We bring our hibiscus trees inside once the weather gets too chilly. We have a grow lite in our basement which keeps them alive all winter. For the past two years, after about 3 weeks inside, they show signs of heavy aphid and white fly infestations, despite our spraying them down with a hose and spraying insecticide on them before we bring them inside. We have even used systemic granules, to no avail. What can we do to prevent aphid and white fly infestations from invading these poor trees? Also, I don't want to contaminate my indoor houseplants. Second question: We have Japanese cherry blossoms on our front lawn which have become overgrown. Is it OK to trim them back this time of year? Thank you.
Wash the plant with a good stream from the hose. Be sure to wash/spray all surfaces well. Wash and wipe off all pot and saucer surfaces as well, paying special attention to the under the lip of the pot. Pick all fallen leaves and debris off the soil surface.
If the plant is really large, you could trim in back by about 1/3 or even one half. Lastly, if you want to be extra attentive, you could finish with a thorough spray of horticultural oil or insecticidal soap.
Give them as much light as you can. During the winter rest period, water only enough to keep the soil from drying out completely.
Monitor for insects regularly. Spider mites can be a problem in our dry indoor spaces. Keep from your other plants until you are sure there are no pest problems. See our publication for more information http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG105_Overwintering_...
Prune the ornamental cherry trees after they bloom.