I just returned from spending two deliciously warm and sunny weeks in Mexico doing as little as possible while the rest of you endured “Snowmageddon” along with some nasty freezes that pretty much put the skids on any productive work in the garden. With that kind of weather I would have expected my garden to look about the same as when I left it 14 days prior, but I discovered some surprising changes.
While I was gone my little Winter Aconites woke up and came into bloom with their bright and sunny yellow flowers. They aren’t much more than two inches tall, but they sure can illuminate a shady bed. The Snow Drops were starting to emerge before I left and are now 6 inches tall and in full bloom in sweet little clumps scattered throughout the garden under trees and shrubs where it will stay cool and shady during the summer when they are asleep. I have patches of daffodils that were nonexistent two weeks ago that are now 8 inches tall and could easily be in bloom in another 10 to 14 days. I also saw a few clusters of tulips that have been in the same place for several years that are already 4 to 6 inches tall. And of course my wife’s Scillas are all over the stinking place, despite my aggressive culling after they bloom every spring.
Bulbs as a whole seem to be indifferent to winter temperatures and will proceed with their growth regardless of whether it is a normal winter or one that is excessively cold. The same is mostly true for perennials. Case in point is Oriental Hellebores. Over the last two weeks mine have gone from just barely showing any sign of potential blooms to vigorous clusters of 10 to 12 inch flower stems, which will continue to elongate over the next couple of weeks until they are in their full glory at around 16 to 18 inches tall. When one is gone for two weeks these transformations become quite evident.
While bloom times for spring bulbs can vary by a couple of weeks, they are mostly predictable and my theory for that is simply that bulbs (and perennials) are more responsive to soil temperature rather than air temperatures. Air temperatures can fluctuate wildly but soil temps (especially in the northwest where the ground rarely freezes) remain fairly constant. In the winter they seem to run about 10 degrees warmer than the air and in the summer 10 degrees cooler. The deeper you go the less variation there is. At around 5 feet deep the soil temperature is basically constant. For an interesting article on soil temperatures check out Cliff Mass Weather Blog, Soil Temperatures and Gardening, Sunday May 6, 2012.
To further reinforce my hypothesis, I offer two more examples from my garden. First, I have a Sarcococca ruscifolia shrub that is growing in my front drive surrounded by asphalt and underneath a deciduous birch tree. It is only 2 to 3 feet tall and is in full fragrant bloom. The protection of the birch tree and the additional heat from the asphalt have countered the cooler air temps, so it is blooming when it normally does. On the other hand, my Cornelian Cherry has yet to bloom (3 to 4 weeks behind schedule and counting) due to the fact that the blooms are 10 to 12 feet above the ground where the cool air is able to slow everything down, irrespective of what is going on down in the ground.