In the Portland metro area, we are lucky. Very lucky, in fact. With the right planning and preparation, we can harvest year-round from our edible gardens. What follow are some of the pieces of information we find most crucial for planning and planting successful fall gardens.
First Frost Date
The #1 factor in planning your fall garden is the first frost date: the first day the temperature drops below freezing, causing damage to frost-tender plants. This date varies in different locations, of course, and some might say it is becoming less predictable overall. However, data collected in past years does allow us to make educated guesses. For instance, the Fall column in the following NCDC chart shows the dates at which there’s a 10%, 50%, and 90% probability that the temperature will drop below 36, 32, and 28 degrees.
According to this chart, it is 10% likely that the temperature will hit 32 degrees (a “light freeze”) by October 24, so it would be reasonable to assume October 24 as your first frost date. More advanced gardeners might want to use the 50% likelihood date (November 15), and push their luck in order to try to get a larger/longer harvest, but the higher the likelihood of hitting freezing, the higher the likelihood that you’ll lose crops to winter damage. The data tables for the rest of Oregon can be downloaded here.
Not only are gardeners working to try to beat the clock before the last frost date: as fall advances and there are fewer and fewer hours of daylight, plants also grow more slowly! To account for this change, there is a formula to calculate when to plant fall crops.
# of days from seeding/transplanting to harvest (varies by variety—look on seed packet)
+ # of days from seed to transplant size (add only if you grow your own transplants)
+ “Fall Factor” (~2 weeks—accounts for slower growth during cool, short autumn days)
= # of days to count back from first frost date to planting date
Here’s an example of how to apply the Fall Factor formula to radishes:
28 days from seeding outdoors to harvest
+ 0 days from seed to transplant (since radishes are direct-seeded)
+ 14 days for “Fall Factor”
= 42 days to count back from first frost date to planting date
Now that you know how to calculate when to plant your fall and winter veggies, you will want to know how much attention they’ll need through the season. You can start to get a good idea of the crops you’ll be able to grow into the fall and harvest through winter by checking out this basic hardiness chart.
As you can see, you can harvest tender, warm-season crops (tomatoes, corn, beans, eggplants, peppers, etc.) up till the first killing frost, then you’ll have to say goodbye and put something else in in their place. On the flip side, you can produce some hardy cool-season crops (kale, turnips, mustard, broccoli, cabbage, collards, Brussels sprouts, Jerusalem artichokes, etc.) well into the fall, while you’ll hold and harvest other hardy and semi-hardy crops (carrots, beets, onions, broccoli, cabbage, etc.) in the garden into the winter.
As the hardiness chart above shows, at a certain point there are some plants you won’t be able to save. But there are many others that will stick it out in your garden through the winter. However, even the hardiest veggies may appreciate some extra protection as the weather cools and the rain starts falling.
The important thing about protecting your crops is to moderate temperature changes and protect plants from wind and excess moisture, NOT to keep them at summer temps. For your fall and winter garden plants, a little chilly is better than too hot; on a sunny day, the greenhouse effect can rapidly cook your plants! Here are some of the many ways you can protect your fall and winter veggies:
A cloche is a mini-greenhouse, usually made with 4 mil or 6 mil plastic clipped over PVC-pipe hoops; if possible, we recommend using pliable (fresh-cut) skinny bamboo instead of PVC, unless your PVC is repurposed! Include vents on the ends of your structure to allow airflow and remove plastic to let rain in on mild days at least every other week.
Cold frames are semi-permanent structures, with glass or plastic on top. You’ll need to leave a cold frame open on sunny days—or better yet, for busy folks, consider investing in automatic openers that will open and close your structure for you as the temperature fluctuates. If it’s really cold, you can insulate your cold frame with straw bales or burlap sacks filled with leaves.
Effective protection for established plants can be as simple as straw, leaves, boxes, buckets, milk jugs, pine boughs, or other material on which frost will form first (before it touches your plants). Be creative! Just make sure it minimizes the impact of raindrops falling on your plants, blocks stiff breezes, and doesn’t become a haven for pests.
Experimenting in the fall garden
Finally, it’s worth mentioning our rationale for listing such very basic information in this post, instead of charts of what to plant when. It’s because conditions vary tremendously from season to season (duh!), and also year to year—especially as global weirding progresses. We hope that providing these basic fall gardening concepts will help you figure out what works for you, and then keep improving upon your methods: that is, treat your gardening efforts as an experiment, understanding that results will vary.
Once you understand the basics, if you want to see how we apply this info and receive monthly updates on specific garden tasks, please sign up for our Newsletter.
For additional information on fall gardening, check out OSU Extension‘s Fall and Winter Vegetable Gardening in the Pacific Northwest or Seattle Tilth‘s Maritime Northwest Garden Guide.