kqcrna said:It usually works well for annuals, but in a cold spring, results can be disappointing.
Karen
I've often thought that "spring-sowing" annuals that need no stratification might give earlier seedlings if the spring-sown jugs were parked inside a low hoop tunnel with loose plastic film, maybe in
partial shade or partial afternoon shade. It would overheat if sealed up, but a vented tunnel would still have an average temperature several weeks ahead of the outside air.
You might lose some of the goal of winter-sowing, that "the seeds know the best time to sprout". That seems like a good plan for perennials with dormancy that needs to be broken, and maybe seeds that do some of their germinating and growing while there is still some snow and frosts. Especoially if the perennials have a temperature requirement close to but a little warmer than your average springtime.
But if you want
annual plants earlier in the spring than you can get them by direct-sowing, you might need more "temperate-ness" or warmth than you get from a single jug's walls, kept in shade.
I bet these two classes of annuals have very different needs if you spring-sow them:
1. cold-hardy plants usually planted BEFORE or near the average last frost date
2. warmth-needing plants planted after all risk of frost is over, and soils are warm.
I wonder which slows most seedlings down more: insufficient warmth in the middle of the day (daily high not high enough), or nights-too-cold (daily low too low). That might vary from species to species. I wonder if anyone experiments with the equivalent of "degree-days" for seedlings?
"Degree days" are something like this:
- Take the days average temperature (maybe just high minus low over two)
- Subtract the base temperature, often 70 degrees.
- The results is how many degrees that day exceeded 70
- (Or, if they track negative numbers, by how many degrees that day failed to reach 70)
- The number of degree-days between two dates is the cumulative sum of each day's number.
For seedlings reaching the point where they are large enough to plant out, I speculate that the interesting statistic might be something like "degree-hours".
- For each hour, subtract 70 from that hour's average temperature.
- Sum up all those degree-hours from the sowing date to the present.
- Maybe each species needs a certain number of degree-hours to make a viable seedling.
Maybe there would have to be different base temps for day and night: some seedlings "like cool nights". If so, then a failure to go BELOW some base temp at night might slow their growth.
Maybe each species stops growing or is "shocked" when nights get TOO cold. There needs to be a similar statistic for "chilling degree hours" to capture the temps that must be avoided, and how much the seedling has slowed down because of overnight chilling.
Now that I think about it, there are temperatures above which some seeds
germinate faster, and temperatures below which some seeds germinate faster. They say that seedlings usually prefer cooler temps than seeds, especially at night.
And maybe the ideal temp for bringing seedlings along fastest varies as the seedling grows up. So there would be a different "ideal daily temperature" and "ideal nightly temperature" for each week or day of the seedling's life.
Maybe it's much more complicated than a simple formula can capture.
Probably just plain
knowing approximately what conditions each species' seedlings prefer most for rapid growth is better than doing a lot of math. I guess we learn this from enough years of experience: this was a good spring for XYZ, so they must have LIKED the warmer-than-average nights.
But if some researchers did the research and came up with ideal numbers and curves for different species, gardeners could use that information and know how best to get each crop as early as practical in their exact micro-climate.
