Plant Quarantine

Plant Quarantine and Pest Prevention Measures

With all that is going on in the world right now, I have been thinking about the ways in which we mitigate risk to protect ourselves and those around us.  I have been thinking the many ways we are combatting this virus, and how there are parallels to be found in the plant world.  In speaking to growers over the years, many of them have remarked that having pests in their collection gives them actual nightmares. 

Fortunately, most institutions have protocols in place to prevent these types of events from happening, but implementing them is often a time consuming and labor-intensive process. Many hobbyists and private collectors do not engage in such rigorous procedures.  Prevention is always preferable, but it is elusive.  The best methods of prevention will depend on the types of plants grown, their location, and whether they are grown inside or outdoors. For this reason, I will only speak to my very limited experience mostly terrarium plants, in an indoor urban environment. I will focus on preventative measures and speak only to the types of pests I have personally encountered. Below are some observations. 

On multiple occasions, I have received plants, placed them in quarantine, and only months later, noticed aphids or mites on those plants.  Even if they appear to the naked eye, to be free of pests, all that means is that they are not infested with readily visible pests.  Snails, aphids, thrips, spider mites, scale, root mealy (etc.) are often inconspicuous at first, and can easily go unnoticed, tucked away behind leaves, in newly unfurling and tightly held leaves, or in the growing medium, for months at a time.   It is, I believe, the responsibility of the grower to do their best to ensure the health of the rest of their collection by quarantining new plants.  It is especially important for those growers who share, trade, or sell plants or cuttings. 

Many plant pests have reproductive cycles that seem extraordinarily long, given their relatively short lifespan.  There are hundreds of aphid species, for example, that can lie as eggs in growing medium or nested under a leaf, for six months or more before emerging.  Some are the size of a pinhead when they hatch out and may require a prolonged examination to be observed, even when present in large numbers.  It has been a somewhat terrifying experience, for me to first notice a novel plant pest. After staring at a plant for a minute or more, rotating it, trying to examine all the surfaces, one small, unidentified insect appears, and then two, and then ten, and pretty soon there are dozens of small dots moving, their legs invisible to the naked eye due to their diminutive size. 

As we have seen on a worldwide scale this year, it is entirely possible, and indeed likely, that there are pathogens being transmitted through seemingly healthy vectors.  When dealing with such a pathogen, it is important to treat all possible vectors as ‘guilty’ until proven innocent.  In my best estimation, the period of quarantine (with close observation) necessary to establish a plant as ‘clean,’ is about a year.  This is based on the longest reproductive cycles of pests commonly found in cultivated plants, and the amount of time it might take an observant grower to notice signs of inconspicuous pests.  This length of time allows insects with long reproductive cycles to become apparent, and for physical manifestation of systemic infections such as powdery mildew and viral infections (most often transmitted by insects).  This can be expedited or lengthened of course. But, this is a metric that I find to be useful. 

Of course, quarantine does not eliminate pests, but ideally, prevents them from spreading to the far reaches of your plant collection.  Inevitably if you work with plants long enough, you will get pests.  Likely, more than one or two types.  Even if you successfully quarantine, it is entirely possible that your healthy plants become infected with a pathogen from the outside world.  It is pertinent to thoroughly wash hands, change clothes, and ideally, shower after doing exhaustive yard work or visiting a nursery, to prevent pathogens from jumping, floating, or otherwise infecting your stock.  Equipment such as scissors or clippers should be sterilized in-between uses, in ethanol alcohol or bleach.  Pots should be sterilized if reused.  Potting mix should not ever be reused.  Hands should be washed when moving from one grow area to another.  As a grower without outdoor growing facilities, I do not have much experience with disease prevention outside the home.  A grower in Hawaii once told me, he wanted to hose visitors down and make them wear hazmat suits, before entering his greenhouse.  This was over ten years ago, and at the time I thought it funny, quirky, and ultimately took it as a joke.  Now, I am not so sure! This type of response seems a lot more warranted, especially considering in greenhouses, pests can roam freely, without the physical barriers of bins, lids, etc. that indoor growing affords.  


Below is my process for quarantining new plants, methods which are useful only in very controlled indoor grow spaces. It by no means prevents all pests from entering the collection, but it limits the possibility for widespread infection and offers treatment for some of the pests that are easily missed even with close examination. 

-Before unpacking new plants, I make sure to have a clean surface to work on. I set up the pots and growing media before handing the new plant material.   This way, I’m not going back and forth between my potting mix and the new plants.  This prevents possible pathogens on the new plants from entering the large potting mix bin, which will come in contact with many other plants. I set up the total number of pots needed, with media, before touching any part of the new plants. 

-Even though many will say to never repot a plant just after you receive it, I like to immediately do so if the plant appears healthy (not an import, not dehydrated).  Regardless of whether the plant is well grown, it is useful to take note of what the media composition is, how much drainage material is utilized, if the root system is well established, and so forth.  It can be very informative! It also makes identifying the possible root and soil-borne plant pests and disease possible.  As Brad Thompson insightfully notes in ‘A Guide to Begonias,’ having all your plants on the same media makes maintaining a collection a lot easier. For example, if you have one pot that drains very quickly and a neighboring pot that stays evenly moist most of the time, it can become easy to over or underwater.  

