Breathing space: a groundbreaker for tree roots

Soil

Several factors can stress trees, whatever their setting - parks, gardens, estates or streetscapes.

Nutrient shortage, surface compaction from footfall and waterlogging, created when water drains off hard surfaces and pathways, can all take their toll on tree health and ability to thrive. Popular measures to ease soil compaction include machines that loosen the ground around roots and beyond, then backfill the spaces created with nutrients and other materials.

Decompaction specialists, Terrain Aeration offer several solutions and work across various settings from back gardens, large estates to royal parks. Using its Terralift, the company can penetrate down to one metre depth, below the root system except major tap roots.

Breaking up soil around roots and beyond allows excess water to drain away to help stop anaerobic conditions and root rot. With its probe the Terralift injects dried seaweed, which helps keep fissures open. Backfilling probe holes with aggregate then provides a semi-permanent aeration/ventilation shaft.

“We aerate around roots using two-metre spacings,” Terrain Aeration managing director David Green explains, “mostly a metre inside and the same outside the canopy drip line, as this is where growing roots lie. Decompacting around roots and injecting air into the soil increases the percentage of oxygen uptake into the root system.”

Visitor attraction Yorkshire Sculpture Park (YSP) found they had a need to call in the compaction experts post pandemic. The park showcases work by British and international artists including Henry Moore, Barbara Hepworth and Damien Hirst, some of which have stood there since the 18th-century parkland at Bretton Hall, Wakefield opened in 1977.

Popular with visitors and curators, especially in recent years, the Lower Park area of the 500-acre estate began to suffer from compaction due to footfall and the weight of sculptures, resulting in build-up of standing water. Hydrology consultants asked Terrain Aeration to inspect the waterlogged area with their deep aeration treatment system.

Mark Chesman, YSP head of estates and parks explains what happened next. “It is unusual for the area to be cleared, but the sculptures there were removed for conservation work and we seized the opportunity as part of a wider project. In all, it took two-and-a-half days and 4,000m 2 of land was treated.”

On site, Terralift kit hammers a hollow probe 1 m into the soil using a JCB road breaker gun.

“At depth, the ground is usually so compacted it needs the hammer to break through,” explains David at Terrain Aeration. “Compressed air is then released up to 20Bar (280psi) to fracture and fissure the soil, with the process repeated in 2 m spacings, to ensure fissures interlink.”

On the end of the air blast, dried seaweed is injected, which sticks to the fissure walls, expanding and contracting with moisture content in the soil and keeping the area ‘breathing’, he adds.

The 3.8 cm probe holes the machine created were backfilled with aggregate to provide a semi-permanent aeration/ventilation shaft and help keep aeration working. The YSP grounds team then overseeded the area to finish.

Limes, beeches and Atlas cedars received aeration treatment at the Yorkshire estate too. Breaking up soil around roots and beyond means excess water drains away from the roots to help stop anaerobic conditions and root rot. Other areas are due for treatment following this first round of work.

Like the Yorkshire Sculpture Park, parks and estates across Britain include historic and champion trees within their boundaries – decompaction can prove a deciding factor in future proofing their livelihood.

Darlington South Park, for example, is home to two of what are believed to be the largest giant Redwoods or multi-stemmed Sequoiadendron giganteum in England. They were planted in March 1863 to commemorate the marriage of the future King Edward VII to Princess Alexandra of Denmark.

Compaction had affected one of the specimens, with top roots visible and unilaterally reaching a long way from the tree and a steep hill down to a river on the other.

The tree was over-compensating to get all its energy. Darlington Council called in Terrain Aeration’s Tree Division to provide deep aeration of the area using Terralift machines.

“The key to successful treatment is usually to work along the dripline, one metre inside and one metre outside, where the rainwater drips from the canopy,” says David.

“Working either side of the dripline reaches beyond the growing roots, increases the percentage of oxygen intake and allows maximum drainage. Rainwater can penetrate deeper, resulting in a stronger root growth and a healthier canopy spread with an aerated soil structure and relief of compaction panning,” he adds. “On receiving significant rainfall, they pick up as they receive more water and nutrients.”

