Home Commentary More on Applying the Stockholm Method of Tree Planting in Montpelier

More on Applying the Stockholm Method of Tree Planting in Montpelier

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The carrots on the left, grown with granite grit and compost, are clearly larger than the carrots on the right, dug on the same day planted in the same garden with compost only, even though the carrots on the right were planted several weeks earlier. The comparison was unplanned. Photo by Dan Hemenway.
The street-tree planting strategy reported by Jenny Blair (“Growing Trees with Stones”) in the June 12–25 issue of The Bridge impressed me. The combination of biochar and compost is an excellent way to secure long-lasting fertility.

Surrounding a tree with cement prevents regular fertility amendments except at the small square of soil around the trunk. Feeder roots generally are concentrated farther from the planting hole if space permits. The system also provides roots with access to air, which they will struggle to find if not provided. 

Tree roots naturally spread far, well beyond the drip line that I was taught was their usual extent. The Stockholm method of street tree planting adopted by the Montpelier Parks Department’s Alec Ellsworth allows trees to send roots much farther than previous street tree planting arrangements. The provision for rain catchment watering is also impressively resourceful. 

I suggest a few possible additions to the Montpelier version of the Stockholm design.

Remineralization

Adding granite dust or grit to the compost can provide minerals important to tree health. It is available free from the Vermont Granite Museum in Barre, and most places where rock is crushed or cut. It is one of Vermont’s most underutilized resources.

Called ‘remineralization,’ fine particles of freshly cut or ground rock may replenish soil minerals lost through extractive farming or supply minerals absent from poor soil. Compost supplies humic acids needed to extract fertility minerals from the rock particle surfaces. Stones vary in their fertility value.

Biochar added to compost in this method binds minerals to its unique carbon structure. Plants can access that natural fertilizer as needed. Biochar will hold fertility in place until the tree roots grow into the extra space under pavement that this method provides.

The feeder roots of most trees are concentrated in the top foot or so of soil. But in street trees, pavement replaces the most fertile zone. To me, this method of planting makes the best of that difficult situation. I think that adding stone dust to the biochar compost mixture may extend the longevity of the trees. I’d recommend a starting ratio of 20:1, compost to rock dust. Pot tests could refine the ratio and determine which of the available rock dusts best work in this situation.

Permeable Pavement?

Persons concerned with maximizing penetration of rainwater into the soil have designed sundry systems that have most of the benefits of pavement but allow some infiltration. I have not encountered information about whether or not tree roots and permeable pavement are compatible. The Stockholm method apparently sidesteps the problem, however.

Essential Soil Fungi

Mycorrhizal fungi are a class of beneficial soil organisms that greatly benefit most temperate-zone plant species. Essentially, their hyphae — fine threads that are the body of a fungus — form symbiotic relationships with plants. These fungi are now widely available as soil inoculants. Most forest, garden, and nursery soils have natural populations of these highly beneficial organisms. While experts suggest that it is a waste of money to buy inoculants for gardens and most landscapes, inoculating the compost used in planting street trees might be good insurance, given the expenses involved. The cost would be tiny compared with the possibility that the tree may live longer. They may be present in the soil or the compost used, but a few dollars spent for tree plantings that cost roughly $10,000 each may be prudent. The fungi species that benefit gardens differ from those that benefit trees.

Phosphate

Phosphate is another amendment that may extend the longevity of street trees. Rock phosphate would be ideal as it is only slowly soluble, providing a small but important continuing contribution to tree longevity. Mycorrhizal fungi are adept in converting phosphorus into a form useful to plants. Phosphorus is vital to all living things and is irreplaceable to photosynthesis. Bone is also a good source of phosphorus, but bone meal is relatively expensive and the slow release of rock phosphate would better suit the situation where soil cannot be amended after the planting and paving are done.

I applaud the Montpelier Parks Department for recognizing an extremely promising approach to establishing trees bounded by pavement, which is inherently hostile to the needs of tree roots.

Robert Kourik, robertkourik.com, reviewed this article for accuracy. Kourik is the author of “Understanding Roots” and other books on gardening and edible landscaping. Remineralization also is beneficial to gardens, etc. Hemenway will email a free three-page PDF illustrating an experience in using rock dust in a vegetable garden to anyone who requests it from permaculturemail@aol.com. 

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