A strange title, and there are many ways to kill shrubs but a recent question seems relevant because we are, or should be, in prime shrub-planting time. The question was along the lines of;
‘I’ve planted a Japanese holly hedge in a trench containing existing chalky topsoil, multipurpose compost and a loamy topsoil. The soil doesn’t appear to be draining properly. What can we do?’

We are always told to add compost to the planting hole when planting and the idea is that it makes the transition for the roots, from the ‘easy’ compost in the pots to the garden soil, easier. The problem comes when a hole is dug in heavy soil and fills with compost. Water fills the hole and no matter how good the compost in the hole, the water runs in and the roots become waterlogged. When roots are constantly in water they suffocate and die. It is an easy mistake to make.

When I started in this garden I was in a rush to get started. The garden was a field. It was previously cut for silage or hay every year, was not ploughed, never had anything added apart from synthetic fertiliser and the soil is low in organic matter and horribly compacted. So some of the first shrubs I planted, before I had dug over the new beds, died. Even though I planted them on slight mounds, the holes filled with water and the roots died. A philadelphus grew well for a year but then, the following spring, having sat in sodden soil all winter, it failed to sprout.

And I hadn’t done the classic ‘dig a hole and fill it with compost’. I had mixed the compost well with the surrounding soil but really all I had done was make a wider planting hole and it still filled with water and, because the water had no where to go, the plants suffocated. I knew I was taking a risk but lack of time and being in a hurry to plant, made me reckless.

It all depends on your soil. In well-drained soil and in gardens that have been cultivated for a long time this is less likely to happen. In my UK garden I left behind where I had free-draining, fine, Fenland soil (boy how I miss that!) I could push clothes pegs in the soil and they would root and grow!

But now I make sure that I prepare the beds well in advance. I dig over the beds, mixing in lots of organic matter and I make sure I cut out the edge of the bed which is basically a gutter in some parts, to keep the water moving away from the beds. I try to dig at least six months before I want to plant and then add more compost at planting time and aim to mulch with organic matter too.

In the case of this hedge in the question, digging out a trench and filling with organic matter is a route to disaster. If the hedge has not been planted long it would be worth try to replant the hedge on a ridge so at least some of the roots are well drained. If there is any sort of slope it may be possible to put a drainage pipe in to move the water from the lower end. Few evergreens can withstand poorly drained soil and it may be that other plants would be more suitable, including cornus as a less formal hedge or hornbeam. I have also found that Cotoneaster franchetii is a very adaptable hedge and is part-evergreen. I have found out myself that hollies are very intolerant of wet soils and Japanese holly (Ilex crenata) is no exception.

how to plant shrubs, making new shrub beds, waterlogging shrubs