Invasive Species

 

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Himalayan Balsam

 

One of the greatest threats to our native ecosystem and domestic gardens is the arrival of pervasive and invasive species. Anyone who has taken a walk along local riverbanks, which are frequently clothed in Himalayan balsam, can see how these can become a real pest.  Himalayan balsam, or Impatiens gladulifera, is a relation of the popular summer bedding plant, busy lizzie. This has colonised huge tracts of river banks, suffocating native plants which our native wildlife depends on for food and nesting. The summertime is when it is most difficult to get in amongst garden plants to weed and sneaky species can get a foothold, so autumn, when you are clearing and cutting back is a good time to look for evidence of unwanted species and dig them out before they become a problem. There are a three categories of invasive plants; the first are the common weeds we all know and battle against year in year out like dandelion, daisy and buttercups. The second category are garden plants like Japanese anemones and Echium that have self-seeded to such an extent that they have become a nuisance, but cause no real damage.  The third category are species which were introduced by horticulturalists as curiosities but quickly naturalized and spread throughout the countryside.  The latter category causes real ecological and structural damage by escaping from our gardens and into the countryside beyond. A classic example of such a pest is Horsetail or Equisetum arvense. This plant has creeping rhizomes which can go down 2m below the surface, making them almost impossible to eradicate by digging. Even when using a pesticide persistent applications are necessary, you may need to call in specialist help. Japanese Knotweed, or Himalayan Balsam, was introduced from Japan in the nineteenth century and since its introduction it has spread nationwide. It can destroy the foundations of houses as it can grow through concrete and tarmac. It also blocks wildlife corridors and clogs up watercourses.

Removal and control of invasive species causes millions of euro a year. Other pests include Bamboos – Philostachys species which form impenetrable thickets, block out native plants and spreads rapidly, again popping up through hard surfaces with ease.

 

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Japanese knotweed

Obviously the best way to fight invasive species is not to plant them in the first place so here are some tips to keep these thugs out:

  • Check a plants habit before you buy it and see if there is a non-invasive alternative.
  • If you have been walking along river banks or areas where invasive non-native species have colonized always wash the soles of your shoes thoroughly before wearing them in your garden. Seeds are commonly brought in on the soles of boots or on animal’s coats.
  • Never add any part of an invasive plant to the compost heap.
  • Invasive aquatic plants have caused considerable problems to our waterways and it is important to stop their spread. Avoid swapping cuttings or fragments from friend’s ponds. Otherwise you may end up with an invasive species in your pond.
  • Wash the roots of plants you buy too – I have a dreadful duck weed problem in my pond which came in on a plant bought from a very reputable garden centre.
  • Learn to recognise seedlings of invasive plants. Dig out seedlings as soon as you see them in spring and before they establish.
  • When weeding out mature plants make sure to remove all of the roots. Plants with tap roots like dandelion species will generate from every little bit of root left in the soil.
  • Rhizomatous weeds such as couch grass, bindweed and ground elder respond well to glyphosate if treated twice over a two-week period. It is very important to avoid drift so apply on a very still day. To treat bindweed, spray the weed killer onto rubber gloves and run your hands along the plant.
  • To eradicate ivy on trees and old walls cut it back to the ground and dig out the woody stump.
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Horsetail

Coppicing for better winter colour

Coppicing refers to pruning shrubs and some trees close to the base of the plant.  Late winter is the best time to coppice and pollard. It promotes colourful young stems, ornamental foliage and rejuvenates plants that tolerate hard pruning.

Coppicing is used to ensure that willows and dogwoods produce a fresh crop of bright coloured stems each winter. It is also a good way to rejuvenate an old shrub which is no longer performing well or has become messy in appearance. To coppice just take a deep breath and cut stems to within 5cm-7.5cm (2in-3in) of the ground, or to last year’s stubs. Do this before new leaves appear in March.

Shrubs and trees suitable for Coppicing

  • Acer pensylvanicum‘Erythrocladum’
  • Beech (Fagus sylvatica)
  • Dogwoods (Cornus), including Cornus sanguinea‘Midwinter Fire’,  and  alba ‘Elegantissima’
  • Elder (Sambucus)
  • Foxglove tree (Paulownia)
  • sericea ‘Flaviramea’ Sweet gum (Eucalyptus gunnii)
  • Hazel (Corylus)
  • Hornbeam (Carpinus)
  • Indian bean tree (Catalpa)
  • Judas tree (Cercis)
  • Lime (Tilia)
  • Ornamental bramble (Rubus cockburnianus)
  • Smoke bush (Cotinus)
  • Toona sinensis‘Flamingo’
  • Tree of heaven (Ailanthus altissima)
  • Willows (Salix), including Salix albavitellina ‘Britzensis’, S. viminalis and S. daphnoides
  • Yew (Taxus)

 

 

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Pleaching trees for a formal garden

 

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Pleaching is a more glamorous use of the skills needed for hedge laying for stock control. This technique is used to create a formal hedge on stilts. Suitable trees are standardised by removing their lower branches and the remaining branches are entwined with those of the tree beside it.