At South Park, the surrounding lawn was aerated with mycorrhizal funghi, which attach themselves to the tips of the tree roots, helping them break through hard soil and leaving the tree more energy to grow and further develop foliage.

“The soil was very hard and dry around the base,” explains David, “so some fractures were done near the tree. Water-storing polymer was injected into the area to help roots retain available water from rainfall.”

Compaction can impede growth of young trees too, says Dr Jon Banks, research lab manager for Bartlett Tree Experts, adding: “It limits root spread, as can old pathways. Over time, compaction pushes vital air out of the soil space. Waterlogging prevents roots from respiring, creating drought-like symptoms as water cannot reach roots.

“About 90% of tree roots lie within the top 60 cm of soil. If that depth suffers compaction, water can collect in the layer above and rot roots. There could be problems deeper down as well, causing wider drainage problems.”

Soil types can also determine levels of compaction adds Jon, “sandy soils have large particles that can maintain air spaces, whereas clay, with its smaller particles, is more prone to compacting.

“Retaining healthy soils after treating decompaction relies on keeping the area free of footfall or heavy machinery if at all possible,” he continues.

“Maintenance really depends on biological activity within the soil. Earthworms and other soil organisms play a key role and if they are absent you may have major problems.”

Symptoms of compaction can be clear enough. “First signs are that the tree canopy is not as healthy as you’d hope for. Surrounding grass may be struggling and digging a hole in the ground near the tree may be difficult. You shouldn’t be getting a workout just to penetrate the ground,” he says.

Cases of deep compaction are not that common, Jon maintains. “Some systems go down several metres, which is not really necessary as only tap roots reach lower levels.

Bartlett use and recommend the Airspade machine for decompaction work. Originally developed by the military for excavating land mines, it has found ready application in the landscaping sector.

Any decompaction process will result in some root damage, however, he contends. “When we need to decompact, the major issue which needs alleviating is compaction.

“Obviously we never want to damage the tree but decompaction removes soil from small roots so we don’t recommend undertaking the process regularly or without good reason.

Prevention is preferable, perhaps applying mulch around trees and deterring people from walking over nearby ground. Mycorrhizae are important postdecompaction. “These can be applied or encouraged by careful soil management but our primary focus is the tree itself,” says Jon.

A process such as hydro-excavation, used in construction, is unsuitable, explains Jon as injecting water into soil creates a slurry, which is more damaging to tree roots than air excavation.

Trees have the ability to recover quickly from invasive decompaction. “We measure tree health before and after Airspading to gauge the response to it. Turnover of fine roots can be as little as three days so a healthy tree will rejuvenate them quickly – anything up to three weeks, although some species tolerate the process more than others below.”

Timing is key, Jon notes. “Avoid the worst of drought times. Winter is better, when roots are dormant and the tree will suffer only a little negative impact. We always water well 24 hours before Airspading as it eases soil penetration.”

Ancient trees merit special consideration, Jon urges. A major current project is underway in Liverpool, where Bartlett are working with the local council and community on a national living monument - the 700year old Allerton Oak Tree.

“Take it slowly is our advice. Study the scale of the problem carefully, then apply procedures that are appropriate to what they are struggling with. “Slower is widely considered better when dealing with tree time, however, it is only appropriate when compaction has also occurred slowly.”

Decompaction can be a complex issue. On one project, Terrain Aeration discovered magnesium deficiency, coupled with low overall soil fertility around trees in a park.

“New technology allowed us to measure light transmission through the leaves to read chlorophyll levels,” David recalls. “The likely cause of the problem was that leaf litter always had been removed going back a century, taking with it natural nutrients from decaying leaves.

“Severe compaction of the surface around the tree had also set in. We injected a tree feed mix containing slow-release general nutrients and magnesium using the Terralift’s ability to inject granular material via its seaweed carrier, forcing it upwards into the rootzone.”

This also avoids losing the nutrients to the grass, which would occur with surface treatment. Two Terralift machines were used to complete the process. Advice from Terrain Aeration for those stress testing their trees is to send root samples from healthy and affected trees, as well as tip samples, in separate, labelled plastic bags to the Forestry Commission or an arboricultural advisor for full analysis. But ensure that a “a complete story” of the trees' circumstances and location, including relevant photographs is given to enable a thorough investigation.