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Deciduous trees. Shoots slender, side shoots zig-zagging; buds alternate, broader (except in T. endochrysea) than the shoot, ovoid, with two or three visible scales. Leaves usually heart-shaped and often asymmetrical, margin usually serrate, teeth often with apiculate tips; leaf often with tufts of hair under the vein axils and sometimes with stellate pubescence under the blade. Petiole sometimes as long as the blade and usually meeting it an asymmetrical angle. Inflorescences axillary, forming cymes of 3- 200 flowers. Floral bracts narrowly rectangular, large and persistent, and fused at least to the base of the inflorescence peduncle. Flowers bisexual. Sepals 5; petals 5, white or yellow; staminodes when present petaloid, alternating with the petals. Stamens many; ovary divided into five parts, only 1 or 2 of which develop seeds. Fruit a small, rounded, ovoid or 5-angled capsule, with a papery, leathery or woody wall usually covered in stellate hairs (Pigott 2012, Flora of China 2018).
Limes are trees of the northern temperate latitudes, just extending into the subtropics in mountains of Mexico and Vietnam. East Asia has the most species, but Tilia is also represented in Europe and the Caucasus, and in Mexico and eastern North America; the genus is absent from the Himalayas and from western North America. No Tilia currently features on the Red List of Threatened Species, but Pigott suggests that T. miqueliana (long grown and naturalised in Japan) is “very rare and verging on extinction” in the densely-populated and intensively farmed eastern provinces of China where it is genuinely native. T. concinna has been described by Pigott from herbarium specimens and from two trees in arboreta in the west, which were collected by Joseph Hers in 1919 in a part of eastern China since subjected to extensive forest clearance; Pigott now considers it likely to be extinct in the wild (Pigott 2012).
Although all the kinds of lime are quite readily recognised as belonging to Tilia, the separation of individual species has vexed botanists for centuries; in cultivation, geographically isolated forms also hybridise promiscuously. For the sake of consistency, Donald Pigott’s treatment in Lime Trees and Basswoods (2012) is followed here, except in the case of the New World limes where the approach of most modern American botanists has been adopted, streamlining them as one widely-dstributed and variable species. This gives a total of 22 species, all of which are in cultivation.
Limes are one of relatively few forest trees which are insect pollinated, and the uniquely structured flowers are deliciously scented. The usually heart-shaped leaves are also a distinctive feature; endearingly like a human heart’s, the outline is typically asymmetrical in a recognisable way, bulging more on one side at the base than on the other. Several species have variants where radial lobes obscure the leaf’s heart-shape; in one, Tilia mongolica, these lobed leaves are the norm, at least among cultivated trees. Autumn colours are yellow, but often muted. Winter twigs can be blood-red in sun, with broad equally red buds, often with one large and one small visible scale, so that they are sometimes compared to tiny boxing gloves. Several Asiatic species show attractive reds and pinks as the leaves flush in spring.
Some limes are sufficiently pubescent for the undeleaves to appear white, making for one of the most strikingly ornamental features the genus has to offer. Confusingly, the feature is less useful in species identification, with some species, such as T. tuan and T. americana in the wide sense adopted here, showing a full range of pubescence from trees with silver underleaves to those which are green and almost hairless. ‘Sun-leaves’ on flowering shoots at the tree’s top are smaller and usually more pubescent than the leaves of saplings or the shaded leaves on lower branches that are easier to study. The foliage descriptions in this account are for sun-leaves, following Pigott (2012).
Another eye-catching feature of the genus lies in the often exquisitely regular teeth fringing the leaves of all species except for the Chinese T. tuan and T. endochrysea in some of their variants; these teeth may be triangular, rounded or step-like and are usually tipped with an emergent vein or mucro. This time, the details of the toothing are fairly consistent and useful in separating the species. Other things to look for include the presence of staminodes (floral structures intermediate between stamens and petals), the number of flowers in each inflorescence, and whether the fruit-wall can easily be broken between finger and thumb. Microscopic study will show that the stellate (starfish-shaped) hairs which form the felting under the leaf in many species vary in their number of arms, with four, eight or sixteen being the commonest norms. Donald Pigott has found this feature to be the most consistently useful in species determination (Pigott 2012).
One Chinese species, T. endochrysea, represents a section of the genus (Endochrysea) otherwise known only from fossils, in which the flower-stalk is attached towards the base of the bract, which in all species helps advertise the flowers to pollinating insects; in other living limes, except for the odd mutant, the attachment is central, allowing the bract to double as a helicopter wing to carry the fruit away from the parent tree (Pigott 2012). T. endochrysea also differs in its fruit, whose husk splits when ripe into five segments.
