To tree or not to tree? Simple rules to successful planting

Guy Watson, from Certhia Consulting Limited, explains why this is the ideal time to be considering new tree planting. Whilst this may be a straight-forward operation, there are some simple rules that will ensure success for the future. The Victorians were great tree planters and we are at the tail end of the benefit of that.

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A large proportion of the trees planted by those far-sighted people are now at the ends of their lives, or have already been removed. During the difficult times that followed the Victorians - two World Wars, the Great Depression of the 1930s - tree planting was very low on the list of importance. Consequently, a gap in the tree population has developed. There is no better time to plant a tree, or as many trees as you can.

The first, and most important, factor is to ensure that the right tree is chosen for the site it is to occupy. There is a huge number of tree species, cultivars, hybrids and forms. Each has its own specific requirements and will give its own benefits. They do, however, require specific soils, associated drainage and pH, space and light. If we do not get this right, that tree may be a very short lived investment. There is no excuse for getting this wrong. A plethora of information is available and most of the reputable tree nurseries provide stunning web resources, catalogues and handbooks to help. Not only do they deal with the practicalities of choosing the tree to suit the site, many of them come with striking pictures of the attributes they offer in terms of leaf colour, blossom type and time, fruit potential, nature conservation interest (especially bees) and much more.

As part of the selection process, diversity and changing climate can be built into species choice. Diversity has many benefits, in particular relating to the incidence of pest and disease. As was seen in the 1970's when Dutch Elm disease took hold, as the trees had genetic identity, it affected all of them. Similar fears are about to be realised with Chalara dieback of Ash, caused by the fungus Hymenoscyphus fraxineus. Whilst Ash has a wide genetic spectrum in the UK, the disease is set to affect a very large proportion of trees in the wider landscape. Maintaining a species diversity in a planting programme gives resilience. In the event of an outbreak, it will affect individuals rather than the whole population, thus retaining a tree stock to build from rather than having to start from scratch.

The next main factor is to choose your supplier carefully. There are very few "tree growers" in the UK and the majority of trees for sale in this country have been imported from mainland Europe - potentially even further afield. Some trees ordered from a UK nursery never pass through their gates: they are ordered through continental suppliers and are delivered straight to site. The plant passport system is supposed to guarantee that plants coming into the UK are pest and disease free. The fundamental flaw in this system is that suppliers issue their own passports. Whilst it is likely that the majority of nurseries abide by the rules, the difficulty in ensuring compliance is enormous. Don't be fooled by a "native" provenance claim either. This may apply to where the seed was gathered. Where has it been since for pre-germination treatment, germination, growing on before final sale? Some of these processes may occur in different countries around Europe. Therefore, soil changes, irrigation water and exposure to continental pest and disease is inevitable. The vast majority of diseases that are of concern are soil or water borne organisms. For example, Xylella, which is spread from plant to plant by sap sucking insects; Plane Wilt, Ceratocystis platani; and all the Phytophthoras. How can these really be recognised at source?

There have been some very obvious imports of pest and diseases into the UK that are directly attributable to this mass movement of plant material. One of the most widely publicised diseases of recent times is Ash Dieback. Whilst this may well have been in the UK for many years, it was spread quickly by imports of young Ash trees from continental nurseries, all under passports. The other pest of public health concern that was definitely imported on planting stock was Oak Processionary Moth (Thaumetopoea processionea). This moth produces several generations of caterpillars. It is the older caterpillars (or older instars) whose defensive bristled backs produce an irritating skin rash and are implicated in breathing difficulties, especially amongst asthma sufferers and the young.

As a result, there has been an increased awareness of the likelihood of risk associated with further imports into the country. Several species of trees can now no longer be imported into the UK. These include Ash, Plane, Sweet Chestnut and Pine. Some can be imported with special licences, or are now only allowed to be imported with phytosanitary certificate or plant passport. Up to date advice on import restrictions may be obtained from The Animal and Plant Health Agency¹. Increased funding into plant health management, including disease resistance and treatment, planting strategies and outbreak control has now been made available.

Ask your supplier about their biosecurity measures. Do they hold imported stock on their nursery for a period of time to check the validity of disease free claims? Or do they grow their own? Importantly, do they adhere to British Standard 8545:2014 'Trees: from nursery to independence in the landscape?'²

This document is a vital source of information for anyone considering a planting programme, providing information on all aspects of species choice, tree production, specifications, handling, planting and aftercare. It should be an essential part of any contract to supply plants for any project.

Nurseries supplying your trees should be able to supply you with information about age, method of propagation and production, pruning (both aerial and root) regimes and, if imported, where from and when, pest and disease identification and control programmes. Do not be afraid to ask! You are the customer and a reputable nursery will be expecting, and should welcome, your questions.

The most important part of any young tree is the root system. It must be well formed with at least four distinct lateral roots developing around the tree. From these must come a mass of fibrous roots, small diameter strand like structures that carry out the uptake of water and nutrients to allow the tree to establish. Well-formed root systems are developed by planned lifting and pruning or undercutting. A carefully grown tree will also demonstrate a well-developed stem taper from the root flare up the main stem. This shows that the tree has had sufficient space on the nursery to grow well and produce a trunk that is strong and supportive. A thin, parallel stem may be a result of crowding and be mechanically weak, therefore not suited to supporting itself in the wider landscape.

