In the Garden September 2015

By Rhiannon Allen

Lately, I’ve been thumbing through old gardening books and magazines. One thing that strikes me is how much gardening advice has changed in the last 20 years. Most of this change is driven by the application of careful scientific observation and research to practical gardening advice, in the same way as scientific advances lead to changes in medical treatment. Two examples illustrate such changes – one that occurred recently, and another that is happening right now.

Tree pruning advice is a prime example of a recently completed change. Decades ago, people would prune tree branches at any old point. Perhaps they would also slather paint or tar over the cut. Pruning in this manner was based on the folk assumption that trees are robust and that, as with fingernails, where to cut was a matter of practicality and aesthetics. And as with human skin, covering a wound was expected to result in faster healing.

But in 1984, Dr. Alex Shigo published a scientific analysis of how trees’ cellular structure protects them from injury and spreading infection. For decades, researchers had been chipping away at folk misconceptions and previously published advice, but Shigo managed to pull this research together in a clear and convincing way, and draw a clear link between botanical facts and best pruning practice.

This paper revolutionized professional pruning, although it took a while for its recommendations to penetrate the practices of home gardeners. I first learned about Shigo’s arguments in a pruning workshop at the Brooklyn Botanic Garden in the late 1990s, in which I was taught to identify the slightly raised collar where a branch joins its parent branch or trunk, and then to cut the branch exactly and cleanly at that collar. This allows the tree to isolate the tissue damaged by the cut and to form a barrier against opportunistic fungi, insects and other nasties. While this advice was new to home gardeners in 2000, all reputable gardening magazines, books and websites now recommend this practice.

A current transition in gardening advice is soil amendments. Everybody had (and still has) his or her own favorite fertilizer. Compost, manure, commercial preparations, kelp, minerals, Epsom salts, earthworm castings, home recipes – sigh. The list goes on and on. Gardening books, magazines and television shows abound with advice. It can all be overwhelming.

And then a presentation to the Point Roberts Garden Club last year changed everything for a number of us local gardeners. Mycorrhizal fungi. Well, I had heard of these before. They are fungi that live in the ground and form symbiotic relationships with the roots of some plants. In exchange for carbohydrates supplied by the plant, these tiny fungi help the plant absorb water and soil    nutrients. When I first noticed these tiny white filaments in the soil a long time ago, I thought they were a sign of fungal infection in the soil. I didn’t realize at the time that they are a sign of healthy roots.

Several decades of agricultural research have now demonstrated that inoculating soil with mycorrhizal fungi results in plants as much as doubling their size in comparison to those grown with no fertilizer. Seedlings planted with mycorrhizal fungi are taller, healthier and more productive than those planted with other soil enhancements. And finally, plants whose soil is rich with mycorrhizal fungi need less water and fertilizer to become healthy and productive.

While mycorrhizal fungi can be more expensive than water and commercial chemical fertilizers, being able to reduce our use of the latter has tremendous benefits for the environment. Since water, phosphate and nitrogen fertilizers that are not taken up by plants are simply washed away into the environment, reducing their use will help us avoid water waste, phosphate run off, water pollution and algal blooms in bodies of water.

So where do you get mycorrhizal fungi, and will they benefit the home garden? Since research is only just beginning to translate into home gardening advice, this is a bit of a challenge. Until more guidelines and products are available, it makes sense to assume that all plants will benefit unless they are known not to use these fungi (e.g., kale, cabbage).

As for buying these fungi, research suggests you should source as locally as possible in order to purchase a mix of fungi that will work with your soil, plants and climate. Since you are unlikely to see a bottle of Pacific Northwest mycorrhizal fungi (usually available in powdered form that you add to your watering can or soil) on your local nursery shelf, try ordering from Fungi Perfecti in Olympia.

In science, there is always more to know. But how exciting it is to see plants growing healthier with each advance of biological and agricultural science.

I wonder what the next installment of research will bring.


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