I decided to cover the little keraji seedling in the ground with a cut transparent plastic water container, as part of an experiment to see if they will be able to survive, or if it would help extend the growing season in this cool climate.
The little keraji seedling has actually put on some noticable growth since being covered with the plastic container. Here it was at the end of August, not having been covered yet:
Here's the transparent plastic container that was put over it:
Here it is on September 20, temporarily with the cover off so you can have a clear view:
And now here's another picture taken today (September 26 ) and you can see it's noticably put on even more growth:
Obviously the plastic container coverings are working.
I think it's helping for three reasons.
First of all, the container holds in the humidity. This may not be an issue in the South, but in this climate when it's warm and sunny it also tends to be fairly dry. As temperatures are cooling down, that ironically is also going to contribute to dehydration since as the leaves are warmed by the sun's light they will be slightly warmer than the surrounding air, and when you have an object that's warmer than the surrounding air a drying effect occurs (analogous to freeze drying). That's one of the reasons most trees in the Pacific Northwest are conifers, by the way, with needle-like leaves for maximum passage of air to cool off so they do not become warmer than the air (which tends to be cooler from the air blowing in from the ocean).
The second reason is insulation that helps the wind from carrying away heat (this is somewhat like the windchill effect). It might not sound like much but just a simple plastic layer that prevents the blowing of air through the leaves can help raise effective temperatures by a few degrees. This is important with the cool climate here and the growing season coming to an end.
The last reason is the greenhouse effect. Sun light that enters hits the plant or the ground as is converted to heat. Light can enter through the plastic more easily than heat can leave, so as a result the temperature inside a greenhouse rises while the sun is out. This can be a pretty strong effect. A simple plastic container may not really function as the most effective greenhouse but I suspect temperatures inside the container may be 5 or 6 degrees warmer than the immediate area outside (at least while the sun is out and shining on it). This greenhouse effect is far less effective at keeping temperatures up after dark, but warmer temperatures in the middle of the day can mean a longer growing season for citrus, since temperatures are about now dipping just below what citrus needs to grow in. The season is already beginning to cool off.
The thing to fight in the PNW is cool temperatures (cold-cool), not so much extreme cold events. That's why I think this type of strategy is probably a lot more effective here than it would be in the South.