In recent weeks a number of Mercury readers have commented on the seemingly poor condition of the casuarina trees in the F6 median strip between Five Islands Road and the Fowlers Road overpass.

Truck driver Peter Simpson, who drives along that stretch of the F6 several times a week, has also noticed the trees' decline.

MORE: Why are trees dying on the F6?

Recently he spotted a contractor in a vehicle doing some spraying on the side of the road, south of the Fowlers Road bridge.

"They were weed spraying on the northbound side from a vehicle," he said.

"The boom from the herbicide was well over a metre off the ground and there was a cloud of herbicide covering all the vegetation, the guard rail plus some of the lane.

"I thought it was a bit horrific really. Driving along and spraying, that was a bit over the top."

Mr Simpson then drew a possible connection between what he had seen and the state of casuarinas on the median strip.

"You look at the ones in the middle, you can tell there has been weed spraying around the base of those, just by the kikuyu there - it's all gone a sickly shade of yellow and it's now dead," he said.

"Just when I saw them spraying herbicide I wondered, maybe that's what's happened to those casuarinas as well."

Roads and Maritime Services, which is responsible for the trees and median strip, hires a contractor to take care of them.

An RMS spokesman said spraying was generally done from a moving vehicle equipped for the task.

"In most cases this has been an effective means of weed control," the spokesman said.

However, he doubted the condition of the trees was a result of herbicide.

"It is unlikely the casuarina trees have been affected by weed spraying as it is not normal practice to spray median strips.

"The condition of the trees varies because of subgrade moisture conditions and soils which vary significantly from one place to another. The trees in some areas seem to be improving.

"RMS will continue to monitor them and seek further specialist advice should it be necessary."