University of Vermont Extension 
Department of Plant and Soil Science

Fall News Article

Dr. Leonard Perry, Extension Professor
University of Vermont
Temperature, and its balance with light, are the two most important conditions for successful overwintering of tender perennials indoors, in addition to proper watering.  Your plants will show symptoms if these conditions aren't to their liking, but these may be confused with other causes, and once they show these it may be too late!  While perennials are hardy in their native climates, in colder regions they may grow as an annual, and so are called "tender perennials."  These include such plants as coleus, cannas, geraniums, and sages or salvia.
Even if plants didn't get inside during fall, but they were left in a protected location, many will withstand some cold and some will even withstand some frost.  If they were subjected to such conditions, try cutting back when bringing into the warmth.  If they are still living, you should see signs of buds or new growth in a week or two. 
If a tender plant you are overwintering inside is losing its leaves, perhaps it is too cold, or cool for too long a period.  With high energy prices, and many turning down thermostats especially at night, such conditions are more common.  Wilted, pale leaves in spite of adequate watering is another sign the plant is too cold.  If a plant such as coleus has lost its leaves, it may be too late to revive it.  Try moving such plants into a very warm area (above 55 degrees F at night and in the 70s during the day), and don't water if the soil is at all damp.
Other plants such as lemon verbena and hibiscus normally lose their leaves, or "defoliate", as part of the normal cycle.  Others that were outdoors in sun during summer, when brought indoors to much lower light in fall, may lose their leaves in the process of making new leaves better adapted in their cell structure to the lower light indoors.
Over and under watering may also cause plants to lose their leaves.  Check the soil with your finger, an inexpensive soil moisture meter you can get at complete garden stores, or look at the surface.  If leaves are falling and the soil feels wet, or is dark, and the pot is heavy, it may be too wet.  If it feels dry, is light in color, and the pot is light, it may be too dry.  If either of these, correct gradually, don't immediately go to the other extreme.  Check plants in saucers to make sure when watering that the water doesn't remain in the saucer for more than a half hour.  If room, place plants on a tray of pebbles you can moisten when you water.  This is great for houseplants too, allowing the water to drain from pots, and keeping humidity higher around the plants.  In general, and if in doubt, keep tender plants indoors on the dry side.
Moisture also is critical for storing of some summer bulbs.  Dahlias and cannas should be kept moist, while gladiolus should be stored dry.  Too much of the other for these may cause them to rot or die overwinter.  Temperatures below freezing may kill them as well, as I've learned in years past when they were placed too near a cold wall with minimal heat.
On the other hand, if plants are in too warm conditions such as next to heater vents or a wood stove, they may have elongated and thin leaves with spindly stems.  This is a sign they are getting too much warmth and not enough light. So the solution is to move to a cooler location with similar light, or increase the light.  The latter can be done by supplemental lighting from lamps.   
Portable clamp-on light fixtures can be used, either to supplement natural daylight or to add light during the night.  Aim for 16 to 18 hours of light per day, with lamps a foot or so away from the plants.  This is enough space to allow the heat from incandescent bulbs to not burn the leaves, yet to provide sufficient light.  If using energy-saving bulbs that emit much less heat, lamps can be placed closer.  Inexpensive timers from garden and hardware stores are used to control such supplemental lighting. 
Keep tender perennial plants, as well as houseplants (many of which can be considered tender perennials), away from drafts.  These could be from doors or windows.  If a sunny day in winter and you open the window for a short time for some fresh air, make sure to remember and shut it.  Even a brief exposure to cold once the temperature drops is enough to injure many tender plants, and is more common than you might expect.
Just as with houseplants, check tender perennials indoors regularly (at least weekly, or when watering), for pests.  Many of these are small, so you may need a magnifier such as for coins, or reading glasses.  Especially check under leaves, the growing tips, and the leaf axils where leaves join the stems.  If you find pests, deal with them then before they spread to other plants and get out of control.  You may find that some plants are just too much trouble, being a favorite of pests.  I've found this the case with lantana and whiteflies, for instance.
Much more on the correct conditions for particular plants, troubleshooting problems then dealing with them, can be found in the excellent reference from Storey Publishing by Alice and Brian McGowan, Bulbs in the Basement Geraniums on the Windowsill.

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