Why will nothing grow where a conifer hedge used to be?
Four things happen under a conifer hedge at once — total dryness, exhausted soil, a mat of surface roots and a thick needle litter — and they need dealing with in that order.
- Difficulty
- intermediate
- Time
- 3 hr
- Read
- 4 min
- Safety
- caution
Short answer
The soil is bone dry, structureless and exhausted, threaded with roots, and covered in acidic needle litter. Remove the litter and as many roots as you can, soak the ground thoroughly and repeatedly, then work compost in and mulch heavily. Test the pH before assuming acidity is the problem — it is usually a smaller factor than the dryness. Give it a full season before planting anything you care about.
The strip where a leylandii or cypress hedge stood is the worst ground in most gardens, and people usually attribute it to something the conifers put into the soil. Allelopathy — chemical suppression by the tree — is a small part of it at most. The dominant causes are physical and they are all fixable: the soil has been under an umbrella for twenty years, and everything followed from that.
Safety
What you'll need
- A mattock or a strong fork
- A hose or a slow-running sprinkler
- Compost or well-rotted manure, in quantity
- A soil pH test kit
- Bags or a skip for the litter and roots (optional)
Step by step
- Understand what actually happened.A dense conifer hedge sheds rain outward beyond its canopy, so the strip beneath it may have had almost no water reaching it for years. Its roots stripped water and nutrients from the same strip. The needle litter built up into a thick, water-repellent mat. And there is now a dense network of surface roots left behind. Four separate problems.
- Remove the needle litter.Rake off the accumulated needle mat. It is water-repellent, it is dense enough to stop anything germinating, and it takes years to break down in place. Composted separately, mixed with plenty of green material, it becomes a perfectly good soil improver — it is only a problem as a mat in place.
- Get as many roots out as you can.The surface root mat is what makes the ground impossible to dig and what will keep competing for water even after the tree is gone. A mattock and patience, or a hired stump grinder for the main stumps. You will not get all of it and you do not need to.
- Test the pH — do not assume.Conifer needle litter is acidic, but the effect on the mineral soil beneath is usually modest, often only a few tenths of a point. Test it. If it comes back below 5.5, lime it. If it comes back at 6.2, acidity is not your problem and liming would be a mistake.
- Water it, properly, before anything else.This is the step that matters most and the one people skip. The soil is likely to be dry to a considerable depth and may have become water-repellent. A slow-running sprinkler or a soaker hose for several hours, repeated over several days, is what is needed. A quick soak with a hose wets 30mm and runs off.
- Check that water is actually going in.After watering, dig a hole and look. If the top 50mm is wet and everything below is dust, the soil is hydrophobic. Break the surface with a fork, add a wetting agent or simply keep applying water slowly over more days, and check again.
- Work compost into the top 150mm.This is one of the rare cases where incorporating rather than surface-mulching is justified, because the soil has almost no organic matter and no structure and you are effectively rebuilding it. Generous quantities — 100mm of compost forked through.
- Mulch thickly and leave it a season.Seventy-five millimetres of compost or composted bark over the top, and keep watering through the first summer. The ground needs time to rewet, for worms to move back in, and for the remaining roots to rot. Planting immediately into it is how the second round of plants dies too.
- Plant tough, drought-tolerant things first.Even after a season the strip will be drier and poorer than the rest of the garden. Start with resilient shrubs and perennials, keep watering through the first two summers, and save anything fussy for later.
Tips
- The claim that conifers poison the soil is largely overstated for leylandii and cypress. Walnut is the genuine allelopathic case in British gardens, through juglone, and that is a different and more persistent problem.
- Rotting conifer roots left in the ground draw down nitrogen as they decompose, exactly like buried wood chip. A nitrogen-rich feed in the first two seasons compensates for this.
- Honey fungus is worth checking for when you remove old conifers — look for white fungal sheets under the bark of the stumps and black bootlace rhizomorphs in the soil. It changes what you can safely replant.
- If a fence went in where the hedge was, the ground beside it will stay dry because the fence sheds rain too. Plan for watering, or plant something that will tolerate it.
- Composted conifer needles and shredded conifer prunings are useful mulch material around ericaceous plants, where their mild acidity is an advantage rather than a problem.
If it doesn't work
Water runs off the strip and does not soak in
Cause: Hydrophobic soil from years of dryness plus needle litter residues — Fix: Break the surface with a fork, apply water slowly over several days rather than in one go, and mulch to keep the surface damp. It rewets, but slowly.
New plants establish then go yellow and stall
Cause: Nitrogen drawdown from rotting roots, plus exhausted soil — Fix: Feed with a nitrogen-rich fertiliser through the first two seasons and keep mulching. It corrects itself as the roots finish rotting.
Ground sinks over the following year
Cause: Old root plates and stumps decomposing below the surface — Fix: Top up with topsoil as it settles. Grinding stumps out before replanting reduces how much of this happens.