Rootgrow Mycorrhizal Fungi Settles a Bare-Root Hornbeam Hedge Along a 10-Metre Line

Rootgrow belongs on living roots

Rootgrow contains living propagules of mycorrhizal fungi. Dry storage against fertiliser can kill them, and that detail changes the planting order for a bare-root hedge. The RHS-endorsed granules from PlantWorks need contact with root tissue within a few days. The usual sequence of dig, backfill, scatter feed on top, then water later puts the fungi in the wrong place at the wrong time.

For a 10-metre line of hornbeam whips set at 30 cm centres, the count comes to about 33 plants. Following the usual pinch per plant gives a Rootgrow quantity of roughly 100 grams. The granules become sticky when damp because they are meant to cling to root, not disappear into loose soil.

Mycorrhizal fungi are obligate symbionts. Away from a living root for more than a handful of days, the propagules run through their reserves and fail to colonise. A trench filled first and dusted from above can leave most of the material 40 cm from the nearest root hair. For this job, 40 cm is enough distance to make the application pointless.

The bare-root method is direct. Open the trench, hold each hornbeam whip at its spacing, fan the roots, and sprinkle the Rootgrow over them while soil is being returned around the root plate. The aim is contact on the root surface during backfilling, before the fine roots dry and before the fungi sit stranded in the trench.

On a dry autumn day, dipping the roots in a bucket of water first helps the granules stick to the places that matter. PlantWorks gives colonisation timescales of several weeks before the fungal network extends measurably into surrounding soil. That clock starts only after contact with living root tissue.

Fertiliser creates the other common failure. High-phosphate fertilisers and mycorrhizal fungi work against each other. Superphosphate, or a rose-and-shrub feed rich in phosphorus, signals to the plant that it has enough P without a fungal partner scavenging for it. Under that signal, the symbiosis may never form.

Bone meal has long been paired with bare-root planting, yet it is exactly the material to leave out of this trench. Skip it in year one. The fungi are the phosphorus-acquisition strategy, so adding a phosphate-heavy feed beside them defeats the point of using the product.

Rootgrow trials show clear benefit on poor and disturbed ground. On a fertile loam already carrying native mycorrhizae, the granules may be acting as insurance against a small risk. That does not change the application method: if the product is used, it belongs on living hornbeam roots.

Setting out the 10-metre line

Measure the run before lifting a spade, then mark 30 cm intervals with a taut line and pegs. Thirty-three whips across 10 metres gives a hedge dense enough to knit within three seasons while leaving each root plate room to establish. A single row is enough for most boundary hedging where the aim is a formal green line.

A staggered double row with a 20 cm offset makes a thicker screen. It also doubles the plant count to around 66 and pushes the Rootgrow quantity to about 200 grams. The denser layout is a planting choice, not a correction for weak establishment.

Hornbeam tolerates heavier and wetter ground better than beech. That is why Carpinus betulus is often the stronger choice on clay sites where Fagus sulks. Bare-root season runs roughly from November to March in most temperate regions, while plants are dormant and the ground is free from frost and waterlogging.

For a single row, a slit trench is enough. Drive the spade in, lever it back, and set each whip against the vertical face so the roots fan downward. Roots that point straight down and outward settle better than roots crammed into a tight folded mass.

Firm the soil around every whip after backfilling. Air pockets around bare roots are one of the earliest causes of failure, and a hornbeam rocked by wind snaps new root threads faster than it can replace them. Water immediately after planting even when rain is forecast, because the water settles soil into root contact.

Mulch and the first summer

A 5 to 7 cm layer of bark or leaf mould along the hedge line suppresses grass competition and holds moisture through the first summer. Leave a 5 cm gap around every stem. Mulch piled against bark stays wet at the collar and encourages rot.

Grass matters because a new bare-root hedge starts with a reduced root system. The whips are running on stored reserves in the stems and on limited uptake from severed roots. Until new fine roots grow, their ability to replace lost moisture is small.

The mycorrhizal network, once established, extends the effective absorbing surface beyond the root tips. It still takes most of a growing season to build. A hedge that went through a frozen January can still be lost in a dry May and June, especially if planting took place late in the bare-root season.

On free-draining soil, 20 to 30 litres per metre per week during dry spells is a realistic target for a hornbeam line. Apply it slowly so it soaks down into the root zone instead of running along the surface. Clay holds moisture longer, so it usually needs deeper soaking at wider intervals.

A rainwater butt diverter fitted to the nearest downpipe can pay for itself across one dry summer on a 10-metre hedge. Hozelock and Harcostar diverters clip into a standard 68 mm round or 65 mm square downpipe and send the first flush into a 200-litre butt. Rainwater has no chlorine and sits at ambient temperature, both of which suit young roots better than cold mains water straight from the tap. One butt refills quickly in wet weather and gives roughly a week of hand-watering for the run when the weather turns dry.

Watering should stop by late August. Autumn irrigation can push soft late growth that fails to harden before the first frosts, wasting reserves the plant needs for winter. The hedge needs time to slow down and lignify before cold weather arrives.

The wrong axe and the right spade

A Fiskars splitting axe belongs with the old stump or root ball cleared from the hedge line; for planting the whips, the useful kit is a sharp spade with a clean edge and secateurs for damaged root tails.

Bare-root whips are machine-lifted, so torn or overlong tails need clean cuts back to sound tissue; a crushed root end invites rot, a ragged tail folds back in the trench, and ten minutes of bright cold wind on exposed roots can do damage that later care cannot reverse.

Once the roots are covered and firmed in, the hidden contact between root, soil, and fungus is the part the closed trench cannot show.