The firm, Tube Lines, has now completed its first year of a Japanese knotweed eradication programme on 176 sites.
Tube Lines head of environment Steve Judd said using the chemical, which is the active ingredient in Tordon, would save £2.5m against using glyphosate treatments for seven years.
The cost of using Tordon, including project management and ancillary costs, would be £264,000 over all the treated sites, added Judd.
"Japanese knotweed has been my nemesis so to actually achieve a 94 per cent kill rate was a big shock to us," he said.
"We will revisit all the sites this year to do a full confirmation."
Tube Lines put together a four-strong team comprising staff from Tordon manufacturer Nomix Enviro, now owned by Frontier Agriculture, and Cleshar Contract Services to carry out the work. Judd said the specialist team was essential as it was difficult to find a contractor trained in using Tordon.
Despite Tube Lines' success, industry experts say it is unlikely to become a widespread solution to Japanese knotweed control because of expense and limitations on where picloram can be applied.
Cornwall County Council's vegetation adviser James MacFarlane said: "It can be very effective but it is more limited in the range of circumstances it can be used than something like glyphosate. It is also more expensive than glyphosate."
Ground Control senior contracts manager Neil Huck said the cost of using picloram was prohibitive. "It is very expensive for what it does and I would question its effectiveness," he warned.
But Judd said that although the actual chemical ended up costing Tube Lines around four times more than glyphosate, the reduced number of applications required meant there were huge savings on labour. "But the cost to a normal landscape contractor would be more," he admitted.
Huck added: "It would be interesting to see if it does work. I have used it before but I found glyphosate more effective. Cost is always a major factor."