Friday, 15 April 2016

Reverted Salix caprea 'Kilmarnock'

A few days ago, I wrote yet another article about these small, weeping trees, featuring questions from two readers, each wanting information about their pruning and management.

Brenda in Georgia has kindly sent me photos of her two "problem" trees, which is fantastic, as now I can make comments that really make sense!

The first thing to say is that I hadn't realised that these were very short trees: I had assumed that they were the "usual"  Kilmarnock type, where the weeping part is grafted onto a short, upright stem of about 5' tall.

However, in this case they were both grafted just a couple of inches above the ground. It doesn't change what they are, or how to deal with them, but it does result in a rather strange-looking "tree". (Sorry, Brenda!)

Starting with the upright one, I'm afraid it does look like a reverted tree: if you look really closely at the base, just above the ground, you can see there is a bulgy section - that's the graft.

I've made an annotated enlargement of the bottom of the stem, to show you what I mean, but first let's quickly run through an explanation of grafted trees.

A nursery worker starts with a small, ordinary, upright tree - in this case probably a normal, non-weeping Salix caprea (Grey Willow). It will have a single, strongly upright central trunk, and it will be good and healthy.

This is called the “rootstock”.  This is chopped off at the required height - normally, about 5' high, but in this case just a couple of inches above ground level. They then take some cuttings of a different plant, a pendulous Salix caprea, and graft them onto the top edge of the rootstock. Usually, there will be a number of cuttings grafted onto one rootstock, in order to get a regular spread of branches: sometimes only one, depending on what was required.

The grafting involves cutting slits in the bark and gently sliding the cuttings into place, then binding them all with grafting tape. If the work is successful, the cuttings will “take”, and will start growing. The rootstock makes a callus over the grafts: in effect, it “heals” over them, and leaves a visible bulge. This is how we know when a tree has been grafted.

So we have a grafted tree with a visible bulge: below the bulge is the rootstock, above the bulge is the decorative, grafted material.

Now, if you have read any of the other articles, you will remember that I mention reverting as a problem with grafted trees - this is where the rootstock sends out growth, which is usually more vigorous than the decorative top section. If the owner doesn't notice in time, these shoots can take over the entire tree - all the energy of the rootstock goes into supplying its own shoot, and the grafted section is left to die.

In this photo, below, I have drawn a sketch with arrows, and the blue line indicates where the graft is.

If you look at the graft area, in the photo above, can you see that there is a dead, rotting stem pointing straight up, and the strong live one is coming off at an angle? From a distance, it looks like an upright stem, but you can see at the very base that originally it was shooting out at an angle. This is very typical of a grafted tree where the top part has failed, and the rootstock has taken over.

If you check between the picture and the sketch, you can see:

A is the rotting remains of the original, grafted top, you can see that it is above the graft line.

B is the new shoot - and can you see where it originates from? Above the graft line, or below it? Yes, it's below.

C is the point on the rootstock where that shoot originates from.

This means that this particular tree has sent up a shoot from below the graft union, which has taken off like a rocket, and is now starting to grow into a perfectly ordinary, non-weeping, willow tree. The bad news for Brenda is that it will grow and grow, until it becomes a full-size willow tree, which is way out of proportion for her lovely garden.

The only course of action here is to dig the poor thing out, and throw it away - it's not even worth planting out somewhere where it can be allowed to grow to fulfilment: that rotted stem is eventually going to spread disease back down through the rootstock, and it will die long before maturity.

Now we turn to the other one: how did you describe it, Brenda? "A tangled mess" was the phrase, I believe! *laughs*

Here, it looks as though the idea was to have the willow tumbling down over the low wall.

However, it's very congested, you can't really see the structure of it, and the ground around it is so cluttered that it just looks a mess.

This next part is a bit tricky, as I have to try not to insult Brenda too badly (!!) so what shall I say? If I were your gardener, Brenda, I would certainly attempt to retrieve it - but it's one of those jobs that is much easier to demonstrate than to describe, so you might not want to do this yourself - although I would certainly encourage you to have a go, on the grounds of "you have nothing to lose!"

I would do exactly what I said in the article about pruning a weeping tree, except that I would be on my hands and knees while I did it.

Firstly I would snip out any dead stems: at this time of year, that means anything which is not putting forth leaves. Then I would snip out,  as close to the trunk as I could manage, some of the lowest stems, the ones that are brushing the ground. Then I would look at what was left, and would consider a little artistic fettling, to take out any branches that were crossing other branches, or which were spoiling the general appearance of the - er - tree. Although I'd probably have to call it a shrub, now that I can see how small it is!

After doing all that, I would clear out all the fallen leaves and debris underneath, and would clean out any weeds or other plants growing under the canopy. I'd probably also spread a thick mulch of something really dark in colour, such as bark chips, in order to show off the plant.

Now, what would I do if it were mine?  (I can feel Brenda wincing in advance!)  Well,  I would dig the whole thing up and put it in a pot, frankly: on a raised stand, so that the branches could "weep" properly, and I would probably mulch the pot with light-coloured gravel or shingle - again, for contrast, so that you can see the shape of the branches in winter.

So there you go - I hope this tailored advice and encouragement is what you wanted to hear, Brenda, and that's my advice: dig out and dump the tall one, and be brave, and have a go at the short one!

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