Overwintering Outdoor Tropicals
Zones 9 & 10
Those rated zones 9 and 10, like red bananas (Ensete ‘Maurelii’) and colourful foliage tropicals, such as philodendrons, dracaenas, tender palms, and hibiscus, really need to be moved to a warmer indoor location as soon as possible.
As night temperatures begin to dip below 10℃, the sooner we can get them indoors, the better. The goal is to transition them inside while the light quality and humidity level are still quite good, and before our home heating system is fired up.
The best place for your plants is to find the coolest location in your home, preferably next to an East or North window, far from heat vents and further still from a fireplace. If the optimum placement is not feasible, a South or West window is better than no light at all, and grow lights or fluorescents can also be a great help.
Once inside, mist the foliage a couple times a day to add some humidity, but make sure the leaves are dry at night to minimize any disease issues. Keep them rootbound in their same containers, and don’t feed them until April, when growing conditions improve.
Zone 8 tropical plants are best left outside until night temperatures drop to about 4-5℃. This also applies to fuchsias, lantanas, angel trumpets, colocasias, and canna lilies. Although they are a little more resilient, they also cannot tolerate a frost and must be given protection. The longer you can leave them outside, however, the lesser time they will spend in a dormant or semi-dormant state.
Putting them in a winter dormancy situation means stripping all foliage off, leaving only bare stems and branches. Canna foliage can be cut low, but the leaves of colocasias must stay on. They need to be kept at a temperature ranging between 5-10℃ and have some access to light. The soil should be kept just slightly moist to prevent desiccation, but not so damp as to cause root rot.
Citrus is a challenging category. They love being outside all summer, but when brought inside, the leaves begin to turn yellow and fall off almost immediately. Indoors, they love it cool and humid, and our houses in winter are anything but.
Keep them in the coolest room in your home, next to the brightest window for best performance. A conservatory or greenhouse that does not freeze is ideal, or even a cool garage with a window for light can work. Mist the foliage as often as you can, and keep the soil just barely moist.
If left outside, they will tolerate a light frost, but not temperatures lower than -2 to -3℃. In warmer regions, such as areas rated zone 7 and 8, they may get through nicely when protected by a product like N-Sulate, which is similar to Remay cloth or floating row covers. Whatever product you use, it must have a 10℃ rating for cold protection.
By far, the most popular outdoor tropicals today are vines, like mandevillas or dipladenias. They are incredible performers in the hot sun and come in colours of red, hot pink, white and soft pink. They out-flower virtually anything else on the patio, but they require warm temperatures, in the range of 15 – 20℃, to overwinter.
By cutting them back fairly hard (down to 25 – 30 cm), placing them in a bright window, and keeping them on the dry side, they should overwinter well. Be sure to check the leaves thoroughly for scale, as it is one of their main pests.
Eucalyptus and Green Banana
Eucalyptus is another plant that has become very popular outdoors but, unfortunately, does not fare well indoors. One of the hardier varieties, Eucalyptus gunnii, is a zone 8 plant that, if the pot is sunk into the ground on the South or West side of your home and wrapped with N-Sulate, can potentially make it through a winter with temperatures of -8 to -10℃.
A cool garage with a window can also be a safe place to overwinter one. The same is true of Musa basjoo (green banana), which is native to the northern islands of Japan. In zone 6, it can be planted in the ground and, after a year or so of being established, can be cut down, frozen in the ground, and still survive to spring. The first year they need to be protected by cutting off the foliage in November and wrapping the trunk with both insulating material and plastic to prevent water from seeping into the stem.
Perhaps the most controversy takes place regarding how to overwinter hardy palms, such as the windmill palm (Trachycarpus fortunei). In a protected location, windmill palms are considered a zone 7 plant. Over time, as they become established, they will tolerate temperatures down to -10 to -12℃. In zone 6, they will need some proper winter protection to survive.
They need to be located on the South or West side of your home with the pots buried in the ground, both for stability and root protection. When the temperatures drop to -8 to -10℃, the top leaves need to be tied up loosely and covered with N-Sulate, then wrapped with plastic to prevent water from getting into the crown and rotting out the centre. It is also a great idea to wrap the the roots with heat tape and the stems with an insulating material, as well. This only needs to be done during a period of cold weather. Otherwise, enjoy their beauty all winter.
Inspecting Your Outdoor Tropicals
Before you bring the plants inside, do a very thorough inspection for any insects or disease problems. Outside, over the summer, they many have provided a nice home for many insects, especially in the soil. A wipe down with a damp cloth soaked in a mild soapy solution, like Ivory Snow, will catch potential insects and their eggs before they become a problem.
As for the soil, though, there aren’t many products available to control soil-borne insects. For crawling species, dust the soil with diatomaceous earth and for fungus gnats, apply about one-inch of washed sand on top of the soil as a deterrent for any adults wishing to lay eggs.