The Importance of Daylength and Light

As winter approaches, the low angle of the sun and the short days and long nights, make every minute of sunshine, precious. Grumble, as we may about these long nights, without them, many plants will not form buds on schedule. I get asked so often about how I get certain plants to bloom which are notoriously shy about blooming as houseplants, and all I can say is that now that I have a greenhouse, I really don’t think about this challenge anymore. There was a time when every October, we would moves the large pots of Christmas Cactus, Poinsettia and Clivia into the dark, cool cellar for a month, to ‘trigger’ bud formation. A move that rarely seemed to work well. But once we built the greenhouse, every plant blooms on schedule, give or take a few weeks.

 I can look back every December on this blog, and see that my Haemanthus should be in bloom this week, (and it is). Clearly, day length is critical, and I would venture to say that it, along with temperature shifts, are the key triggers for bud formation with many plants. It does make one wonder, do humans respond to seasonal shifts in temperature and light? I mean, beyond the known benefits of light exposure ( which, I must admit, a sunny day in a winter greenhouse does wonders for those suffering from seasonal disorders, which Joe has, and I think that I do not). But I wonder if there are any other benefits from humans being exposed to natured elements that we not miss in our climate controlled homes?

The Poinsettia we see at the supermarkets are artificially subjected to short daylength periods earlier in the season ( draped with black plastic).  At home, it is almost impossible to get a plant to rebloom unless you are prudent about daylength ( even a streetlight or a lamp can throw off the photo period). The above plant I have is actually the wild form of Poinsettia, or EUPHORBIA PULCHERRIMA. As you can see, it is just starting to form colorful bracts, which is normal for late November. In the wilds of Mexico, where this plant is native, the same thing is happening. When I lived in Hawaii, I remembered old Hawaiian cottages that looked very much like New England cottages, but instead of blue Hydrangeas, red Poinsettias were in bloom in January. 

The plants we see now everywhere are not only hybrids, but drenched in growth retardant, which keeps them short enough for retail ( even shelves on shipping trucks are kept at  certain height to maximize profits). So good luck finding a tall, old-fashioned poinsettia unless you know of a good florist who grows their own ( rare these days). The wild form is weedier, but I am fine with that. My only concern is as the weather cools down, the greenhouse will start to stay at around 40 degree’s which is too cold for this genus, which prefers to stay above 50 deg. F. So I may move this, and a tree aloe which is forming a bud, into the studio for the winter, to see if that helps.

Speaking of light quality, I try to keep single pane glass as clean as possible, since I believe that light quality in winter, is one reason why I have good results with some of my winter growing bulbs. Since I must use bubble wrap insulation on the roof, my greenhouse is constructed with single pane glass, which may see very impractical given that I live in Massachusetts and winter temperatures can reach well below zero degrees F. for a couple of months in the winter. You may notice that many nurserymen and wholesale growers use poly hoop houses, those greenhouses that look as if plastic bags are stretched over hoops. These are usually inflated, with fans blowing air between the layers of plastic, which forms an insulating layer between the freezing temperatures outside, and the warm air inside. With glass, heat passes through quickly, and with single pane glass, even more so.

Still, I prefer the single pane glass for both aesthetic reasons and for the light quality, since South African bulbs require almost desert-like sunshine in the winter. So although I may tape bubblewrap insulation on the roof, I leave the front of the greenhouse un-insulated. I sited the house by looking where in our yard, the sunshine still hit on the shortest day of the year, so even though we have very tall trees, I can still get 7 hours of direct sunshine in the winter.

Light quality is affected with poly film plastic, so even the bubble wrap is not ideal, but it helps me feel better about my propane bill! Since I keep many of the bulbs near the front of the greenhouse, I feel that if I keep the glass clean, the light quality is even better. Today, it was warm enough to wash the glass again ( I had done it in late August, but you can see how dirty it gets in just a few months!). It takes about three washings on either side to clean most of the streaks off, since  the low angle of the winter sun seems to pick up every streak.