Summer 2025: Trees under stress
- hhtreewatch
- Aug 1
- 14 min read
Updated: Aug 5
It's the first day of August, and the heavy rains of the past few weeks have finally broken the long heatwave. For some of our young street trees the relief from heat and drought has come too late. For the second year running, trees that had put on healthy new leaves, flowers and fruit in spring have suddenly wilted and shrivelled up in early summer. In some cases they've recovered; in others they've died.
We'll take a look at the casualties further down this blog, but first it's worth exploring what's actually been happening to cause these failures.
The symptoms
The first sign of tree stress is usually wilting, dull-looking leaves, drooping from the branches and looking limp. If the weather conditions don't improve the leaves will start losing colour all over, turning yellow or pale brown, and eventually they'll become brown, curled up, stiff and brittle. Twigs and branches can then start to die back and leaves fall prematurely, potentially killing the whole tree.
Sometimes only the edges of the leaf change to brown, a disorder known as leaf scorch. This isn't fatal, although if scorch occurs year after year it will weaken the tree and leave it vulnerable to disease and insect attack.
Heat or drought?
We're not professional arboriculturists, so we can only record what we observe and read about, rather than offering a precise explanation for these incidents. It's also not always clear whether the culprit is the intense heat we've been experiencing or the prolonged drought. But in most cases the outcome - and the solution - are identical.
It's easier to point the finger at drought. It can take young street trees three years to put down deep roots after they've been transplanted from the ideal growing conditions of the nursery into the hostile environment of a city pavement. They can't benefit from rain-fed groundwater like older specimens, and their new habitat is harsh. The paving, asphalt and concrete around them absorb heat from the sun and the soil is poor, full of impediments like cables, pipework and rubble. It lacks nutrients and the mulch that can keep it moist, and vigorous weeds in the tree pit syphon away any surface water.
Sometimes, though, it's not just the soil that's dry; moisture in the air also drops sharply in hot or windy weather, and this rapidly parches the foliage. Water flows upwards in a tree, from the roots to the leaves, and evaporates from the leaf surface in a process called transpiration. If the atmosphere is hot and dry, the rate at which water is lost speeds up, and if it's not replenished the leaves will wilt.
So heat and drought are closely linked when trees suffer, but whether it's one cause or the other, the answer to stress is always the same: water.
Every new tree is given a green watering bag that can deliver an exact amount of water each week, dripping slowly into the soil and promoting root growth. But as the climate heats up relentlessly, more will be needed. Southwark's contractors commit to fill each 55-litre bag twice a month in spring, four times in June, July and August and once in September. But that's not enough to keep saplings hydrated when the temperature climbs into the 30s, especially when you take into account the taller, heavier, thirstier trees that the council plants nowadays.
Our volunteers can make a critical difference to a tree's prospects by giving it extra water when the going gets tough, starting when the first leaves appear and carrying through into autumn. You'll find watering guidance here, with extra advice on rainy days on this page and tips from professionals at this link.
Resilient species
Climate change and global heating aren't going to go away, so any new planting we undertake should ideally be focused on trees that will cope with the challenges. These will come in many forms: longer, much hotter and drier summers, warmer and wetter winters, fiercer and more frequent storms, unprecedented wildfires and sometimes dangerous floods.
We've got a list of drought-tolerant trees on this website and many of the named species are already widely planted by Southwark council: hawthorns, crape myrtles, medlars, maples, alders, Turkish hazels, ginkgos, privets, whitebeams, service trees and mountain ash, hornbeams, planes, pagoda trees, limes, sweet gums (Liquidambar), disease-resistant elms and the related zelkovas. We've got examples of all these on our streets and we'll update the list as we gather more information.
But some of our best-loved flowering trees, like cherries and most magnolias, don't make the cut. Nor do many familiar natives. The birches are all intolerant of drought and the council is no longer planting them as street trees. But in their place we can expect to see more exotic species, from regions more accustomed to drought and high temperatures, and maybe new cultivars, bred to thrive in the new conditions.
In the meantime, here's the casualty list for the past 12 months or so in Herne Hill, starting with the five trees that unfortunately didn't survive. You'll see that there's a definite pattern of sudden deaths among young trees in the summer months. We'll begin with the first one affected in this way.
