Showing a series of pictures of problematic landscape builds, Andy Sturgeon, told the event in Henley-on-Thames organised by soil consultancy Tim O’Hare Associates, that "one of the biggest problems I’ve had in my career in terms of plant failures is drainage".
At one project for a stately home owner, he tried to combat the badly-drained and compacted land by creating a series of mounds with imported topsoil, with paths around for people to travel on, a design borrowed from RHS Hyde Hall in Essex.
But despite there being a good landscape contractor on-site progress was stymied by the site manager allowing people to "trample over this soil for days".
"Control of your soil on site during construction is important," Sturgeon said, saying that at this site, it was instead "chaotic". Even good imported topsoil can be ruined if it's trampled over during a landscape build or mixed with rubble, he said.
At another site, where the client was in such a hurry, the design was constructed "like a Chelsea garden" with everything except the most mature trees were imported pre-grown, many plants died because of too much compaction.
"It was heart-breaking in a way but it fundamentally meant that the whole area was a disaster. We had no time to start again, we tried to lift areas up but we lost a lot of balls and a lot of small trees, 50% of balls died."
In the end he had to go back in and dig up plants to replace the soil.
"In a way it’s been a disaster but it looks quite pretty. It took three or four years to establish, it’s been so painful," he said.
Another really common problem, Sturgeon said, was old roads which had been covered up by topsoil.
He added: "With big commercial schemes it’s really nailed down, with private gardens there are a lot of holes in people’s specification. For me it’s about really nailing it down.
"With a lot of these projects you’re not being paid for administering the contract or visiting the site and there’s a problem there."
For more on SoilsCon16, see next week's issue of Horticulture Week, out 14 October.