Winter gardening expert Eliot Coleman observed that when the hours of sunlight drop to less than 10 hours per day, plant growth comes to a crawl or stop. He called this period of time 'Persephone Days'. In Washington, DC, our Persephone Days span from November 17 through January 24. By using the "days to maturity" on seed packets (also found on the informational tags included with purchased seedlings) and counting backwards, we can plant our fall and winter vegetables with enough time for them to reach a harvestable size, well ahead of the period of shortened daylength that might slow their growth. At which point, the garden becomes an outdoor refrigerator that maintains our harvest at the peak of freshness.
Hours of sunlight isn't the only important factor for plant growth. Soil temperature is also important and it's critical for seed germination, which is why a soil thermometer is a useful tool to have.
I took this photo yesterday (Wed, Nov 28) at 9:46am. The reported air temperature was 34F while the soil thermometer, inserted approximately 2 inches deep, shows a reading of 45F.
Woohoo, 45F! That means I can still plant beets, broccoli, cabbage, carrots, cauliflower, celery, chard, garlic, leeks, lettuce, onions, parsley, parsnips, peas, radishes, spinach, and turnips, right? I mean, these are all crops that germinate at soil temperatures lower than 45F.
...Not so fast, there's a page 2 in that University of California publication I linked to above. As an example, let's look at carrots, which are reported to take 50 days to germinate when sown in soil that's 41F. Counting from today, it would take to at least January 18, 2019 for carrot seeds to germinate...and it takes at least another 2 and a half months for the roots to reach a harvestable size. But that doesn't mean that I can expect to harvest carrots by the end of April if I plant now. My garden is not a controlled laboratory and I cannot expect the soil temperature to remain constant at 45F between now and January. It's going to get colder, perhaps as cold as 32F or lower. And at a soil temperature of 32F, those hypothetical carrots won't germinate at all.
My soil thermometer will come in handy after our Persephone Days. Warmed by the longer periods of sunlight, this is when the soil temperature will begin to rise. At a soil temperature of 50F, there's a much more reasonable 5-7 day wait for lettuce and turnips to germinate. I will also be planting radishes, which have an 11 day germination time at 50F. Babyleaf mustards, broccoli raab, and bok choy aren't listed (I'd assume they weren't tested), but I think it's reasonable to expect that their germination time would also be about a week or two, given their overall similarities in behavior to lettuce, spinach, cabbage, and turnips. I will certainly let you know my results.
So what is there to do in the garden between now and the end of January? On days when the air temperature is above freezing and the greens show no signs of frost, I will harvest as needed. Watering is all good for now since it rained on Monday, but our gardens still need water even in winter, especially during dry spells. Most of all, I will be reviewing my notes from the past season of weird weather, thinking strategically about how to adapt my gardening practices. In January, I will begin to purchase seeds and determine which to start indoors under growlight. I may even give winter sowing in plastic takeout containers (upcycled mini greenhouses!) a try.