The lost trees
The first to go was a white-barked Himalayan birch (Betula utilis jacquemontii) at the top of Ruskin Walk, the second tree down from the postbox at the corner of Herne Hill. The leaves emerged as normal in spring 2024 but then died in early June. It was replaced in January this year with a silver birch cultivar, Betula pendula ‘Select’. Even the silver birch was wilting from the hot, dry weather of the past couple of months, but it's now being watered by a neighbour.
Next was a Kanzan cherry (Prunus serrulata 'Kanzan') outside 110 Burbage Road, seen below with our street leader Tyrrell Evans. It was one of many stunning Kanzans along the street and its gorgeous blossoms had featured in our May 2021 blog. But in July last year the fresh green leaves of spring began to fade and the deterioration was swift. It was removed in the autumn, a short six years after being planted.
A few weeks later, in August, a similar story was unfolding just around the corner on Stradella Road, opposite no. 105. The victim here was a native common hornbeam (Carpinus betulus 'Fastigiata'), a shapely, upright little tree that had been planted in July 2022, at the peak of another fierce heatwave when the temperature reached over 40C for the first time. Again the leaves started to change colour to yellow and green, and within four weeks the whole crown was chestnut brown. Hornbeams keep their leaves right through the autumn and winter and this one was no exception. It's still standing in its tree pit but the branches are now totally bare. This has been an unlucky location: the previous occupant, a Southern nettle tree (Celtis australis), was brought down in Storm Eunice in February 2022.
The next to succumb, in September last year, was the American sweet gum (Liquidambar styraciflua) next to the Dulwich Mead flats on Half Moon Lane, close by the bench at the 37 bus stop. This was a bigger tree than the other victims, although by no means full-sized, and the autumn colours this species is famous for had already revealed themselves. But the leaves quickly browned, dried out and curled up and the tree was eventually felled in February 2025.
The last of the confirmed tree deaths was the Swedish whitebeam planted in December 2023 at 17 Burbage. This was a cultivar (Brouwers) with a narrow, ascending form, just like the Norway maple (Acer platanoides 'Columnare') planted at the same time a few yards away at no. 15. Both trees took to their new environment well, and the maple is still looking lush. In spring this year the whitebeam's glossy, scalloped leaves, its creamy flowers and small oval fruits were looking perfectly healthy, but in June it declined sharply. It appeared to us that the watering bag was blocked, but by then it was too late to do anything about it. The tree has now been taken down.
Trees that came back from the dead
Sometimes trees that you think are doomed can surprise you and spring back to life. A case in point is the common medlar (Mespilus germanica) at the front of the big open space on Sunray Avenue, by the pedestrian crossing. This is another tree planted at the height of the 2022 heatwave, in July that year, three or four months after the planting season should really have wound up. It was severely stressed from the outset and its leaves rapidly turned brown. But the residents around the green began to water it regularly and the following month a couple of dozen fresh green leaves began to emerge in the crown. It struggled to produce fruits for a while, but now it's covered in them. Our other medlar, at the foot of Hollingbourne Road, has been flourishing ever since its arrival in December 2022, thanks to neighbours on both sides who've been watering it.
The Turkey oak (Quercus cerris) in front of 79 Red Post Hill, in the corner of the big crescent-shaped lawn, also had us worried. After it was planted in February this year, six months after an old lime tree was felled on the same spot, the fresh green foliage quickly faded to a tan colour. But it recovered after a few weeks and promises to be a big and striking tree, distinguished from the English oak (Quercus robur) by its narrower, more deeply lobed leaves.
Our only lingering concern for the health of the young oak is the mass of suckers that have shot up from the remains of the stump of its predecessor, the small-leaved lime (Tilia cordata). Unless they're removed, these shoots will be competing with the oak for water, nutrients and growing space. Coincidentally there's a self-seeded Turkey oak in the planting bed a little further up the hill, outside no. 87, which if it's left alone looks set to take over from an old holly beside it.
Clinging to life
Between nos. 97 and 99 on Burbage Road stands a stricken Northern Japanese magnolia (Magnolia kobus), one of a pair planted on the street in November 2023. There are a few more of these kobus cultivars on Casino Avenue and they're interesting trees, with small white flowers in early spring and extraordinary fruit in late summer. November is an ideal time for planting saplings, when they're dormant and have time to settle in before spring arrives, and this one, like many of the trees we've described, had a promising start.
