arboriculture

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Living Structures: PLEACHING

A plant is considered to be ‘inosculate’ if it is self-grafting; if the branch of one individual will, as the result of gentle abrasion, form a living bond with the branch of another individual, or with another branch of the same plant. When this grafting is aided or initiated by humans, the process is called 'pleaching.’

In mediaeval Europe, in areas where annual flooding endangered human settlements, the pleaching of inosculate trees was employed as a solution to what otherwise might have been an insoluble problem. The trees were planted on a grid, like a small orchard. As they grew, branches were pruned and trained along this grid, so that eventually the branch of one tree met that of its neighbour. At that point, an incision was made in the bark of both branches and they were tied together, like blood brothers or sisters. The analogy is deserved, in that not only did these branches grow together to form one member, but their support activities (condition of water/minerals and sap) merged, thereby joining the life processes of the neighbouring trees.

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Diagram: McGill Architecture // Photo: Angus Kirk


More on living sculptures and grafting

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Photos courtesy of Strange and Wonderful Things, Rare and Exotic Seeds, and Stan Shebs.

Chiranthodendron pentadactylon aka Devil’s Hand, Monkey’s Hand, and Mexican Hand Tree. Native to southern Mexico and Guatemala. Hardy in zones 9-11.

IN DEFENSE OF TOPPED TREES

A Permacultural Perspective

Most arbourists these days will tell you that tree topping (the practice of pollarding large trunks and branches on mature trees) is an excessively destructive process: it’s largely been replaced in the trade with spiral thinning.

There are a variety of reasons for this advice against tree topping: all having to do with the long-term health and integrity of the tree. The caution against topping trees is relevant for gardeners, landscapers, horticulturalists, arbourists, and city planners, but not neccessarily a hard and fast rule for permaculturalists.

The reasons for this have to do with differing ideas about the function of a tree:

  • In a heavily-landscaped garden or city street, there may be a few trees that constitute a significant investment, and the aesthetics and maintenance of those trees tie in to property values. Topping compromises the health and durability of the tree, which is fundamentally an economic concern.
  • In the permacultural ‘food forest’-style garden I’ve built, I currently have 50+ fruiting trees, and I am planting and grafting new trees every day, so it’s not a big deal if one dies. I use pruned lumber for a variety of purposes, and sometimes I deliberately do things that impair the health of a tree – such as planting parasitic mistletoe – in order to create wildlife habitats.

Last year, I topped one apple tree, and did some drastic ‘spiral thinning’ on another. Both survived, with the topped tree sending up much-maligned water sprouts, and the spiral-thinned tree leafing out as usual (albeit with a little sun scorch on the leaves).

Topped Crabapple, with grafted water sprouts

Drastically spiral-thinned canopy on a Skovfoged apple

I’ve noticed that on the topped tree, fungus has begun to be established on the two stumps. Far from being dismayed by this, I am actually overjoyed! If you want to understand why fungus on my apple tree is welcome, look no further than the first picture accompanying this piece of writing.

Topping can stimulate the development of tree hollows, which are vital habitats for bees, beetles, birds, and tree-dwelling rodents like squirrels.

Tree hollows in living trees are cavities where the heartwood has been degraded by fungi or bacteria. Trees compartmentalise decay, and the only actively growing or vascular tissues of a tree are concentrated towards the bark. This means a tree can potentially be almost completely hollowed out, but still continue living.

Bees in a tree hollow

Topping provides an opening for fungi and bacteria to create a cavity in the tree, and thus creates habitat. The excessive growth of water shoots is also a benefit, as timber is a valuable resource for mulch, hügelkultur, and carbon-sequestering soil building.

I’m doing my best to preserve these cavities in trees in the forest garden, even in trees that otherwise seem doomed. In the case of my European Ash tree with developing fungal lesions, instead of getting rid of the tree, I’ve done a bridge graft around the lesion. The hollow will continue to form, but the vascular pathways will remain intact.

Topping trees, like sheet mulching, can essentially be an accelerated way of promoting ecological succession in a forest garden. Fungi and bacteria also have a place in the ecosystem, and naturalistic garden maintenance means embracing those forces, rather than trying to control or eliminate them. We can always plant new trees, but bringing species back from the brink habitat loss-induced extinction is a little harder.