The fruit of other limes can float for several days (Pigott 2012), meaning that trees often grow near streams. Stream valleys may also offer the steep, rocky topography which can protect trees against browsing herbivores; limes have unusually palatable leaves and large numbers of browsing animals are one significant factor which can limit the genus’ natural regeneration. Additionally many species have thin bark, making them vulnerable to animals that strip or rub the bark, especially in cultivation and when young.
Dying bees are often found underneath flowering limes and a suspicion has persisted that the flowers are toxic. Ongoing research has failed to identify any toxins, but the flowers do often produce caffeine, confusing bees and encouraging them to return again and again to flowers after their nectar has become depleted, until they starve (Koch & Stevenson 2017).
All limes prefer a base-rich soil and most like some summer heat. However, they are shade-tolerant and coppice strongly, so that they are typically features of ‘climax’ woodlands. The tendency of the foliage particularly of the Common Lime, T. × europaea, to support huge populations of sap-sucking aphids, whose sticky droppings or ‘honey-dew’ have been reputed to corrode the paintwork of cars parked underneath, is one of the genus’ few demerits.
Lime timber rots readily, but several species are very long-lived. New roots are produced with facility, both from branches that touch the ground and within the decay of the original trunk. Across central Europe, but not in western France or in Britain, ‘village limes’ are often the oldest tree in the neighbourhood. The T. platyphyllos at Kasberg, Bayern, Germany, is supposed to have been planted in 1012, and the one at Schenklengsfeld, Hessen, Germany, has a stone underneath it recording, reliably or otherwise, a planting date of 760 (Pigott 2012). Pigott observes that the area of distribution of these ancient trees is coterminous with the region from which Celtic culture expanded in the first millennium BC.
Wild limes (T. cordata, T. platyphyllos and their natural hybrid T. × europaea) are now rare or at least local in England and Wales, though they still dominate a few ancient woodlands. T. cordata in particular is toward the northernmost limit of its natural range and seldom ripens fruit in Britain’s short, cool summers, although it grows further north in continental Europe, reaching well into southern Sweden (Pigott 2012). Analysis of pollen deposits has suggested that limes were the commonest trees in the much of the primeval ‘Wildwood’ of the English lowlands (Rackham 2006); an apparently sudden decline in abundance around 5000 years ago coincided with a period of falling summer temperatures but was probably due largely to the introduction by early farming peoples of very high levels of grazing, which would have turned dense woodland over a period of generations into open grassland and would particularly have affected the palatable lime.
The even-grained, uniformly pale timber of limes is considered the best wood for detailed carving, and was the substrate for the famous carvings of Grinling Gibbons (1648-1721). The strong fibres of the under-bark (‘bast’) were traditionally used for making ropes and mats.
Limes are best grown from seed which should be collected around October when the husk turns from green to pale brown, and sown immediately. Seeds that have dried on the tree or have been stored will tend not to germinate until their second spring, some taking longer (Pigott 2012). However, open-grown seedlings, when several lime species are accessible to pollinating insects, are quite likely to be hybrids.
Nearly all of the species are currently commercially available in the UK and are popular collectors’ plants, but information is often lacking about the scarcer oriental limes’ cultivation in continental Europe or in North America. Although they are forest trees, limes are perhaps best planted in isolation where the gracefully domed crown of the tree in youth has room to develop freely, and when flowering will begin all over the canopy from a relatively early age. Their typically very consistent growth makes limes a good choice for formal avenues, provided that there is room for each tree to develop to maturity.
No doubt to the relief of purists, the genus has produced very few exuberantly distinctive garden varieties: no purple limes, very few attractively variegated forms, and nothing strictly fastigiate or spectacularly weeping. Of the many named clones the great majority are selections made for their vigour, good form or disease resistance, rather than plants which can be identified in the field or which demand to be grown for their unique qualities. In all these cases it should be remembered that the regular habit for which the clone has been selected is a youthful feature which will be lost as the tree ages.
In the UK, there are National Collections of Tilia at Thorp Perrow Arboretum in North Yorkshire and at Peasmarsh Place in East Sussex (an arboretum created since 1976 by Terence, Lord Devonport, who has generously sponsored the preparation of this coverage of the genus). Another nearly comprehensive collection of species was built up at the Cambridge University Botanic Garden by Donald Pigott as Curator from 1984 to 1995. Collections outside England include the Nationaal Lindearboretum (National Lime Collection) developed since 1988 at Winterswijk in the Netherlands, and the lime collection in the Morton Arboretum at Lisle, Illinois, planted from 1924.
