Friday 9 August 2013

The art and science of pruning (Week 18, Thursday and Friday)

There are three basic rules to pruning:
  • avoid pruning; the best way to go is to choose a species suitable for its location, & the available space, right from the beginning,  then to get a good specimen of the species;
  • look at the plant as an individual: by all means look it up, learn what to do with it, then observe how it grows, flowers, fruits - each specimen in its environment is different, and applying rules from books is not that straightforward;
  • do the least damage possible to the plant: use the right tools for the jobs, sterilised, sharp and in good working order; cut misplaced branches when they are still small and can heal quicker, make clean cuts that allow the plant's natural defences to kick in quickly, cause no damage to the bark (its physical protection), and leave no snags for pests & diseases to creep in through
That's the basics. And thanks goodness plants are generally rather more resilient than the theory of pruning principles makes them, otherwise there would be so many dead & diseased plants out there...

... that said, plants in cultivated environments need some managing, in terms of

Green wood blade (the raker
tooths helps to clear the cut)
  • health (dead and diseased wood needs removing, by the way, there are specific saw blades for dead wood)
  • formative pruning, for shape 
  • pruning for productivity (flowering and fruiting)
  • rejuvenating neglected plants
Pruning principles make sure pruning is done in the least damaging way for the plant. There are plenty of things to consider, and piles of books to describe them, but a quick summary could cover:
Dry wood blade (peg tooths)
  • work in clean and safe surroundings, for H&S but also so you can see what you are doing; remove the cuttings as you go, so as to get a clear view of the plant at all times, and from different sides
  • remove first of all all dead and diseased material; crossing branches are the next thing to look at, as rubbing causes bark damage; but remember to leave a framework of old wood to sustain new growth, which otherwise will likely flop;
  • make clean cuts: smooth surfaces (that heal faster) with tools appropriate to the task and in good working order, sterilised to prevent the spread of diseases
  • cut above an outward-facing bud (so the plant does not get congested with the new growth), with a slant gently sloping away from it to avoid moisture accumulating around it; however, you should make a flat cut to protect opposite buds with the blunt thick blade of your bypass secateurs (bypass is better than anvil, as the latter sometimes crushes the stem - see also these pictures).
  • by the same principle, always cut a branch back to where it meets another, without leaving snags, 
A bad cut: leaving snags and at an odd angle,
it will stand out like a sore thumb

  • good aestheticsis never far removed from the plant's health, so maintain the shape as natural as possible, that means: cut to a branch that is - at the crotch - no less than 1/3 of the diameter of the one that it originates from (this a. makes sure the branch that becomes the new leader has the strength to take the spurt of growth that will ensue b. will look more natural and not like you have axed the plant) and never cut where branches meet at odd angles, for example 90°
  • cut big branches in stages, and, to make sure they do not tear the bark under their weight on falling, use a two stage method: an undercut (some 5 cm) behind the first cut you are going to make, both of them some length away from where you want your final cut to be; that will allow you to have more control of the operation and make a clean final cut

A bad cut: dieback of snag and  cut at the
joint with a branch that was too small
  • when removing whole limbs, cut parallel to the branch collar and as close to the ground as possible (I mentioned this before with explanatory pictures) - by the way, V joints are weaker than U joints, so if you have to choose between which branches to cut, you know which one to go for!

The biology of clean cuts and target pruning goes broadly like this.

Parenchyma cells and wound healing

Parenchyma cells are general-purpose cells with thin walls that form so-called ground tissue, filling in between other types of plant tissue. They are alive, and because they do not have a narrowly defined, specific function of their own, they can be easily repurposed by the plant to form callus when a wound is inflicted to it.

A jugged surface slows down callus formation in two ways: first, there is more surface to cover; second, if parenchyma tissue around the wound has been crushed, the plant does not have ready cells on the spot to repurpose...

Cambium and compartmentalisation of decay 

Cambium is where plants produce new tissue. There are two concentric layers of cambium in a branch section: the cork cambium between the cork & the phloem, and the vascular cambium, that runs between the phloem and the sapwood. Cuts need to leave those areas intact: that is why we don't cut flush to the trunk any longer.

Where a branch joins the stem, the vascular tissue branches out as well to carry water & nutrients into it, and so does the cork cambium. These areas of branching, the branch collars, are where the plant is able to "compartmentalise" the wound: the way plants naturally heal. That works a bit like those sci-fi film spaceships that, having been hit by the enemy, shut out whole peripheral areas to save the core of the ship. Similar areas are present in the rootstock, where the stems join that come out of the ground.

If we do not cut flush to the stem and through the branch collar, the main stem will continue to work properly (so will the rootstock of ground branches). However, if a snag is left, the cambium in the branch may never manage to heal completely. Snags often die back to the main stem anyway, but in the meantime, the area will have been exposed to pests and diseases, with weakened defences, the damage possibly increased by moisture accumulating inside the wound.

Sometimes, however, the collar is not clearly visible, it's rather long or the branch's angle to the stem is odd and not easy to work with. The risk of damaging the bark (which is a physical barrier to access by pest & diseases, beside containing cambium), and the cambium areas, is high.

So one might decide to make a cut perpendicular to the branch instead (so the wound has the least possible diameter), as close as possible to the stem, but leaving a little stump.

I found a rather good tree pruning guide from the Central Bedfordshire Council. My sources are first and foremost my colleagues, to whom I owe a lot, Brown's book on pruning (which they suggested, and that describes pruning plant by plant) and the RHS practical guide to pruning & training.

All of this to say that I had the opportunity in the last two days to have some pruning training and join in in a session on renovating pruning, in which Rossana explained about our historical collection of Philadelphus and its renovation programme, and then I got to try what I had learnt on a Deutzia that needed summer pruning (deadheading and formative pruning).

Deutzia before

Deutzia after

I felt rather a lot of pressure during the task, but I was satisfied with the result: the plant was airy, open in the centre, it had flowering stems for next year (Deutzia flowers on second year wood) and a pleasant shape, not weighted down by the massive seedheads from this year's flowering.

1 comment:

LINDA'S said...

lovely post many thanks for sharing