Where Should I Plant My Hedge or Trees to ALSO Benefit Water Quality?

By MIKE O CONNOR Agri-environment Scientist, IRD Duhallow

The tree planting season (Mid-November – March) is upon us and more opportunities than ever exist for landowners and community groups to plant trees and hedges. Many farmers are receiving payments for planting trees and hedgerows through ACRES, the new national agri-environment scheme. The Native Tree Area Scheme has been recently opened which will allow farmers to plant 1ha of native trees on their land without a licence or 2ha if they are adjacent to a watercourse (with generous payments in return). Many dairy co-ops are also offering suppliers trees to plant on their farms, for example through the KERRY Evolve Programme. This provides a great opportunity to harness the multiple benefits of trees, including for water quality, provided they are planted in the right place!

Benefits of Trees

The benefits of trees and hedges are multiple for both farm and environment. Hedgerows were once the boundaries of fields and towns and prickly whitethorns and blackthorns created stock-proof barriers. Today, trees and hedges still provide important shelter for livestock from wind, rain and sun. These benefits will become more pronounced as we move into the future and climate change intensifies. In addition, there is a body of evidence to support the importance of shelterbelts for soil health.

Hedges and trees provide obvious benefits to biodiversity and climate, through providing a habitat and sucking up carbon. But the key focus of this article will be the importance of hedges and trees for water quality!

Planting trees along an excessively eroding stretch of a river or stream is a nature-based solution to stabilising riverbanks. Preventing this excessive erosion will save the farmer from losing land and will reduce the amount of sediment that enters the river, carrying phosphates with it and smothering insects and fish eggs.

Creating a ‘buffer’ involves planting in a place that will intercept water as it moves over the land. This is because phosphates are transported overland by water (unlike nitrates which leach through free-draining soil). A common buffer would be

a ‘riparian buffer’. This involves planting trees in the riparian (riverbank) area. However, a buffer does not have to be in the immediate riverbank area. A hedgerow planted in the right place will also slow down water as it moves through a field, long before it can ever get to the river!

A riparian buffer strip

Planting an eroding riverbank

Planting The Right Tree In The Right Place

Knowing where water flows off a field, allows us to strategically target where trees and hedges should be planted to intercept overland flow. The EPA have created Pollution Impact Potential (PIP) maps that are openly available on www.catchments.ie. These show areas at higher risk of runoff, as well as how water (potentially carrying phosphates) moves off the land (known as nutrient flow pathways) and where it enters a watercourse. While this is an important resource that helps us to put the right measure in the right place, nutrient flow pathways can often be identified on the ground, and farmers will often know the path water takes as it moves through a field during heavy rainfall and where it enters a watercourse.

Planting a hedgerow through one of these nutrient flow pathways will disrupt the flow of water and allow the hedge to take up more phosphates before they can get to the watercourse. Alternatively, trees can be planted in a block at the point where the overland flow enters the watercourse. This is a very effective way to reduce phosphate runoff. Phosphate is the dominant nutrient that runs off into rivers in poorly draining areas. It is also much more efficient in terms of land-use than planting an entire riparian strip, if water is running off the field at specific points.

PIP Map showing nutrient flow pathways and entry points

The point at which water enters a drain, planting here would disrupt the overland flow of phosphates

What species should I plant?

Whitethorn (Hawthorn) is the classic hedgerow species! But it is better for biodiversity if you can plant a mix of species. Other suitable species include Blackthorn, Hazel, Holly and Dog Rose. These should be planted at a density of 5/m2, coppiced to one inch after planting (to allow them to thicken), and protected with a biodegradable membrane to protect them from being outcompeted by vegetation. Each year, take 1.5 inches off them until they are at least 6ft tall. Leave one tree per 10m/30ft grow to maturity.

For tree planting, if you are planting in an area that will realistically be wet, such as a riverbank or a nutrient flow pathway, you should select trees that like ‘wet feet’. Suitable native trees are Alder, Birch, and Willow (Sallys). Willow is naturally rich in rooting hormones and will put down its own roots if a slip taken from a tree already on the farm is driven into the ground. Willow is particularly useful for planting on riverbanks or drains to prevent erosion.

Willow (Sallys) planted in a nutrient flow pathway

For more Info, contact Mike O’Connor at 029 60633