By Rhiannon Allen
While non-gardeners often assume that summer is the busiest gardening season, experienced gardeners know that this is not strictly true. Autumn can be a busy time, with most gardeners needing to be active and organized because freezing temperatures impose non-negotiable deadlines.
The first deadline is the arrival of the first frost, and the second is the onset of a hard freeze. The first frost has already arrived, three weeks ahead of its average onset. (Woe betides the gardener who left wanted plants outside that die with the slightest touch of frost!) Right now, gardeners are coping with the anticipated arrival of a hard freeze (air temperature below 28F/-2C for more than four hours), which is highly variable in its onset.
A concept that governs a lot of choices gardeners make in autumn is “hardiness.” If you are an experienced gardener, you are familiar with this concept already. But just a few words to explain it to those new to the game.
USDA Plant Hardiness Zone is a numerical system indicating a region’s average annual minimum temperature. The lower the number, the colder a region gets in winter. We are zone 8a, which means our minimum winter temperature ranges from 10 to 15F (-15 to -12C).
In other words, we do get a hard freeze in an average year, although certainly not as hard as the zone 4 mid-west and prairies endure. This system is also used to designate the minimum temperature a plant will tolerate before dying. Hardiness ratings for plants are easily obtained using an internet search.
The notion of tender vs. hardy fits into this overall framework as a colloquial way of referring to how cold a plant can get before it dies. Specifically, a “tender” plant will succumb at 32F (0C). Healthy plants termed “semi-hardy” can survive at this temperature for a short period of time, but will likely succumb to a hard freeze.
A “hardy” plant will probably survive a hard freeze, its survival dependent on exactly how cold it gets, how stressed it has been and how long the freeze lasts. If you are not sure whether a plant will over-winter outside here, check to make sure that it is rated as hardy or tolerates zone 8a or lower.
Given that plants differ in hardiness, you can see that gardeners must organize priorities this time of year, and, in fact, need to be more organized than at other times of the year.
Semi-hardy plants (e.g., some popular potted plants) tolerant of a light frost might be able spend our anticipated mild 2019–2020 winter outside, but if possible should at least be moved to warmer or more sheltered locations such as against a south wall, under the eaves, or into a greenhouse until they can be moved back to their home location after our anticipated last frost date at the end of March in the new year.
If the plants are not movable, you can try mulching around them very heavily or even wrapping the entire plant in a protective horticultural fleece or burlap, making sure that the wrap is closed at the top to prevent cold winter rains or snow from soaking the top of the plant. Smaller plants can be protected by placing a cloche over them.
This advice applies primarily to perennials. These relatively long-lived plants are worth saving from one year to the next for the repeated pleasure they will provide. In contrast, annual plants are not worth protecting from the frost. No matter how much you like them, chances are that they will die a natural death midwinter at the latest, even if you bring them indoors. Harvest seeds if you want, but any way you do it, you are looking at replacing the plant outdoors next year anyway. Just leave them outside as mulch, critter food, or insect shelter instead.
For me, the most important task to be accomplished before the first hard freeze is fall planting. This means that trees, shrubs and hardy perennials that I need to relocate or get out of their pots should be planted in their permanent homes now. Similarly, hardy bulbs like garlic, shallots, daffodils and tulips need to be planted now so that they can spend the coming months slowly establishing their root systems and preparing for spring eruption. Make sure that you plant them in good, enriched composted garden soil while you are at it.
This is also a good time for harvesting most root vegetables, like potatoes and beets. While some vegetables like parsnips reputedly taste better after a frost, you cannot dig up root vegetables if the ground is frozen solid. And personally, I think that beets have a better taste and texture if harvested before frost.
Now back outside, because I have a lot of gardening to finish!