Lime blossom

This line of mature common lime trees Tilia x europaea stands beside Waskerley beck, on the northern edge of the Demesne picnic site at Wolsingham in Weardale. I would guess that they are about a century old. 

During the last week of July they were in full flower and on a still, warm morning I could detect their wonderful fragrance, which has a hint of lily-of-the-valley about it, from about 100 yards away, long before the trees came into view.

Thousands of insects were attracted to the scent too. Standing beneath the trees I could hear a constant low-pitched hum, mostly from the hundreds of bumblebees that had come to feed on the nectar-rich blossom, but also from countless flies, hoverflies, wasps and honeybees that find the flowers irresistible.

For many years there have been reports of bumblebees dying after feeding on lime nectar, though there has never been a clear causal relationship demonstrated between the tree's nectar and bee deaths.   I've been interested in this bee death phenomenon since I lived in Kenilworth in the 1970s, where there was a lime (not a native lime species, I think) beside the bus stop that always seemed to have a few dead bumblebees underneath it at flowering time. This is not something I've noticed under lime trees here in Durham and when I walked under these trees at Waskerley beck there was a carpet of dead blossom on the path, but no dead bees. 

I'd been keeping a lookout because I was reminded about the bee deaths in a recent research paper by several authors from the University of Oregon in the United States  which seems to dispel a long-held and oft-repeated notion that mannose in lime nectar, a sugar supposedly toxic to bees, is responsible for the bee mortality. The paper suggests that the deaths mostly occur in cooler weather because bees simply can't get enough energy from the lime nectar to keep their muscle metabolism going and die of energy deficit. It has been very warm here throughout the lime flowering period, so maybe that's why there were no dead bees under these trees.

The really fascinating part of the paper is that the authors found an alkaloid in the lime nectar called trigonelline that apparently affects bee learning and memory. The authors suggest that this changes bumblebee behaviour so that they become addicted solely to foraging on lime nectar late in the season, forsaking other sources, even when the tree doesn't provide enough energy to keep them airborne in cool conditions.

Lime nectar doesn't seem to be toxic to honeybees, because they produce an excellent, delicately-flavoured honey from it. This brand is made by honeybees in Roumania and it's hard to imagine beekeepers allowing their hives to forage on lime nectar if it killed their bees.

Lime flowers also make a refreshing tea, that has a reputation as a soothing treatment for sore throats. In his journal entryfor 25th. July 1790 the famous parson-naturalist Gilbert White, learning that the French found lime blossom tea soothing for coughs, hoarseness and fever, described making his own brew and finding it to be “soft, well flavoured, pleasant, saccharine julep, in taste much resembling the juice of liquorice”.

Although lime trees are generally a wonderful source of pollen and nectar for insects, and their foliage is food for many more, common lime, which is a natural hybrid between small-leaved (Tila cordata) and large-leaved (T. platyphyllos) limes, has a major fault as a tree for planting in avenues or in city streets.

As the tree ages great knobbly, cankerous burrs appear on the trunk base (and sometimes on the upper trunk) and these sprout a forest of shrubby growth. Suckers also develop from the roots that are near the surface and all this herbage usually has to be hacked back annually when the tree is grown as a landscape or cityscape feature.

Yet, despite this trait, which Alan Mitchell, a famous tree expert,called 'deplorable', the hybrid tree has been planted in far greater numbers than it's better looking parental species. The reason for this is that those suckers and sprouts that have to be removed also root easily, allowing the tree to be mass-propagated at little cost.

To quote Alan Mitchell again, " this tree is the worst possible one for planting in streets, squares and odd urban corners and high in any list of those to avoid in avenue plantings  ... [but] .... has long been the favourite one for all of these usages. The results include shaded upper windows, severe pruning or lopping; broken and lifting pavings; corroded cellulose surfaces on cars parked beneath and avenues abandoned to the park cattle and new lime-free ones planted".

The damage to cars that he's referring to is due to the millions of sap-sucking insects that feed on the leaves and send down a constant rain of honeydew, which coats cars like a thin veneer of toffee. I can see his point - I once parked our car under a lime tree at Wells-next-the-sea in Norfolk, then later drove home behind a lorry carrying live chickens that shed feathers; after a mile or two the car had a good covering of chicken plumage stuck to it.

Honeydew that rains down on lime foliage also sticks to the upper surface of the leaves, which soon become covered in a growth of sooty mould, spoiling their autumn colour.

In terms of honeydew production, lime is no worst than sycamore, whose leaves are infested with aphids, but planting it in avenues, parks and streets tends to make lime more conspicuously mouldy.

Just a few minutes' walk away from those hybrid limes beside Waskerley beck, in the cemetery of Wolsingham parish church of St. Mary and St. Stephen, there is a wonderful, ancient specimen of one of the parent species, small-leaved lime Tilia cordata. It is the handsomest tree in the churchyard.

This is a rare native tree in Durham, only existing in a few places in small numbers as a wild tree, and not very often planted. It is at the limit of its natural climate range here, only setting viable seed in the longest, hottest summers, but as the climate becomes milder it's likely that it might begin to set seed regularly and become a more familiar sight.

In this species the tassels of flowers are held above the foliage, giving its domed crown a golden glow.  Here, amongst the tombstones, it has had time and space to live life to the full, to realise its natural size and shape, protected from browsing animals so that the tips of its arching limbs almost touch the ground. Urban common (hybrid) limes, or those planted in avenues, rarely grow unmolested in this way. 

The picture above shows the way in which the flowers of the common (hybrid) lime dangle below the leaves, while the picture below shows the flowers of small-leaves lime held above the leaves, which are indeed smaller than the hybrid.

Underneath, the leaves of hybrid lime have tufts of pale hairs at the base of the main veins (above), while in small-leaved lime (below)  there are golden hairs that extend along the main veins. In large-leaved lime the whole underside of the leaves is downy.

This churchyard Tilia cordata is a very fine tree, with a spreading crown that provides cool shade on hot summer days, and when it flowers the sound of the insects feeding on its flowers is soporific.

Small-leaved lime does produce a few basal sprouts from the trunk, but only a few - nothing like the forest of vegetation sprouting from burrs that can hide the lower trunk of its hybrid offspring.