Find out about the groups of UK trees we highlighted as our tree of the month during 2011. We'd particularly like you to identify and record these trees for the urban tree survey.
Some capture the essence of a season, others are at particular risk from disease, and some are non-native species that may cause issues if they spread from gardens into the countryside.
We've summarised the key seasonal features to look out for. You can continue to survey each of these trees throughout the year.
Holly (Ilex species) berries
Hollies are a large group of deciduous and evergreen trees and shrubs found in almost all parts of the world. There are around 400 species, with most growing in Asia and the Americas.
The most well-known is the common or European holly, Ilex aquifolium, one of only three native European species. This is one of the plants most associated with Christmas, perhaps because a holly tree laden with bright red fruit set against glossy green leaves is one of the most cheerful objects in the winter landscape.
There are several hundred varieties of I. aquifolium available to plant in the garden. These vary in shape, the amount of spines and the colour of leaves, bark and berries. Nearly all varieties have male and female flowers on different plants, so in order to get berries both a male and a female tree must be planted.
Strawberry tree fruit ripens after a year, at the same time as flowers appear. © Mnolf, Creative Commons Attribution-Share Alike 3.0 Unported license
Native to the Mediterranean regions and southwest Ireland, this small tree or large shrub is grown throughout warmer parts of the UK. It is one of the few members of the heather family (Ericaceae) that thrives in soils containing lime.
Sprays of small white bell-shaped flowers are produced at this time of year, as last year’s fruits ripen orange-red.
The fruits may superficially look like strawberries, but they do not taste as good - the species name unedo is thought to come from the Latin ‘unum edo’ meaning ‘I eat one only’. However, the fruits can be used to make jam and flavour alcoholic drinks, such as the Portuguese liqueur Medronho.
Horse chestnut fruit. The green husk contains the familiar shiny brown seeds, conkers. © Andrew Dunn, Creative Commons Attribution-Share Alike 2.0 Generic license
Shiny brown conkers produced by horse chestnut trees are one of the delights of autumn in Britain. These majestic trees have graced our streets and parks for hundreds of years, since the common horse chestnut (Aesculus hippocastanum) was introduced in the 17th century for its attractive candles of white flowers.
But their appearance is suffering as they come under attack from an invasive moth, the horse chestnut leafminer (Cameraria ohridella). The moth’s caterpillars ravage the leaves, turning them prematurely brown in summer.
Although such infestations don’t appear to be life-threatening, the aesthetic damage is increasingly prompting councils to replace horse chestnuts with other species. However, there is a potentially fatal disease that affects horse chestnuts. This disease, known as bleeding canker (due to the rust-coloured fluid that oozes from the bark of affected trees), is also becoming more widespread. So are the days of horse chestnuts in Britain numbered?
Field maple fruits occur in pairs, with the 2 parts in a straight line © Jean François Gaffard, Creative Commons Attribution-Share Alike 3.0 Unported license
This large group of trees is easy to spot. Typical maple leaves are broad and flat with 5 deep lobes. The fruits have a thin membranous wing and occur in pairs on a single stalk.
Look out for the UK's only native species, the field maple (Acer campestre), not to be confused with the sycamore (Acer pseudoplatanus) which naturalised from mainland Europe several centuries ago and can be an aggressive woodland weed. To tell them apart, look at the fruit - the 2 parts are set at an acute angle to each other in the sycamore, but they are parallel in the field maple.
You may also spot one of the many cultivated maples introduced from northern Asia and North America. Many assume beautiful red or yellow tints as autumn arrives.
Leaves and acorns of an oak (Quercus species) © Robert Vidéki, Doronicum Kft., Bugwood.org
There are over 500 species of oak around the world. They mostly grow in temperate regions and a number of different species can be cultivated in the UK.
The most well-known species, and one of our most-loved trees, is Quercus robur, the English oak (although it is native to all parts of the British Isles).
Oak trees can be easily identified by their familiar acorns, which vary only a little between species and can currently be seen on mature trees. Many species also have distinctive lobed leaves.
Flowers of the tree of heaven, Ailanthus altissima © Jan Samanek, State Phytosanitary Administration, Bugwood.org
An attractive and exotic-looking tree, Ailanthus altissima has become popular in streets, parks and gardens in the UK since its introduction from China in the 18th century. Greenish flowers are followed by brightly-coloured twisted fruit.
The fast-growing tree can rapidly reach a height of 25 metres or more. It can produce strong suckers that crowd out other plants and toxins that prevent germination of other species nearby. As a result, the tree is considered an invasive weed in many countries, including the USA and Australia.
In Britain, the tree of heaven is currently restricted mainly to parks and gardens in the south and east of England. But due to our increasingly mild climate and the tree’s invasive nature in other countries, there is a risk that the species could start spreading across the country and pose a threat to native plant diversity.
Leaves of Tilia x europaea, a hybrid of 2 native British lime trees © www.istockphoto.com
Lime trees, Tilia species, are large deciduous trees. The common lime that we see all over the UK is actually a hybrid of our 2 native limes, T. cordata and T. platyphyllos.
At this time of year they have a dense foliage of heart-shaped leaves and sweetly scented flowers, or may even already bear tiny dry fruit. Their flowers attract many bees and they are popular with beekeepers.
You may notice the ground beneath lime trees becoming sticky with a sugar-rich substance called honeydew. This is produced by aphids, which often infest lime trees.
Yellow-leaved form of Robinia pseudoacacia © www.istockphoto.com
The false acacia, Robinia pseudoacacia, originates from America. Due to its attractive foliage, it is a very popular tree in gardens and public spaces. Its popularity in Britain surged when the yellow-leaved form, ‘Frisia’, came on the market as suitable for small gardens.
However, some countries consider the false acacia an invasive species. Major issues are the plant’s ability to regrow when cut down and to form dense groves that can be detrimental to native vegetation. There are reports that it is spreading on waste ground along railway lines in the UK.
Mature trees produce spectacular streaming clusters of fragrant white flowers in May and June.
Blossoming cherry trees
From our native wild cherry to beautiful ornamental cherries introduced from Japan, China and Korea in the 19th century, over 20 different cherry species from around the world are grown in the UK, as well as countless hybrids and cultivars.
Along with other members of the genus Prunus, such as plums, almonds and peaches, they add beauty to parks, streets and gardens around the country when they bloom from March to early May.
Flowers of the European white elm, Ulmus laevis © Vellela
Since the late 1960s Dutch elm disease has killed over 20 million mature elms in the UK. Those that survive may hold the key to propagating a new generation of disease-resistant elms.
Small clusters of reddish or purplish flowers appear on these deciduous trees from March to April, before leaves have appeared.
Elms provide important habitats for many invertebrates, lichens, mosses and fungi. For example, they are the only known host plants for larvae of the white-spotted pinion moth, Cosmia diffinis.