To me, local wildlife is included in the permacultural ethic of ‘fair share,’ as in, they too deserve a fair share of the space and harvest from this garden. You may be surprised at who moves in, when you make them welcome!

Tree Hollow Dwellers

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Ginkgo biloba

Ah yes, the Ginkgo. Isn’t it an angiosperm because of its broad leaves? Wrong! It’s a gymnosperm. It’s also a monotypic plant, which is just a fancy word for saying that it is the only one in its taxon. The leaves can be bisected or not and the yellow fall color is unparalleled.

The ginkgo has been used for centuries as a medicinal plant, often being associated with good memory. On top of this, the plant has no known diseases or pests! That’s great!

“But what’s the downside?”

Well you see, the Ginkgo is also dioecious, meaning that trees are separate-sexed. The male tree is great! The female tree is too….except for the small fact that it’s fruit stink to high heaven. Seriously. I have had people tell me that the smell has nearly knocked them down before. Overall, however, it’s a very wonderful plant!

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Treating Fire Blight

Fire blight is a bacterial disease that primarily affects the young, rapidly-growing shoots of pome fruits (especially pears and quince, but also apples and firethorns, and to a lesser degree, hawthorns, serviceberries, loquats, mountain ashes, and other related species in the subtribe Malinae).

It is spread by pollinators like honeybees during flowering, and pinkish orange streaks develop in the wood as a canker develops. If left over many seasons, the infection moves from the canker towards the roots.

The name ‘fire blight’ refers to the burnt appearance of leaves, bark, and fruit at infected sites.

Younger trees–and rapidly-growing tissues–are more vulnerable to infection. Infected wood can’t be treated with the application of antibacterial and antifungal copper sprays: and must be pruned out. However, an older tree tree can live with the disease.

The tree above is my Clapp’s Favourite European Pear, on an espalier. As sad as I was to cut off 10 fruiting spurs, it’s much better for the long-term health of the tree that infections are eradicated as soon as they appear.

The canker becomes active in spring, dripping a sap laden with bacteria down to the soil, and also slowly moving towards the trunk and roots, thus continuing the life cycle of the pathogen. Instead of allowing that to happen, I am burning the infected tissue and converting it into useable ash and biochar, both excellent soil amendments.


Read more: Fire Blight Identification, Life Cycle, and Management

#Pyrus #pruning #plant diseases #fruit trees #microorganisms

Grafting, from start to finish

Roughly three months ago, I started adding wood from a number of different pear cultivars to my mature ‘Conference’ pear tree.

This graft is a ‘whip and tongue’ graft of a ‘Marie Louise D’Uccle’ pear scion.

I peeked under the electrical tape binding several of my attempted top-working grafts about a month ago: the white masses of callus tissue forming between scion and stock were the first indicator that at least a few of the grafts I attempted were successful.

This one was by far the most healed at that point, due in no small part to the amount of vascular cambium tissue contact inherent in this type of graft: my other grafts will be slower to establish, because many of them are cleft grafts.

The next indicator of a robust, successful union is when the scion begins growing leaves of it’s own: I observed a month ago that this scion was well on it’s way to photosynthesising and branching out.

Over the growing season, the grafted tissue expands as the scion and stock coalesce.


Soon, it will be one branch among many, producing one of several different kinds of pears on a single tree!

This is my first whip and tongue graft, so I couldn’t be happier it worked.

Before actually trying grafting at all, I was struggling to understand it. It seemed complex when I was looking at line drawing diagrams, trying to understand how to align things and recognise different kinds of tree tissues: I was fundamentally not expecting it to work.

I think the best thing you can do to learn these sorts of skills is just try: I had an easier time learning from real photographs, video tutorials, and practising it myself.


More on grafting

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Photos courtesy of Daniel FuchsRasbak, Brosen, and Eike Wulfmeyer.

Rhus typhina aka Staghorn Sumac and Velvet Sumac. Native to North America (distribution map). Hardy in zones 4-8. Look at that foliage. Bask in it for a moment. I mean, uhh, this deciduous shrub is dioecious. It grows well in dry and poor soil, but spreads somewhat aggressively via multitudes of seeds and rhizomes.