The climate’s changed, there’s no doubt about that, but whether it’s cyclic or just a blip, I don’t know. I’m not a scientist, I’m a pragmatic, practical tree grower. What I would say is that a short time ago seasons used to be almost equal periods — winter and summer were quite well balanced, which is what you’d associate with a temperate climate like ours. Now you can’t predict when one’s going to start and another’s going to finish.

For the last five years in the southeast we’ve been in drought conditions. Spring has come four or five weeks earlier than normal, and autumn has come very late. Last year every tree was still in full leaf halfway into November. Winters now are too warm and short, so our trees aren’t having a dormancy and shutting down properly for a long enough period where they can rest and absorb moisture from the winter rain. And there’s been a shortage of rain in winter, which is the important time for rain because that’s what charges your soil and tops up the aquifers, which is what trees need. Plus the summers have been very dry as well.

We have 14,000 trees here, and I’d say 20% are stressed, which is quite high. Just like in humans or animals, you can tell an unsprightly tree: it looks run down, it has small leaves — or a lack of leaves — dying branches, fungi; its crown is thinning; it really is struggling. God, it breaks my heart. I am emotional about it. I find it hard going around the gardens and seeing trees just struggling to grow. Sometimes I’ll put myself in the position of a tree: “What are your problems? What do you need?” I worry about them.

When trees are under stress from moisture deficit they become very vulnerable to pests and diseases, and we’re seeing more potent forms of both creeping in. Horse-chestnut leaf miner, for example, is a larva that tunnels between the leaf surfaces. It only came into this country in 2002, but it’s likely to decimate the horse-chestnut population within quite a short time. Bleeding canker has a serious effect on oaks and is very difficult to control.

We’re doing everything to keep our trees alive. A lot more attention now goes into the way we plant trees to get them off to a good start. We plant in autumn as well as spring now, which gives them longer to establish; we’re using decompaction work to moisten and break up the parched ground, and constantly feeding and mulching too. We work so hard to try and give them a lift, but there’s only so much you can do.

A lot of the old trees are starting to die. My single favourite tree here at Kew is an English oak which has been around since the 1700s. But it’s got a problem with bleeding canker and woodworm now. It’ll probably not live beyond next winter. I’m gutted; I really love that tree.

We can’t afford any further extensive losses of native British trees, but it’s already happening, which is sad. Beech is migrating north because it doesn’t like the climate. Even the birch is finding it too dry. The elms have gone — Dutch elm disease took all of them away just a short time ago. Mountain ash will go too, soon. The oaks are dying from bleeding canker, and for some reason we’re really struggling to grow holly.

Before too long, when man plants trees in this country, he’ll select exotic species more suited to this changing climate. We’re already growing species we couldn’t grow before. Ten years ago it would have been unheard of to see an olive tree in this country; now you can go into any garden centre and buy whatever size olive tree you require.

This year we had what I’ve been wanting these past five years: good, steady rain throughout the winter. The trees responded really well to it, which is quite a relief — it was a lifesaver, actually. This summer has been pretty wet too. But we still have to expect dry weather and drought again at some stage. We need to look further afield for new species, not just relying on British natives. We’ve started going to the Mediterranean region, North Africa and Mexico for things like evergreen oaks, and the eastern seaboard of the USA for many of the red and black oak species.

And so, over a very short period, the treescape of Britain will change. Eventually the British treescape is going to resemble that of North America. The giant redwood, for example, is becoming the dominant tree on the skyline in Surrey, Sussex and Kent. And you can guess how I feel about that.

I think the only good that comes from all of this is that it’s a wake-up call — if there had been no visible signs of global warming, nobody would have taken any notice. Hopefully, people will take some course of action now, whether it’s recycling, burning less fuel, or whatever.

The damage has been done. We’ve made a rod for our own backs with what we’ve done to our planet, and it doesn’t matter what we do now. We might slow things down, but I don’t think we can change the course we’ve embarked on.

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