A tender perennial is defined as a perennial, a plant that can live for many years, that is not winter hardy (will die in winter in one’s particular climate).
Sometimes these plants are called annuals for the sake of simplicity, but they are not true annuals, which cannot be live more than one year.
This includes pelargoniums (so-called annual geraniums), many tropical ferns, tuberous begonias, some impatiens, fuchsias, tender sages, and others. Some even broaden this category to include tender shrubs such as hibiscus or citrus, but they should not properly be called perennials. For simplicity, here we will emphasize the most popular types of plants.
The basic concept of overwintering tender perennials is to manipulate the environment of the plant just enough so that it can behave like a perennial instead of an annual. The most important point within this is minimum temperature exposure. The gardener should start monitoring their plants in late September to mid-October, watching for night temperatures that are coming close to freezing. Take those steps to protect your plant before the risk of a real freeze.
The Portland area is usually considered to be USDA hardiness zone 8, which is a reference to how cold it gets here. For those perennials that are almost hardy here (USDA zone 9), sometimes a cold frame is enough, or a minimally heated enclosed environment.
Some pelargoniums, fuchsias, and many assorted zone 9 plants can be kept in a semi-protected spot most of the winter in a container, but be ready to move them to a frost-free environment in case of a severe winter storm. Such a move should be temporary, just so the plants are not subject to very low temperatures. Pelargoniums, for example, can handle frost but not heavy freezes the way the (hardy) geraniums can.
Some might wonder why you should not just move the plant in to the house for the winter, growing it like any other houseplant, and this often happens. The end results are mixed, depending on the conditions the plants receive and the types of plants used this way. Keep in mind that if you keep a sun-loving plant at room temperature all winter in a relatively dark living room, it will probably live and try to grow, but the growth will be very weak and floppy, resulting in an unsatisfactory appearance. Also, plants under such stresses can have exaggerated weakness to over- or under-watering.
A cool but frost free greenhouse is probably ideal for most of the plants that fall in this category. That said, most of us don’t have a greenhouse, so... Here are a few tips on individual plant types and ways to keep them over the winter. Other areas for overwintering tender plants include garages, basements, and covered or enclosed porches.
If you choose to overwinter Pelargoniums inside, here are some tips. Zonal and other such geraniums can be kept in a normal brightly lit indoor location. They would prefer it cool, but don’t need it. What they absolutely need is to dry fully between waterings. Some even go so far as to water them no more than 2-3 times all winter, though the goal here is not to speak in absolutes.
Some people leave them outside in a covered (no rain) location for the winter, and only move them in during severe freezes. This can be successful as well, as long as the plant is not too wet and you do catch it before the freezes.
As an alternative, the gardener can cut back their pelargoniums hard, and store them in the basement or garage, still keeping the soil mostly dry. Scented geraniums tend to make fairly good houseplants.
Fuchsias are challenging to overwinter without a greenhouse environment. Note that a sun room or enclosed porch can work preXy well, but a normal living room style of location usually leads to a very sparse, unattractive plant that may not be worth your while. If you have a greenhouse or similar environment, then simply keep them cool but not freezing, moderately (not fully) dry between waterings, and pinch any excessive growth. Some fertilizer in spring will perk them up nicely.
Most tender ferns make fine indoor bright light houseplants. They should be kept moist but not soggy, and don’t be too surprised if they do some rapid shedding of leaves shortly after being moved. They will generally survive this and re-grow well given time. Asparagus ferns (not edible asparagus) can also make good houseplants, but can handle things a little cooler than some ferns, and prefer it somewhat brighter as well.
Alternately, you can let tender fuchsia go mostly dormant until they loose their leaves, then cut them back to the basic woody structure and store the pots in a cool, frost free area, such as a basement. Water sparingly.
Allow plants to naturally turn yellow in autumn weather, then cut off all the above ground growth. Dig up the tuber, dry it, and store it dry in a cool but frost free place, such as a basement. In late April or so, replant the tubers in fresh soil and regrow for a new season.
For further reading, there is a great article on this subject on the website belonging to Fine Gardening Magazine. It can be found at Overwintering Tender Perennials and Tropicals.
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