Leaf-margin entire or with distant tiny teeth
Leaf-margin quite closely and regularly toothed, or lobed
Leaf-margin with a few lobes/large teeth
Leaf-margin unlobed, with regular small teeth
Leaf significantly longer than broad; flower-stalk separating from flower-bract near its base; fruit splitting when ripe
Leaf about as broad as long; flower-stalk separating from flower-bract near its centre; fruit not splitting when ripe
Tilia mongolica (and some of its hybrids)
Sports of Tilia platyphyllos, T. tomentosa, T. amurensis, and some hybrids of T. mongolica
Teeth around the leaf-margin consisting mostly of a whisker/peg tip
Teeth rounded or triangular (serrate), with or without a short whisker/peg tip
Whisker-tips of teeth > 2 mm long
Peg-tips of teeth < 2 mm long
Leaf-margin scalloped between stout peg-tips of teeth
Short peg-tipped teeth form tiny steps on the leaf-margin
Flowerheads with 2 - 5 flowers
Flower-heads with 9 - 17 flowers
Underleaf coated partially or fully with a dense or sparse felt of stellate hairs
Underleaf hairless, or with a thin cover of simple hairs, or only with tufts of hair at the vein-joints
Any stellate felt on twig or leaf-stalk soon shed
Twig and leaf-stalk with a persistent stellate felt, often dense and white
Young twig very slender (< 2 mm thick); flower-bract with a long (5 - 25 mm) stalk
Young twig usually more than 2 mm thick; stalk of flower-bract seldom more than 10 mm long
Leaf large (often more than 150 mm long), and rather oblong (significantly longer than broad)
Leaf about as broad as long and seldom more than 100 mm long
Leaf with small, step-like marginal teeth; fruit with 5 bold longitudinal ribs
Leaf with approximately triangular, moderately deep marginal teeth; fruit scarcely ribbed
Fruit almost as broad as long; flower-heads with 2 - 5 flowers
Fruit about twice as long as broad; flower-heads with 9 - 17 flowers
Underleaf densely felted and brilliantly white
Underleaf grey or greenish and rather sparsely felted
Fruit more or less spherical
Fruit consistently longer than broad
Leaf rather triangular, with a long steadily tapered upper portion
Leaf rounded, with a short or more suddenly pointed tip
Marginal teeth of leaf with concave sides, forming scallops between long peg-tips
Marginal teeth triangular or with convex sides
Flower-heads with 4 - 10 flowers
Flower-heads with 15 - 20 flowers
Young twigs usually more than 2 mm thick; leaves usually more than 80 mm long
Young twigs very slender (< 2 mm thick); leaves often less than 80 mm long
Leaves notably large, often more than 150 mm long
Leaves seldom as much 150 mm long
Leaf rather oblong and significantly longer than broad; tufts under vein-joints neat and compact
Leaves about as broad as long; hairs extend along the main veins from their junctions
Underleaves with zones of sparse stellate felt extending away from the vein-joints; flower with staminodes
Underleaves with simple hairs and/or with compact areas of hair around the vein-joints; flowers without staminodes
Flower-heads with 2 - 5 flowers; fruit with bold longitudinal ribs
Flower-heads with 8 - 14 flowers; fruit not boldly ribbed
Marginal teeth of leaves tipped with very short whiskers, or none; fruit not boldly ribbed
Tilia × europaea
Marginal teeth of leaves tipped with obvious whiskers/pegs; fruit with bold longitudinal ribs
Upper leaf surface glossy
Tilia dasystyla, T. 'Euchlora'
Upper leaf surface matt
Mature leaf hairless except for tufts under the vein-axils; leaf surface fairly flat
Tilia dasystyla subsp. caucasica
Mature leaf often with simple hairs on both sides; surface softly rugose with sunken minor veins
Leaf oblong (significantly longer than broad), on a stalk much shorter than the blade
Leaf about as broad as long, on stalks sometimes almost as long as the blade
Leaf very small (< 35 mm wide); winter buds with 3 visible scales
Leaf 25 - 70 mm wide; winter buds with 2 visible scales
Winter buds with 3 scales visible
Winter buds with 2 scales visible
Underleaf often glaucous; flowers with long stamens extending past the petals; staminodes usually absent.
Underleaf pale green; flowers with short stamens not extending past the petals; staminodes present