The brittle gums that will line Canberra's light rail route are being progressively shipped out of Yarralumla Nursery, but of the 1500 being grown, only 1200 will be planted along the corridor.

That's because Yarralumla Nursery general manager Chris Ware has set the bar high for these trees, after shallow roots, rot and fungal infestations caused the premature demise of the previous ones.

While the trees along Northbourne were always going to be cut down and replaced as part of the construction of light rail stage one, their removal was brought forward after a storm in January 2017, where a tree crashed onto a car.

Around 200 of the new trees have been planted along the light rail route.

These trees have been grown especially so their roots dig deep into the earth, Mr Ware said.

Planted in bags and weighing several hundred kilograms each, Mr Ware said they irrigate the trees with a "teardrop" wetting pattern to encourage dense root growth.

"From time to time we cut open the bags and have a good look at the inside to see what's growing, because the last thing we want is for that bag to be planted and the soil to crumble away, because that would ruin the root system that we've developed. So, we're looking for a good, well developed, well rounded root system internally in the bag," Mr Ware said.

The trees will eventually be transplanted into three-metre by three-metre pits along the light rail route, roughly 10 metres apart, Canberra Metro environmental and approval manager Patrick Giltrap said.

They will be hand-irrigated until they are established, with long tubes feeding water into the base of the pit to encourage the roots to grow down.

"The previous trees were irrigated at the top along the grass whereas we're driving the water down," Mr Giltrap said.

"There's been a lot of lessons learned, [about growing trees] I think in Canberra, over the last 50 years."

At their full height of 15-20 metres, the trees will one day tower over the six-metre powerlines.

But at around five metres now,  there's the risk errant branches could shoot off sideways and interfere with the wires.

That's why only the straightest trees will make it onto the truck, Mr Ware said.

"What we don't want is to see these branches that go out at funny angles that will grow towards the powerlines or towards the road, depending on where it's planted," Mr Ware said.

"One of the problems with growing [this species is] that it will often throw a branch out to the side and make its own mind up that that should be the dominant branch, but then goes off at a 30 degree angle."

If, once planted, the trees do start branching towards the powerlines, Mr Giltrap said Canberra Metro would undertake "formative pruning".

Some trees that haven't met quality control have been disposed of, while others have been planted in other parts of the city.

A proportion will be keep in reserve as replants, just in case.

'The nursery industry goes on about a 10 percent loss rate ... so we grew more than what we always knew we'd need. Nothing worse than saying to my client that, 'Oh, we're two or three trees short ... that would be a very bad look, wouldn't it?'," Mr Ware said.

Trees on Northbourne Avenue, the entrance to Canberra, are regarded by many as vital to the character of the bush capital.

The announcement they would be cut down to make way for light rail caused outrage, with protestors placing white crosses where the felled trees once stood.