However, unlike its twin over the road at no. 106, it appeared to have died suddenly in June 2025, when the weather was at its hottest and driest. A closer inspection showed it was in fact still alive, despite its hopeless-looking crown. Carefully scraping off a thin layer of bark with a penknife revealed that the cambium (the tissue that thickens the tree stem year by year) was green and healthy. So our Burbage volunteers are continuing to water it to boost its chances of survival.
Cases on housing estates
We've definitely lost one magnolia in the past year, over at the Hillcrest estate on Sunray Avenue. It was one of three 'Heaven Scent' magnolias planted along the back wall in February 2023, a remote corner that's easily overlooked. There does seem to be a pattern of losses or near-losses in the grounds of some housing estates, perhaps because the trees there are beyond the reach of the watering lorries used by the planting contractors.
The Pynnersmead flats near the foot of Herne Hill are another example. Two of the maples planted on the estate's wide lawns have suffered noticeably from lack of water. Like the medlar on the Sunray open space, both arrived very late in the hot, dry 2021/22 season, between May and July. The first is a Cappadocian maple (Acer cappadocicum) on the grass behind the low front wall, where the foliage turned brown earlier this month. The ground underneath it was dry when we checked and the green bag was rotten. The second is a red maple (Acer rubrum 'Red Sunset') in a very isolated position near the bins behind the block of flats. This has got extensive dieback and only a couple of the lowest branches are still in leaf.
By contrast, two other Pynnersmead trees planted in spring/summer 2022 have come through the heatwaves almost unscathed. Significantly, they're both drought-tolerant species: the Japanese pagoda tree (Styphnolobium japonicum) behind the Cappadocian maple and the Rebona elm (Ulmus 'Rebona') next to the maple along the boundary wall. The leaves on the pagoda tree are still green but it has some dieback and is noticeably smaller than others planted in Herne Hill at the same time, like the one at the Half Moon Lane end of Holmdene Avenue and another on Carver Road where it meets Herne Hill. But the Rebona, a modern cultivar that's resistant to Dutch elm disease, dwarfs its neighbours. It replaced a sycamore that was another victim of Storm Eunice.
The Delawyk estate, where it can be puzzling for visitors to navigate the many pedestrian paths and scattered lawns, has also had problems with young trees lacking water. On a small patch of grass between nos. 57 and 63 there's an unusual and interesting crab apple, planted in March last year. It was happy enough for a few months and the he fruits were developing well, but by August 2024 it was looking very feeble. This year, thanks to the efforts of a near neighbour, it's clearly on the road to recovery. It took us a while to identify the tree, which has clusters of creamy flowers and a pale brown, smooth bark, much like a whitebeam or rowan. But we're pretty confident it's a a Yunnan crab apple of the veitchii variety (Malus yunnanensis veitchii): a rarity from China that we don't want to lose.
Elsewhere on Delawyk, on the lawn between the front of no. 44 and the rear of 54A, is a new tree known for its glossy, mahogany-coloured bark: a Tibetan cherry (Prunus serrula tibetica). Dating from January this year, it's a like-for-like replacement for an earlier Tibetan cherry that died here in 2023 after barely a year in the ground. We put that loss down to dehydration, and by July this year it seemed like the new Tibetan, a bigger, sturdier tree than its predecessor, was going the same way. The leaves were limp and faded, but curiously the watering bag was always full. It turned out that the tiny holes at the bottom of the bag had somehow become blocked, so nothing was reaching the soil. A few pricks with a skewer got the water dribbling through again and we're optimistic that the cherry will survive.
Up at the top of Ruskin Walk, there was a problem with a different kind of cherry: a Prunus 'Umineko', sometimes known as the Snow Goose cherry, grown for its white blossom and its neat upright form. There's been a pair of these growing successfully just uphill from no. 8 since the spring of 2021. This summer, though, their fortunes diverged. The higher one became sickly while the lower one continued to thrive. The difference, we think, was a a plant called field thistle entirely smothering the tree pit of the weaker partner.
