When you bring a new houseplant into your home, it’s always wise to put it in isolation at first so as not to accidentally bring back insect pests that can spread to your other houseplants. After all, mealybugs, whiteflies and aphids don’t appear by magic, the result of spontaneous generation. These pests always come from somewhere and travel most easily on other plants.
And don’t rely on the fact that your source seems above reproach: no nursery in the world will claim that there aren’t sometimes harmful insects on its plants. After all, they bring in thousands of plants from all sorts of wholesale greenhouses: they can’t check each and every one. Look just as suspiciously at cuttings or divisions your grandma or best friend offer you: they are just as likely to carry unwanted passengers as nursery plants.
How long should the isolation period last? Some insects, such as whiteflies, multiply quickly and if you’re observant, you’ll likely notice their presence within a week or two of bringing the plant home. Others are sneakier. That’s why I like to use the term “quarantine”, from the French word quarantaine meaning 40 days. Formerly boats suspected of transporting the plague were isolated for forty days before passengers were allowed to debark. That’s a decent time limit for isolating plants as well.
Keeping New Plants Separate
Ideally, the new plant should pass its quarantine in a room where there are no other plants, but that just isn’t possible in homes where there are plants in every room. There is, however, an easy way out: you seal them inside a transparent plastic bag. Do move the bag back from any sunny window, though, because if it sits in direct sun, the temperature inside will go through the roof and could kill the confined plant.
Don’t worry that your plants will “run out of air” in a sealed plastic bag: plants produce oxygen and absorb carbon dioxide during the day, then do the opposite at night. Thus, they produce all the air they need to breathe well.
Attention, though. While the plant is isolated inside its plastic bag, it’s unlikely to need any watering, because leaves on plants sealed inside a bag lose almost no moisture to transpiration and therefore the roots, not needing to compensate for water loss, use little water.
When the Forty Days Are Up
Before releasing the plant from its isolation, examine it carefully. Look especially at the leaf axils and under the foliage. If you can, it’s even best to remove the root ball from its pot to see that there is nothing going on in the potting soil. If you see insects such as scale insects (which resemble small bulging shields), mealybugs (much like tiny balls of white cotton on leaves and stems) or root mealybugs (cottony wool among the roots), always the most insidious insects, the best thing to do is to toss the plant.
If you insist on keeping an infested plant, keep it in isolation and treat it again and again until you no longer see any insects. It may take years of treatments to eliminate some particularly pernicious pests.