West London Coroners Court heard at the resumed inquest, held before a jury, that accounts manager Wilson's fellow New Zealander Tessa Marshall was walking through the gardens with her three year-old daughter Ruby and Wilson. Marshall then heard "a crack like a lightning crack" as she and Wilson chatted while passing a 30m Cedar of Lebanon tree on the way to the Kew children's play area.
She described how a branch fell and hit Wilson, who was pronounced dead soon after, with a post mortem finding she died of multiple injuries including a severe head injury and multiple
The police passed the investigation on to the Health and Safety Executive after finding no evidence of criminality.
A 30mph (50km/h) "squall" had hit the gardens on 23 September, Kirkham said.
He said there are 14,000 trees in the gardens, with the 25m tall cedar planted in about 1730, making it a Kew 'heritage' tree.
He added that the weather in summer 2012 had been wet, with the wettest June since 1910, but there had been very little rain in September until the day of the accident.
He said the branch had fallen 22m bringing two others down with it, "catapulting" 16m across a path to hit Wilson.
Kirkham, who has worked at Kew since 1978, said the tree showed no sign of decay or defect.
He told coroner Elizabeth Pygott: "I think the branch failure was triggered by the wind squall. It gusted to 30mph and that would be strong enough to move branches. That and a combination of 5mm of rain falling during the hour up to the branch failure [caused the branch to break]."
He said he did not attribute the branch fall "purely to summer branch drop", because the accident happened in September rather than summer, it was not humid, the tree was not deteriorating, the branch broke at the trunk and not along the branch and it was an ascending then level branch. If those factors had been in place, that would have been typical for the phenomenon to happen. But he admitted Lebanese cedar is a susceptible tree to summer branch drop, which he explained happened after heavy rain but usually on a calm, humid day. The tree takes up a lot of water that causes changes to the wood structure and more weight in the branches. Kirkham said there was not enough conclusive evidence yet about the phenomenon.
He said he uses the International Society of Arboriculture tree inspection system on every tree at Kew. This has 'hazard ratings' which means he would embark on remedial action to lower the rating. He said Kew looks at every tree "from root to shoot" - a type two inspection - and not just a "walk past" type one inspection.
All trees are inspected at least every five years.
The cedar tree has been inspected every two or three years since 1998 but since the incident is on a one-year inspection regime because of "previous failures".
The tree "failed" again in a 28 October 2013 storm and has since, in December 2013, had two more branches removed and others pruned. Kirkham said the September 2012 branch fall had opened up the tree to wind damage.
He said of inspections before the accident: "We wouldn't have done anything different. The inspections we did would have identified any need to reduce the leverage effect." He added: "I still believe we've done the best for that tree."
Kirkham said because the branch that killed Wilson fell 16m from the tree, fencing around the immediate vicinity of the tree would not of prevented the death.
He said fencing the 1,500 trees at Kew potentially suffering from summer branch drop would mean "closing half the garden" and would mean closing the whole of Kew from June-September.
Kirkham said summer branch drop warning signs would not "achieve anything" and would be "alarmist", would "put people off coming into the gardens" and that the phenomenon is very rare.
Earlier, HSE inspector Steve Kirton told the court the branch had fallen 22m. He said sudden branch drop was not mentioned when he spoke to Kew when he started investigating the day after the accident.
John McLinden QC, representing the Wilson family, who weren't in court, suggested to Tessa Marshall that Kew could have put up signs warning about the danger of sudden branch drop, similar to Kew's existing oak processionary moth warning signs, which he showed the court.
Marshall said if there had been signs: "I would have read it and taken notice of it."
McLinden said: "The reason Kew didn't put up the signs because they thought it futile and wouldn't make any difference or might deter them [visitors] from coming into the gardens."
Marshall said she would have liked to have had the choice to act upon a sign: "The worst case scenario happened so i would like to have known."
She agreed with McLinden that it was just a "fluke" she and her daughter weren't killed too.
Patrick Blakesley, for Kew, suggested to Marshall "warnings of the tiniest risk" were unnecessary when 13 million visitors had been unaffected in 10 years and 66m visitors since a death in the 1950s.
Marshall said warning signs "perhaps" were necessary "when I'm looking after the safety
of my daughter." Blakesley asked her if she had walked through an unmanaged wood or had
let her daughter run free at Kew since and Marshall said she had.
The inquest resumes on 12 June. Tree consultants David Lonsdale and Jeremy Barrell are still to give evidence.