We've often warned about allowing opportunistic weeds or wildflowers scattered as seeds to take over the pits of young trees, since they can easily soak up any available water in the soil. It's possible that the unwelcome guest here was impacting a tree already suffering from heat and drought. We pulled up all the thistles and their roots, forked over the soil, took out a lot of stones, added some tree bark compost and tried pouring on water. But the ground was so rocky and compacted that it could hardly absorb more than 10 litres over the course of an hour. So we attached an unwanted, slow-release, watering bag and found a neighbour happy to fill it every week or so. The cambium is still green, so that gives us some hope, but this is another work in progress and we won't necessarily see any results until next spring.
Defence mechanisms
Trees are naturally capable of repairing themselves after damage and protecting themselves from the spread of disease and decay. Dehydration is more challenging, but some species have methods to defend themselves against heat. Our familiar London planes (Platanus x hispanica) shed bark all year round, resulting in their characteristic patchwork of colours - grey, brown, green, yellow and white. This helps them get rid of harmful parasites, pests, fungi and pollutants from heavy traffic that have accumulated in the bark. But in prolonged heat and drought the process accelerates and long sheets fall away, revealing a new creamy yellow skin within. This seems to be a response to shrinkage in the girth of the tree, but it also allows the plane to maintain its health and vigour in the adverse conditions.
Another large tree, the silver lime (Tilia tomentosa), reflects intense sunlight by turning its leaves through 180 degrees, showing the silvery or pale yellow underside instead of the darker, heat-absorbing top side.
Sun lovers
Finally, let's have a quick look at a few local trees that actually thrive in hot, dry conditions. The first is a Japanese privet (Ligustrum japonicum) outside 27 Herne Hill, right at the top of Casino Avenue. It was only planted seven months ago but it's put on a great show of pale yellow flower spikes this summer. Privets come highly recommended in textbooks. Cassell's Trees of Britain and Northern Europe says of the closely related Chinese privet (Ligustrum lucidum): "This tree thrives in urban conditions and seems to survive on compacted dry and even eroded soil. It will withstand damage and abuse by people, domestic dogs and grass cutting machines, and also tolerates high levels of air pollution."
We've lost a few of these glossy evergreens in recent years: a rather spindly Chinese privet stood on this very spot until it was knocked down by Storm Isha in January 2024. There were also two outside Bessemer Grange school on Nairne Grove, but only one remains. Three more used to live on Milkwood Road, at the back of Pizza Express, but they outgrew their location, with its narrow pavement overshadowed by high brick walls. Nowadays there's just one large privet left there, the stump of another and a recently planted fastigiate (upright) hornbeam.
The second sun worshipper is a complete contrast: a tiny tree that's survived seven or eight years at another busy road junction and is finally shrugging off its inferiority complex. It's the amazing Persian silk tree (Albizia julibrissin) on the pavement by the zebra crossing across Village Way. It's endured repeated attacks by scale insects and frequent leaf loss, and while its stem still measures only an inch or so (3 cm) across, it's put in a huge effort this year to produce a full crown of astonishingly beautiful pink and while flowers. They're even growing in the lower branches that only appeared around a year ago. It's still got a long way to go to rival the enormous silk tree outside the Crown and Greyhound pub in Dulwich Village, but it might yet surprise us.
Third and last is a tree that really ought to be planted on our streets as the climate heats up: the Koelreuteria paniculata. It's got a number of vernacular names, like China tree, golden rain tree, varnish tree and pride of India (it's actually a native of Korea). But they're all rather fanciful and we tend to just use 'koelreuteria'. It likes full sun, it's drought-tolerant and it produces long strings of bright yellow flowers in summer that develop into strange and extraordinary fruit. These 'capsules' or 'bladders' are delicate, three-sided, papery containers, like miniature lanterns, each holding three seeds. They start off pale green or pale yellow, maturing into pink and later bronze and hanging on the tree throughout winter.
We have three koelreuteria in the Southwark side of Herne Hill and one on the way, but none yet planted on a street pavement. You'll find one in Sunray Gardens, just behind the railings close to no. 2 Elmwood Road; another, chosen by public vote in March last year, in the circular planting bed outside the main gate to Brockwell Park, where it's competing with a lot of weeds; and a third behind the wooden fence around the grounds of Judith Kerr school, next to the crossing over Village Way. We're scheduled to get a fourth this coming winter on the lawn behind flat no. 1 in the Carterscroft estate, near the roundabout on Red Post Hill. It will replace a wild cherry that was felled there in autumn 2024.
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