Trees and shrubs: improving health of declining specimens

Though problems with newly planted trees and shrubs may be expected, well-established trees and shrubs after years of growing well can start to decline. This can be a worrying time for the owner so we look at what can be done.

Air-spading

Air-spading

Quick facts

Suitable for: Declining trees and shrubs
Timing: All year round
Difficulty: Moderate to difficult

How to tell if a tree or shrub is in decline

Symptoms that a tree is suffering from ill health include;

  • Stunted growth
  • Poor leafing up - coming into leaf late or sparsely
  • Premature leaf fall
  • Yellow or browning leaves
  • Dieback and shedding of branches

If the tree has already failed, see our advice page on Why has my tree or shrub has died?

And for additional causes of leaf damage in woody plants see here.

Causes of decline

One or more factors may be contributing to the decline of a tree. These include old age, unfavourable growing conditions or poor horticultural practise (what we might term 'environmental' stresses) or a pest or disease problem. While little can be done about the age of a tree, other causes are worth investigating.

Environmental

Stresses that lead to tree and shrub decline can happen quickly (within a single season) or build gradually for many years. Here are some common ones;

Pest or disease

Some causes of dieback, such as that observed in the golden-leaved Robinia pseudoacacia 'Frisia', are simply not yet fully understood. But others are detectable;

Diseases

Plants stressed by poor growing conditions may be more vulnerable to diseases. Some conditions such as waterlogging can also favour disease like Phytophthora root rot.

Top growth diseases such as leaf spots, rusts and scabs just weaken the plants or make it unsightly, others including box blight, ash dieback, bracket fungi, Phytophthora ramorum and P. kernoviae can be fatal to the plants. 

Root diseases such as honey fungus, Phytophthora root rot and verticillium wilt are potentially fatal.

Pests

Pests of top growth can weaken plants contributing to decline e.g. viburnum beetle, euonymus scale, wisteria scale, cypress aphid. However, pests such as elm bark beetle spread potentially fatal Dutch elm disease.

Soil pests seldom cause significant damage to soil-grown mature specimens, though vine weevil larvae could be a problem for trees and shrubs in containers.

How to stop or reverse decline

Get an assessment

If a tree or shrub is of high value and the cause of its decline not immediately apparent, it is worth investing in a professional assessment. Large specimens often need further on-site assessment to determine if the tree presents a risk and if any treatments are worth considering.

Organisations and professionals offering disease diagnosis

Conventional treatments

Try improving the growing conditions around the tree or shrub. For more details see section ‘Remedying specific environmental problems’.

  • Remove excess soil from around the stem or trunk base (the 'root collar')
  • Remove vegetation from around the base of the tree or shrub to a minimum of 1.2m (4ft) diameter. Apply fertiliser and organic mulch
  • Limit traffic (foot and vehicle) over the rooting zone especially when soil is wet
  • Consider installing drainage
  • Water in drought conditions
  • Consider soil analysis and rectify specific nutrient deficiency

Alternative treatments

Mechanical relief of soil compaction
Use of compressed air-tools (air-spade, Terravent or Terralift) can be beneficial for relieving compaction in the main rooting area and for removing excess soil from around base and root collar of trees and shrubs. Such treatments may be worth considering if trying to save a high value or hard to replace tree. An initial assessment by a tree specialist will consider the tree’s safety and then the likely success of remedial work.

Soil amendments
Bio stimulants have the potential to improve biological activity in the rooting zone. It is claimed that some can mobilise the tree's or shrub's natural defences against diseases. Such products include biochar, mycorrhizal fungi, rock dust, wormcasts and trichoderma. This can be combined with using air tools.

Pure wood mulches
Studies in the UK have assessed the disease suppressant properties of some pure woody mulches which leach a range of chemicals and compounds into the soil when shredded and applied as a mulch. The interface of soil and mulch is where diseases are often suppressed. This area is one of high biological and enzyme activity as well as diversity of fungal organisms. Many trees and shrubs root shallowly within this mulch/bark interface.

Species under consideration are for example European beech (Fagus sylvatica), common hawthorn (Crataegus monogyna), silver birch (Betula pendula), common cherry (Prunus avium), holm oak (Quercus ilex), English oak (Q. robur) and willow (Salix species). Though this research is at an experimental stages, results suggest that such mulches may have the potential to improve health of declining trees or reduce the likelihood of root disease infection especially if combined with other bio-stimulants such as biochar.