-New plant material is inspected from root to new growth.  If the plant was shipped potted, it is removed and the root structure is closely examined, and the roots are thoroughly washed with water until all organic media is removed. Two of the most common soil-bourn pests are soil aphids and soil mealybugs.  The mealybugs are easier to spot, as they leave white powdery wax on the sides of the pot and throughout the soil.  It can appear at first glance like mycorrhizae.  The root aphids are trickier to spot if their numbers are low.  They are small and off-white, often congregating where the root mass is thickest.  Pulling apart root-bound plants is necessary to ensure that there are no pests hiding in the heart of the mass. 

root mealy (will sometimes live on aerial roots in high humidity)

-After the plants have been thoroughly cleaned with water I soak them in Spinosad for 15-20 minutes.  “Spinosad is an insecticide based on chemical compounds found in the bacterial species Saccharopolyspora spinosa.”  It is approved for use in organic agriculture and does not damage even the most sensitive Begonias.  It kills spider mites, thrips, leaf miners, and some beetles and caterpillars.  Spider mites and thrips are some of the pests most commonly encountered in Begonia culture, and some of the most insidious, so I like to give the plants some protection against these two especially.  An air stone can be added to break the surface tension of the water, preventing some smaller insects from floating and surviving. While ideal for indoor plants, it makes a poor candidate for outdoor plants due to its toxicity to non-target insects.  


-After the Spinosad soak has been completed I take another look at the plants and examine the soak bucket for any insects.  If all looks good, the plants are potted up in their new pots and left in a sealed bin for 1-3 days without strong overhead light.  For new acquisitions, ambient light seems preferable to bright light for the first few days.  Since the plants are recuperating from shipping as well as repotting, when I put them under grow lights I place a sheet of printer paper over the opaque lid of my growing bin to shade the plants further.

-All plants from the same source are kept in mostly sealed bins, housed on a rack kept separate from the rest of my grow space.  Ideally this is a different room, but, that’s not always possible.  Beginning one or two weeks after receiving a new plant, I treat a second time with Spinosad, the next week with insecticidal soap.  If root mealy is detected at any point, those plants undergo a separate, targeted treatment using hot water baths or alcohol dips.  Usually I follow up with a second round of insecticidal soap within a few weeks.  As I add new plants and inevitably new pests to my collection, I expect my quarantine and pest management strategies to change and improve, but these are my current methods. 

Preventative Treatments for Propagation Material, and the Allocation of Mother Plants

As a kid, I remember visiting orchid and tropical plant greenhouses in upstate NY.  Every so often I would come across a plant that even amongst the perfectly grown specimens laid out on benches, that stood out.  Upon closer examination, they almost always seemed to be marked in sharpie, often on red or green tags, with ‘specimen plant’ or, ‘not for sale’.  Behold the mother plant! These plants are important to any grower, not only for their aesthetic value but also for their use as future propagation material.

Pest detection is inherently more difficult with larger plants.  There is more surface area to examine, more hiding places, and a better chance that the plant does not immediately show signs of hosting pests due to its impressive size and health.  Additionally, if a plant is maintained in an open-air greenhouse where pests can roam freely, the chances of infection are heightened.  To reduce the opportunity for pest transmission, propagation material can be treated with the same care as new plant acquisitions.  A full inspection of the mother plant helps identify any possible pathogens, and prevent the propagation of infected stock.  If a plant looks in poor health, it should not be used for propagation, other than to establish a backup plant. To avoid root borne pathogens, for Begonias and other plants propagated easily with leaves, only leaves, and no rhizomes should be utilized.  I do a Spinosad soak and setup all potting materials ahead of time (same methods as with new plants).  Propagation is an excellent way to establish new, clean plants, as the risk of transmission when dealing with such a small amount of plant material is lessened significantly, especially when that material is treated intermediately. 

Keeping newly propagated plant material in proximity with other newly propagated plant material whose mother plants have been closed in close proximity, offers another level of protection.  For example, in a greenhouse, two Begonias sit next to each other on a bench.  If one plant is found to have a pest shortly after leaves have been used for propagation, the pot next to it has a higher chance of hosting that same pest, even if this is not immediately apparent. Suppose, a month later, that neighboring plant is found to have the same pest as the first.  If the intermediate pest treatment is unsuccessful in killing the pest on material taken for propagation, ideally the plants produced from these two pest infected plants are confined to only one propagation area (a bin or tray).  If they are distributed in multiple propagation areas, conceivably plants that did not start off with pests, might now have them. Ideally, propagation material should be kept singly, one bin or tray per plant, but, this is often not possible or an efficient use of space.  Still, keeping the total number of propagated plants per propagation area low is one way to minimize opportunity for pests to spread to clean plants. 

The methods above are ones that I have found to be compatible with the often sensitive nature of Begonias.  These are, of course, many shortcomings in these methods.  There is no perfect system.  But, quarantine and preventative measures are essential tools in preventing pests from becoming widespread in a collection and preventing their introduction to other growers through shared plant material.



Thompson, Brad. A Guide to Begonias.