Garden School:


Garden School:
Teaching this week: Rose pruning (as always!) and leaf mold.

Friday, 6 November 2015

How to get Couch Grass out of Bearded Iris

Here we go again: a new Client says "oh, could you just weed that clump of Iris for me, I've done all round it but there's a bit of grass I can't get out, in the middle of it."

A "bit" of grass?

Hmm, this is what I could call a full-on infestation of weeds in general but couch grass in particular.

This was a Bearded Iris, by the way, but this applies to any rhizatomous Iris.  They often need to be sorted out as part of the autumn tidyup (or "autumn slaughter" as I call it), and this is the time when you discover that they have grass and weeds poking up all around and through them.

It's easy to get into this situation:  Iris (plural of Iris, anyone? Irisis? Irises?) tend to form dense clumps, and if a blade of grass manages to infiltrate, it's impossible to get at it without damaging the rhizome, so there is a tendency to leave it "for now" and by the time flowering is over, and you get around to dealing with it, you find that there is a nasty infestation of grasses along with general weeds: and if you are really unlucky, the grass is couch grass.

What's the answer? Unfortunately, the only answer is to lift the entire clump, but it's not all bad news, as it gives you a great chance to remove any unproductive rhizomes, along with any rotting ones: this means you can space them out more evenly when you replant, and there will usually be enough to either start some new clumps elsewhere in the garden, or to give away to friends. (Or to pot up, for selling or swapping on GreenPlantSwap next year, of course!)

It sounds like a big job, but it's not so bad: I use a border fork, which is smaller than a normal one, and can more easily be pushed in amongst the other plants without damaging them. Get a sheet of plastic to protect the grass before you start, and work your way round the edge of each clump, levering gently each time you push the fork in: don't aim to rip them all out in one go, aim to loosen the soil all the way round first, so as to do minimum damage to the roots.

When the clump is loosened, lift the whole thing out and plop it down on the plastic sheet. Repeat for as many clumps as you have.  Now you can dig over the whole area, loosening the soil, and removing every scrap of couch grass root, along with any other weeds or unwanted plants.

At this point, stop and assess your soil: if you think it looks a bit impoverished, you could take the opportunity to improve it a bit but be warned, Iris don't like rich soil, they actually like things a bit lean and mean. Too much "goodness" in the soil, particularly nitrogen, will just result in lush massive leafage, and not much on the flowering side, so don't get too carried away.

Having prepared the bed for replanting, you can clean up the rhizomes. Shake off all the soil, get a sharp knife (or an old pair of kitchen scissors) and trim off any damaged rhizomes - sometimes you can't avoid tearing them as they are lifted, but all is not lost, as you may be able to tidy up the torn end. Look for any that are squishy or mushy, and trim them off, too. This season's flowering stems can be cut right off as well: they won't flower again from the same point, but each flowered rhizome should be forming new "arms" which will flower next year. I usually remove any feeble or spindly bits, as they are unlikely to do much, and I look closely for signs of pests: if I see any rhizomes with what look like woodworm holes, I cut off the damaged section and discard it.

This should leave you with a good selection of neat, firm rhizomes, thumb-thickness or more: firm, light-coloured, and with at least one good fan of leaves. Some people will trim off any dead roots - they are the dry, dark, skinny, ropey ones, as opposed to the fat white wormy ones and the wire-like ones - but I usually leave them,  as they can help to anchor the Iris when you replant it. If your rhizomes don't seem to have any roots at all, or only have little tiddly things, then there is one option you can try - pop them all into a shallow dish with an inch or two of water, propping them up so that the bottom of the rhizome is only just in the water.  Leave them outside but keep the water topped up, and in a couple of weeks they should have grown a whole mass of strong new roots.

Before replanting, work out which way the sun shines on the bed: the rhizomes need to be baked by the sun in late summer, in order to make good flowers the following year, so the trick is to orient the rhizomes so that the non-leafy end is pointing towards the sun. This might look a bit regimented at first, but by next season they will be putting out new growth at odd angles, and they will quickly look natural again.

At this point, I usually chop the top leaves down into a neat fan shape (left), to make it easier for them to establish themselves without being blown over by the wind. Iris seem always seem to be a massively top-heavy plant, and although they will need some leaves to photosynthesise, it is more important that they get their roots growing away, which is not possible if they are constantly being pulled out by windrock.

Now comes the clever bit:  replanting them in such a way that the rhizome is on top of the soil, but so that the roots are able to hold it in place.

Are you ready for this? It's easier to show than to describe, but here we go:  firm down the soil, decide where the first rhizome is going to be, then scrape out a trench to either side, patting and forming a firm central ridge of soil to support the rhizome, like an earthen plinth. Plonk the rhizome down on the top of the ridge, parting the roots and draping them down either side of the ridge. Now backfill the trenches with loose soil, patting it down firmly to hold the roots in place. Behold! Your rhizome is now sitting proudly above soil level, but is not wobbling and rocking to and fro.

The acid test is to water them in, once you have finished: if any of them fall over, you've done it wrong! If that happens, just lift out any fallen plant, and re-do the plinth-and-trench, bringing in more soil from elsewhere in the garden if you need to.

Here is the final job: nice clean rhizomes, nice clean soil, nice neat (ish) rows, big ones towards the back, littler ones towards the front, all pointing at the sun, which chose not to shine on this particular day, but when it does, these Irises will soak it up.

As a general rule, it's good to carry out this operation on overcrowded clumps of Iris every four or five years: there is no hard and fast rule, just do it when they start to look as though they are climbing over each other to get out of the soil.

Best of all, having dug the whole bed over thoroughly, this Client won't be troubled with couch grass again - as long as they keep an eye on it, and pull out any new growth as soon as they see it, instead of leaving it until it is so bad that they have to call in the professional!

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