be placed immediately in garbage bags, which should be then tied and promptly discarded.
Cover all porous walls with clean plastic sheets. If using “panda” type sheets, ensure that the white surface faces in towards the grow room, for better light utilization. This helps keep the wall surfaces clean and hinders the spread of any pathogens lurking inside the pores of the walls. It also makes the application of insecticides and fungicides more effective.
Alternatively, use a wall covering like fiberglass reinforced plastic (FRP). The food industry uses FRP and it can be easily cleaned and sterilized.
Seal floor and wall joints with silicone. Bathroom silicone caulk found in hardware stores works fine for this purpose. Sealing stops the ingress of insects and also prevents insects that may already be inside the room from hiding in existing crevices.
Have everyone working in the grow area wear protective clothing (for example reusable or disposable coveralls) and have it washed or changed at least on a weekly basis. This practice is particularly important for large facilities. The risk of contamination is generally proportional to the number of people working inside.
Put disposable gloves on while handling plants inside the grow facility, including for trimming and drying. Using gloves both reduces the risk of contamination of the product and makes hands much easier to wash afterwards.
Implement wearing disposable booties inside, especially if contamination has been a problem in the past. Shoes can also carry “hitchhikers” on their sides and this measure is the same as in hospitals.
Do not allow any house pets to walk around. Same reason as for not letting house pets inside the grow facility in the first place. Allowing even a thoroughly cleaned pet to roam around the grow room would also let the pet spread around whatever contaminants may lie on the floor.
Check your plants daily for signs of pest or pathogen infestation, using a 10x magnifying glass on any suspiciously looking areas. Remember to look at the underside of leaves as well. The early signs of many pests and pathogens spreading are easy to miss. The more an infestation advances, the harder it gets to fight and the more lasting damage it does. By the time a problem can be seen from across the room, it may be too late to save the affected plants.
Invest in a dehumidifier if the air in your area is very humid. High relative humidity of the air promotes fungal pathogen growth. In some areas (especially near large bodies of water), the average air humidity may be too high for a typical AC system to handle. In such cases, installing at least one dehumidifier in each grow room can be a great investment.
Sterilize your grow containers and rooms after each harvest. This practice “resets” whatever contaminants may be present in the room (even if not manifesting as an outright infestation) to the lowest possible levels.
Use sticky traps for insects, check them often and change them as needed. Keep some fly strips hanging. These can also be an early warning sign if you see any insects on them.
Apply insecticides and fungicides consistent with your grow type and stage on a regular basis (for example, biweekly). It’s a great practice to establish a schedule for the preventive application of your favorite pest control formulations and stick with it.
Periodically wash and disinfect all tools and vessels used in the grow room. Keep your tools cleaned and sanitized. For example, use diluted bleach for all non-metallic tools or vessels and rubbing alcohol on metals. Other disinfecting formulations based on hydrogen peroxide, peroxyacetic acid, and quaternary ammonium salts are also acceptable at the right dilution, done by following all manufacturer instructions and safety precautions.
Limiting the Spread of Existing Contaminations
Remove promptly all leaves and small branches showing signs of infestation with pests or pathogens and dispose of them in a tightly closed plastic bag (do not store them inside the grow facility for any length of time). Oftentimes, an infestation would start in a single spot and spread out from there. Removal of that “hot spot” can effectively stop it in its tracks.
If a single plant is showing signs of advanced infestation or disease but the nearest plants around it look OK, consider taking that plant out (along with its grow container) as soon as possible and disposing of both in tightly closed plastic bags. It may be heart-wrenching for a grower to destroy an entire fully grown plant (or two) but that may still be preferable to ruining the entire crop by letting an infestation spread. This is particularly true in the case of aggressive pathogens such as grey mold, which can spread like a wildfire around the grow room and ruin a crop in a matter of days.
Keep insecticides and fungicides ready to use on short notice. Whenever an infestation is already at work, time is of the essence for any countermeasures. Stocking some common insecticides (like pyrethrum) and fungicides (like copper octanoate) saves precious time for trips to the store or mail delivery.
Apply insecticides or fungicides that are consistent with your grow method and stage to both the plants showing signs of pests or fungal disease and to all healthy looking plants in the same room. Keep in mind that you may not be seeing the full picture and such treatment should never be limited only to the visibly affected plants.
Consider using biological means of defense (such as predator mites or ladybugs), especially if growing organic. There are a variety of such means available commercially. They may not be compatible with chemical insecticides, but biological means can be very helpful in managing persistent problems with pests like spider mites and aphids.
The author thanks Randy Shipley (VividGro) for reviewing the first draft of this article and making helpful recommendations.
Emil Radkov, PhD, is the V.P. Science and Agronomy with VividGro, Inc., in Chicago, Illinois. Direct correspondence to: firstname.lastname@example.org
How to Cite This Article
E. Radkov, Cannabis Science and Technology2(1), 40-42 (2019).