Trees are invariably supplied in the following forms.. ...all of them have advantages and disadvantages:

• as bare root: literally lifted from the field with no soil;
• rootballed: where the tree is lifted from the field with field soil attached and wrapped with hessian or similar. Larger rootballs must be strengthened with wire netting or similar;
• containerised: lifted from the field and put into a container for a period to allow root development in controlled environments; or
• container grown: grown in containers and moved up as required for continued healthy growth.

Bare root trees are generally cheaper, but are vulnerable to drying out if not cared for properly. Both bare root and rootballed trees may have limited fibrous roots if they are not prepared well. It is estimated that a poorly managed bare root or rootballed tree may leave 95% of its fibrous root in the ground when lifted due to poor nursery practice. Containerised trees can be confused with container grown. Where a tree has been containerised, ask how long for. If it has been lifted, put in a pot and sold, there will have been no time for the new roots to develop and you have simply bought a bare root tree in a pot. Containerised and container grown are more expensive, weigh more, but are more resilient and can be stored, and planted at different times of the year. The type of trees you require should be discussed with your supplier to ensure you get the right trees at the right time and are aware of transport and storage requirements. This means that your trees will grow on in their final planting location.

Handling on the nursey, and during transport, are vital components of this process. Your nursey may be many motorway miles from your location. A bare root tree left exposed for even 15 minutes can suffer considerable desiccation. As a result, its functionality is irreparably damaged. Ensure your trees will be covered, supported and protected during transit. All parts of the trees should be protected by opaque sheeting to prevent drying on road journeys, even when dormant. Co-extruded bags should be used to cover root systems, white on the outside to reflect heat and black on the inside to keep dark. Journey time and lorry time should be keep to a minimum. The lorries unloaded as quickly as possible into a suitable plant handling area where the trees can be watered, root systems covered and protected as necessary, and trees supported in an upright position until planting.

Trees must be inspected for quality during delivery. Check root systems, ensure the rootball is intact, not split or loose. The aerial part of the tree should demonstrate an evenly branched, undamaged structure with all branches and the terminal bud being alive. The tree should have a specific leader and the whole should be upright, straight and balanced. All bark must be intact, larger trees handled poorly with slings or straps can easily have significant bark abrasions that will affect long term growth and development. You supplier should be informed about any tree that does not meet the specification and should be rejected.

Once you are happy you have the correct tree and it is of good quality, plant it as quickly as you can. The digging of a tree pit may seem simple and it is, do ensure that these golden rules are followed. In clay soils, and some others, check that the sides and base of the pit are not smeared or glazed. This polishing of the faces of the pit can create a significant barrier for roots trying to grow out of the pit into the surrounding soil. Do not mix top and sub-soils. Replicate the horizons in the pit as far as possible. Use soil ameliorants sparingly. The use of additives such as mycorrhizal inoculants, water retaining gels, bio-stimulants etc. have been advocated but trials do not seem to have come to any specific conclusion as to their efficacy. Get a good tree, prepare your pit well, plant and look after your tree; it will extend its root system into surrounding soils and grow to the site constraints it has. Providing all the nutrient and moisture within the pit a tree needs will not encourage that development and, so, independence. New trees do not require fertiliser, unless soil tests show a definite nutrient deficiency.

Ensure you plant your tree at the correct level. It is better to plant too high than to bury it. The root flare should be clearly visible: make sure rootballed trees show their root flare, the same for container grown trees - visible root flare is vital. Once a rootballed tree is positioned, the hessian and wire should be loosened. The wire should not be galvanised and will, with the hessian, rot away. If the wire encircles the stem at any point, this should be cut away.

Provide appropriate support for your tree: be this a traditional stake and tree tie, an angled stake, cross bar or underground - or over ground - guying. Any stake should be kept as low as possible to allow stem flex and development. They should be installed prior to the tree and the roots arranged around it. Stakes must not penetrate rootballs or containerised or container grown trees. In these cases, an angled stake, or double stake and cross bar, is appropriate. If underground guy cables are used, adjustment may be necessary and the cables should be near the surface. Any support should be kept on for the minimum amount of time. It should be assessed regularly for need and removed as soon as possible. Significant damage can occur from unadjusted or retained tree ties, and rubbing on stakes. Protection from animal damage may also be required in the form of guards and tubes to prevent browsing or gnawing. The type of guard will depend upon the likely predator in the locality.

Newly planted trees should be saturated to field capacity at planting and a mulch of organic material added to a depth of 50-100mm, but not touching the stem. Further irrigation may be needed to transition the tree into its new environment. The amount and timing of any application will be dependent upon soil moisture capacity and climatic conditions. Water should be applied slowly to allow it to percolate down into the pit. In very dry times, trees may need watering 2-3 times a week and may need to continue for two summers after planting.

All new trees should be checked at least annually for health, condition, stake and guard adjustment, mulch replenishment etc., do not just assume it will grow!

Tree planting is an investment. Make sure that you give it full consideration, allow lead time for discussions with your nursery to ensure the trees you want are available, visit your nursery, ask questions and choose your trees. Then, the actual trees from the nursery delivered to you will be in fine condition ready to plant and become the next generation of our tree population.

¹https://www.gov.uk/government/organisations/animal-and-plant-health-agency
²http://shop.bsigroup.com/ProductDetail/?pid=000000000030219672