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Monday, January 12, 2015

How Do You Sterilize Pruning Equipment?

Q. You often discuss the need to "sterilize" your garden tools to prevent transfer of disease. How do you "sterilize" these items?

A. A. What I mean by “sterilize” is exactly what is meant to a medical doctor and for some of the same reasons. Not sterilizing pruning equipment before its use is a terrible oversight. People overlook cleaning and sanitizing equipment because people don't understand why it is needed.
99% of the time unsterilized equipment is not a problem. It’s that 1% of the time when it becomes a problem. These are the times I receive questions about the dieback in olive, mulberry, silk tree and perhaps even ash trees.

Diseases are transmitted. Several important diseases are transmitted on pruning equipment. These include sooty canker (fungal disease of mostly ornamental plants), fire blight (bacterial disease of
Mimosa or silk tree die back due to Verticillium wilt.
many ornamental plants and fruit trees), crown gall (bacterial disease of many woody plants), slime flux (bacterial disease of many plants), Exocortis (viroid disease of citrus and tomatoes), bacterial spot (Xanthamonas, a bacterial disease), sudden oak death (fungal disease), figure mosaic virus, rose mosaic virus, tobacco mosaic virus, Fusicoccum viticolum on grape (fungal disease), Pierce's disease of grapes (5% transmission rate, bacterial disease) and others. I did not include tropical plant diseases like papaya ring spot virus and banana wilt.

Sooty canker on ash.
Cut flower growers and florists also realize the importance of sanitizing knives and pruners to prevent infections from entering the cut flower and promoting its longevity.

Whenever we enter fresh plant tissue with a pruning shears or saw the equipment needs to be sharpened, cleaned and sanitized. Adjusted and sharpened pruning equipment provides a narrow point of entry which minimizes plant damage around the cut. The concept that gardening tools should be kept clean, adjusted and sharpened is less controversial since this makes sense to people.
Equipment should be adjusted, sharpened and sterilized at the beginning of a pruning day. Equipment used for pruning should be sterilized for the same reasons we sterilize hypodermic needles and scalpels. Several important diseases are transmitted on pruning equipment. You can read more extensively about this topic this week on my blog.
Sanitize and sterilize pruning equipment. Here I am using an alcohol wipe after cleaning, sharpening and adjusting the blades.
Unlike a medical procedure which usually enters the body in one location, pruning involves entering the plant multiple times at different locations. So when pruning we must be concerned about transmitting a disease from plant to plant and the possibility of spreading a disease on the same plant to multiple locations.
If trees are healthy, then there is no reason to sterilize or sanitize pruning equipment between cuts or between trees. If the disease is present or you suspect a disease, sanitize between every cut to prevent the disease from spreading within the tree.

Sterilizing methods have been researched and there is some disagreement about what works best. Sterilizing solutions recommended include household bleach, Pine-Sol, rubbing alcohol, trisodium phosphate (TSP), and household disinfectants. 

Household bleach (ex: Clorox): 25% solution (1 part bleach + 3 parts water)
Pine oil cleaner (ex. Pine-Sol): 25% solution (1 part cleaner + 3 parts water)
Rubbing alcohol (70% isopropyl): 50% solution (1 part alcohol + 1 part water)
Denatured ethanol (95%): 50% solution (1 part alcohol + 1 part water)
Trisodium phosphate (Na3PO4): 10% solution (1 part Na3PO4 + 9 parts water)
Quaternary ammonium salts: use as directed on product label
Household Disinfectants (Lysol, etc): full strength

Personally, I have used alcohol and even a cigarette lighter when nothing else was available.
By the way, bleach can be very corrosive to steel. When using bleach, oil your equipment at the end of the pruning day.

Sterilizing and sanitizing solutions have a life span. Dispose of these solutions at the end of the day and reformulate them again when needed. If there is a lot of pruning and equipment is particularly dirty, then sterilizing solutions will need to be reformulated more frequently. 

One excellent method that reduces disease transmission from a piece of equipment is air drying it.  Unfortunately, when you are pruning the cuts are made fairly rapidly and the blades never really have a chance to dry between cuts which increases disease transmission potential, particularly bacterial, viral and viral-like diseases.

How often should you sterilize equipment? Equipment should be adjusted, sharpened and sterilized at the beginning of a pruning day. If trees are healthy then there is no reason to sterilize her sanitize pruning equipment between cuts or between trees.

When you encounter a tree that is diseased or you are not sure if it does have the disease, I would recommend sanitizing between every cut to reduce the potential from spreading it within the tree.

Workers who are moving between properties must sterilize equipment between properties at a minimum. They should be taught that if a tree or shrub looks unhealthy, they need to sterilize between cuts. This is when a rag for wiping off debris and cigarette lighter that can fit into your pocket becomes handy. Any attempt is better than no attempt.

Root pruning. Sometimes I recommend pruning roots of plants. When cuts are made on roots of plants it is important to keep the pruning cut exposed to the open air for 24 to 48 hours before allowing it to come in contact with the soil again. This helps prevent disease transmission from the soil to the plant of soilborne diseases which are numerous.


  1. I would add hydrogen peroxide to the sterilizing agents either at 3% it comes in or diluted in half to 1.5%.

    I have used alcohol based sterie-wipes used by diabetics as convenient wipes for pruning sheers. I disagree with diluting 70% alcohol preps. Alcohol is a poor disinfectant (except for enveloped viruses) but a superb cleaning agent and hence its effect is to remove pathogens from a surface rather than kill. The 70% is optimized (or a compromise) in both ethanol and isopropanol (rubbing) for pathogen removal versus tissue damage in people. I am not sure at what use concentration surface removal of pathogens starts to decline.

    Bleach from the store generally runs around 3-5%. If used it should be diluted less than 24 hours before use (preferably and disposed of after that. A 1:5 dilution should be effective against spores. A 1:10 dilution against anything else. Rather than just plain water, I would dilute it in a 1:10 dilution of vinegar which acidity amplifies bleach's killing power (WHO recommendation for Bacillus spores). As you indicated while very effective, bleach is corrosive to steel.

    I use a plastic pickle jar (Boar's Head with lid) to dip my shears in between trees. My only known infected trees are pears (fireblight) and with them I try to remember to do it between cuts.

  2. Comment from Tom Spellman at Dave Wilson Nursery to me in an email.... He is talking about the disinfectant needed on pruning shears and saws when removing infected limbs and shoots.

    Hey Bob, just a comment on the section where you mentioned Fire Blite. Studies have shown that the only reliable disinfectant for sterilization is a 50/50 mix of household bleach and water. Alcohol and other disinfectants have proven to be almost useless.