Remedying specific environmental problems

Here's some more guidance if you have managed to narrow the likely causes of decline to one of the following;

Deep planting or build-up of soil level

For optimum growth, the root flare (first roots on the trunk) should be just below soil level. Planting too deeply may cause establishment problems soon after planting, but it could also become a problem later on. Over time the soil level can build up around the base having similar effect. 

Solution: With a spade or trowel, carefully remove excess soil from around the base to expose the root flare. Take care to avoid damaging larger roots and the roots of trees and shrubs that are prone to suckering (root damage can stimulate or aggravate suckering). Unless the roots are very close to the surface, also reduce the soil level in the main rooting area under the canopy. Cover the area with mulch such as composted bark or wood chippings, but keeping the root collar free from mulch.

With high value trees and shrubs, consider hiring in a professional with air-tools that use high pressure air to remove excess soil and mulch from around the trunk and main rooting area with minimal damage to the roots. At the same time bio stimulants may be added to the soil. See 'Alternative treatments' above.

Compaction

All soil can become compacted, but clay and silty soils are the most vulnerable. Compaction causes poor gas exchange in the soil meaning the roots quite literally asphyxiate, leading to root death and poor top growth. The soil structure can be damaged by heavy foot traffic or machinery, especially when the soil is wet, causing compaction. Compaction can lead to problems with poor drainage and waterlogging.

Solution: Avoid walking on or working wet soils. Divert paths away from the base of ailing specimens. Remove all vegetation including turf from the main rooting area under the canopy. This eliminates the need to enter the area for regular mowing or weeding, as well as competition from other plants.  Cover the area with an organic mulch and replenish as needed. The microbiological activity can help to gradually improve the soil. Consider applying general fertiliser to the rooting area in spring to boost growth. Using compressed air tools (air-spade, Terravent and Terralift) can help to reduce compaction in the main rooting area. See 'Alternative treatments' above.

Waterlogging

When soil is saturated its air pockets are filled with water instead of air and roots can ‘suffocate’ and start rotting. Wet conditions also provide favourable conditions for certain soil borne diseases.

Solution: It is difficult to improve drainage around established plants but in areas prone to waterlogging consider installing drainage. Where practical consider moving smaller trees and shrubs to a better suited position. To reduce further soil structure damage avoid compaction.

Drought

Though well-established trees and shrubs can generally cope with a degree of drought stress, extremely dry conditions can result in significant leaf loss or premature leaf fall. Drought stress may contribute to or speed up decline, if the tree or shrub is suffering other problems already.

Solution: Apply mulch when soil is moist to reduce evaporation. In spring apply a general fertiliser to boost growth. However, avoid overfeeding that encourages soft, lush growth that is more prone to drought damage.

Lack of nutrients

Poor growth of established ornamental trees and shrubs is seldom caused by lack of nutrients in the soil. The problem often lies with poor uptake of nutrients due to the growing conditions such as drought, compaction or waterlogging or is caused by a root disease problem. However, they may suffer from specific nutrient deficiencies if soil conditions are unsuitable. 

Established trees and shrubs do not generally require regular fertiliser, but fruiting trees and shrubs and shrubs with a long flowering season such as roses benefit from annual feeding.

Solution: Aim to identify the underlying problem and address it. Consider soil analysis. Application of a general fertiliser as directed by the manufacturer can boost growth of stressed plants. Preferably apply fertiliser early in the growing season. Where possible, remove any vegetation from under the canopy before feeding. In the long term is it worth investing in organic mulches as these will gradually improve the soil’s texture, reducing leaching nutrients and evaporation from the soil leading to better nutrient availability.

When to remove a declining tree or shrub

Taking the decision to fell or remove a mature tree or shrub is often not an easy one, especially if it has sentimental value or is performing a service such as helping to provide screening or support wildlife. A couple of steps can be taken to determine if the time is right;

  1. Monitor: try remedies as described above, looking for signs of improvement or further decline. Making notes and taking photos can help with the assessment.
  2. Consider removal: if improvement is lacking or the tree or shrub becomes unsafe. If it fails completely it will need to be taken out. Removal of larger specimen should be carried out by professionals.

Dealing with woody waste

The cut wood can be usually used for wildlife log piles or shredded and turned into mulch.

Studies indicate that wood and bark chip mulch made from most diseased trees is unlikely to transmit pathogens to the roots of healthy trees, growing under good soil conditions and provided such mulches are not worked into the soil. The RHS does, however, recommend that basal parts of trees infected with honey fungus, Phytophthora root rot, or parts of trees infected with stem diseases such as verticillium wilt or coral spot, are not used, to reduce the risk to nearby plants.


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