As with espaliers you can now buy trees which have been ready-pleached; the branches of these are trees have already started to be trained and are tied to a bamboo frame. In order to create a formal hedge on stilts you need to plant your trees in an evenly spaced row. Pleaching is not for the faint hearted as it is extremely labour intensive. You need to constantly tie in new stems to the tree’s neighbours, prune regularly and loosen ties which have become too tight.

When to pleach

Always select young, whippy plants that are more easily trained for any pleaching.

Plant in winter and during the early years also prune in the winter when the plants are leafless and dormant.

Train and tie new shoots in over the summer.

Once pleached trees have reached their full extent, prune in the summer, pruning to shape the tree growth and reduce its vigour.

How to pleach

Plant trees in single or parallel rows 1.2m (4ft) apart in the row and at the same spacing between parallel rows

Allow 2.4-3m (8-10ft) between rows to provide a pathway

During the spring and summer growing season, tie in new shoots to the supports. Horizontal stems should be plaited or tied in with those of neighbouring trees

Unnecessary or ungainly shoots should be pruned back to one or two buds from the base during early autumn or winter. Pinch out leading growths to encourage bushiness.

Trees suitable for pleaching

  • Generally trees used for pleaching are deciduous. The most suitable evergreen for pleaching is the evergreen oak (Quercus ilex).
  • Lime (Tilia platyphyllos ‘Rubra’ T. x europea ‘Pallida’
  • Hornbeam (Carpinus betulus)
  • London Plane (Platanus x acerifolia)

Pollarding trees

 

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Pollarding refers to pruning a tree’s branches right back to the trunk or stem so that it will produce a dense growth of new shoots. It is often used in cities and parks to inhibit growth branches which might cause an obstruction.

Trees suitable for pollarding

  • Beech (Fagus )
  • Black locust Robinia pseudoacacia
  • Catalpa Catalpa
  • Hornbeam Carpinus 
  • Horse chestnut Aesculus  hippocastanum
  • Linden Tilia
  • London plane tree Platanus x acerifolia
  • Mulberry Morus 
  • Redbud Cercis canadensis
  • Tree of Heaven Ailanthus altissima
  • Willow Salix

How to Espalier a fruit tree

Tb0c95bb73a0d1cb3f9db6718cef5a750espalier-typeshe practice of espaliering fruit trees was brought to Ireland in the seventeenth century by Huguenots fleeing persecution in France. Training apples and pears as espaliers, not only saves space in the garden but also ensures a good crop, keeps trees fairly disease free and creates a good architectural vertical feature in the garden.

The espalier method means growing a fruit tree or ornamental shrub while training its branches to grow flat against a wall, supported on a lattice or strong wire.

When planning to grow a tree as an espalier, make sure you buy one on one of the following rootstocks, this can be complicated so ask in your nursery for advice when buying a tree. Pears trees are generally grafted onto quince stock. You can often buy trees with one to three tiers already formed or grown from one-year-old trees.

Planting and initial training

Firstly you need to establish a training system against a wall or fence. Erect horizontal wires 35-45cm (15-18in) apart between posts, or straining ‘eyes’ on walls or fences.

Trees should be planted 3.75m-6m (12-20ft) apart, according to their vigour.

When planting a new ‘unfeathered maiden’ – garden parlance for a one-year-old tree with no side shoots, cut back the main stem to 30cm (1ft) from the ground

Allow the top three buds to grow out in spring, train the top one vertically up a cane, the others two to canes at 45 degrees to the main stem. In November, lower them carefully until they are horizontal, tying them in with soft twine

Cut back the vertical stem to within 45cm (18in) or the lower arms, leaving two buds to form the next horizontal layer and the top bud to form the new leader. If growth is weak, prune back the horizontal branches by one-third to downward facing buds

The following year train the second tier in the same way as the first. Cut back competing growths on the main stem and side shoots from the horizontal arms back to three leaves above the basal cluster

Repeat the process until the trees have produced their final tier and grown horizontally to fill their allotted space. Then allow two shoots to grow, tie them to the top wire and cut them back to within 2.5cm (1in) of their base the following winter

Remove the blossom in spring, for the first three years, so all of the energy goes into plant growth.

Prune trees after fruiting by cutting back the side-shoots growing from the horizontal leaders to three leaves from the basal cluster, 7.5cm (3in) long. Shoots from previously pruned side shoots should be cut back to one leaf from the basal cluster, 2.5cm (1in) long

If any secondary growths arise after this pruning, cut them back to the base in September. Side-shoots on the vertical stem are best removed completely

Clusters of fruit buds may need to be thinned after seven or eight years of ‘spur thinning’, alternatively, a few can be done each year in the dormant season (November to March), reducing over-complicated spur systems to one or two fruit buds.

Trees and shrubs suitable for espaliering

  • Pears
  • Cercis
  • Laburnum
  • Crepe Myrtle (Lagerstroemiae)
  • Apples
  • Parrotia persica
  • Flowering almond, Apricot, cherry, peach, plum (Prunus)
  • Cotoneaster
  • Pyracantha