There’s something just a little bit sad about a field of Christmas trees in January.

I’ve grown Christmas trees for 45 years on a small scale. We sell about 70 trees each year on a cut-your-own basis. We plant about 80 trees each spring that I grow from seedlings in the garden. As I ski through our little field after the last customer has cut their special tree, I always wonder how the next generation of trees will grow enough to be ready to sell next December. But they always do.

The explanation for the visual size estimation is simple geometry. Think of a Christmas tree as a cone that grows both higher and wider each summer. The volume of a tree in January that is five feet tall and four feet across the base is 20.9 cubic feet. That tree will grow both taller and wider next summer, and it will be five feet ten inches tall and five feet across the base, for a volume of 38.2 cubic feet. In other words, the tree will not only be 10 inches taller, but its volume will also have nearly doubled in one growing season!

What does a tree need to grow? Christmas trees are a crop, planted and harvested regularly. The timeline is ten times as long as a corn crop, but it’s the same idea. All crops absorb nutrients from the soil and some of those nutrients are removed when the crop is harvested. Ergo, nutrients must be added to the soil to replace those removed. But a tree is not a corn plant, and the nutrient cycle is more complex than it seems.

As a Plant and Soil Science major at UVM, I took a course called Soil Fertility and Management. I first heard mysterious terms like Cation Exchange Capacity and adsorption (as opposed to absorption). I learned how the size and shape of soil particles affect fertility, and I even got reasonably comfortable with chemical formulas and equations. There was lots of research to absorb, nearly all of it related to chemical fertilizers and crops. 

The science around tree fertilization is not nearly as well understood, where it exists at all. I hit this wall when I planted my first Christmas trees about 45 years ago. I was still at UVM, taking those soil classes, and I wanted scientific recommendations on soil fertility and Christmas trees. I took soil samples and sent them to be tested, only to learn that there were no recommendations for Christmas trees. There is more known today and UVM now has recommendations, but research into fertility and trees is a tiny fraction of that dedicated to crops.

I grow trees differently than wholesale growers. For a wholesale market, trees need to be uniform, thick, and deep green to appeal to demanding urban customers. To get that sort of consistency, growers spend a lot of time and money to control “weeds,” insects, and diseases. To be sure the trees have plenty of nutrients, they spread commercial fertilizer around each tree.

My market is local, and the experience of getting a tree is as important as the tree itself. Our field is small and tucked into a corner of the woods. That’s good for the experience but means the trees near the woods don’t get as much sun as they could use. My attitude is that trees each have their character. Because my customers choose and cut their own, they are free to look for one that appeals to them in a natural setting. Most people put their Christmas tree against a wall and don’t mind a flat side; some look for such a tree. Trees that are not chosen will continue to grow next year, get pruned once again, and will usually appeal to someone next Christmas. I don’t strive for extreme uniformity. I allow a naturally round tree to be round, and a naturally skinny tree to be skinny. I don’t worry about insects, because my field is next to mixed woods with lots of natural fir trees that have evolved their defenses and I prefer to let natural controls take care of problems. I don’t consider the grass, legumes, and forbs that grow in the field around the trees to be weeds. We mow them two or three times each summer but I like to let them flower in August and September to give the pollinators a chance.

Managing fertility for trees is still mysterious. Most growers assume that they need to apply nitrogen, phosphorus, and potassium every year. The best way to judge soil fertility may be to look at the trees, how fast they are growing, and how green the needles are. To be scientific, one can analyze the needles for their nutrient content. Oregon State University has developed a fairly sophisticated system correlating nutrients in the needles to nutrients in the soil. Their research has measured how much is in the tops, the trunk, and the roots, and they have charts to help growers decide how much of the major nutrients to add to the soil.

I’ve never done needle analysis, though it would be interesting. Over the years I have fertilized with wood ashes, theorizing that they are the residue of trees and should contain some nutrients that will help trees. Wood ash has a long history of use in New England, for industrial purposes as well as an agricultural soil amendment. Ashes contain a lot of potassium, a fair amount of calcium, and a wee bit of several micronutrients. They are high pH, and must be used sparingly so as not to upset the balance in the soil.

I believe that the various plants growing under and around my trees play an important role in long term soil health and the availability of nutrients. Some of them are legumes, harvesting nitrogen from the air. Others have deep tap roots, pulling mineral nutrients from deep in the soil, below the reach of Christmas tree roots. The mat of organic matter, living and dead, absorbs and distributes water and is a continuous source of slowly released nutrients. The diverse plant community around the trees is also host to a wide spectrum of creatures, all of which are part of a web that serves to keep the soil in balance and the insect population diverse.

Then there is the magical world of mycorrhizae, the hidden helpers of plant roots. Mycorrhizae are fungi that form a symbiotic relationship with plant roots, and research is proving how essential they are to tree nutrition and health. They are perhaps the most important reason that I believe in growing my trees in as close to a natural setting as possible. But mycorrhizae are a whole topic unto themselves, so for now I’ll ski through the balsam fir trees that I have raised from infancy and be happy about their role in my neighbors’ Christmas traditions.