Tips for Improving Drainage in Small Gardens.

2nd July 2024

With the recent weather events, we have been asked for tips on improving drainage in our small gardens to make them more resilient to heavy rainfall.

The good news is that the measures we take to make a garden less prone to bogginess will also help it withstand periods of drought, so it's a win-win!

Our housing developments are built on hard, compacted clay, which takes water much longer to seep through when dry and is prone to waterlogging when wet. To make matters worse, the clay dries quickly during hot weather, becoming rock-hard and aquaphobic. This means that when the heat is followed by periods of rain, water is not easily absorbed into the ground, causing it to pool and overflow our drains.

This clay was dug out of a garden in Hobsonville Point.

Add lots of rich organic matter.

You may have heard us say that improving soil quality is so important!

Organic matter introduces many beneficial microbes that slowly break down organic matter. Over time, the organic matter binds with the soil particles, forcing them apart and creating better aeration and space for water to seep through. You can add organic matter to the soil by incorporating compost, mulch, manure, leaves, or other organic materials.

Water-tolerant Plantings.

Luckily, a good selection of plants suits various soil conditions, from always boggy to damp to occasionally boggy.

For boggy soils.

If your area is consistently damp or boggy, choose wet-loving plants that absorb excess water and improve soil quality.

From left to right:

Apodasmia similis (oioi grass), Selleria radicans, Carpodehus serratus prostrate, Papyrus King Tut, Carex secta, Iris sibirica.

For damp but free-draining soils.

These plants (except the Canna tropicana) are best suited to those shady sites on south-facing fencelines or under trees where soil takes longer to dry out due to the lack of light. Think forest floors or tropical underplanting. The Canna tropicana likes sunlight but prefers consistently damp soil.

From left to right:

Asplenium bulbiferum, Blechnum silver lady, Blechnum penna-marina, Liguleria reniforms, Canna tropicana, Alocasia nigrescens.

For soil extremes.

For soils that experience more variation throughout the year, there are good options that can tolerate a range of soil conditions from dry to damp. These tend to be New Zealand natives that have adapted to our "four seasons in one day" type of weather.

From left to right:

Carex virgata, Phormium surfer, Libertia grandiflora, Dianella nigra, Leptospermum scoparium, Leptospermum Burgundy Queen, Hebe stricta, Arthropodium cirratum.

Build raised garden beds.

There are many benefits to having raised garden beds, but the most important is that it allows us to add premium topsoil. This soil is less compacted, providing good drainage and more space for the roots of your plants to grow deeper into the ground and access more nutrients.

At this stage, we can dig and turn the clay on which the beds are laid, creating more space for the new soil to blend and improve the old.

If you have a limited budget or building skills, raising the bed 15cm off the ground will provide some benefit by allowing you space to add compost and mulch, which will improve your soil over time. Big box retailers like Mitre 10 sell rolls of garden edging for the DIY gardener, or you could consider corten steel edging if you prefer a higher-end look.

However, we would opt for a minimum of 30cm for the best results.

Permeable pavers, stone and gravel.

A permeable hardscape can drain water directly through itself. This means the drainage system is built into the hardscape and requires little or even zero extra drainage features to keep the surface from pooling or flooding.

Permeable hardscapes decrease stormwater runoff, allow rainwater to permeate the ground, and help water your plants. To achieve permeable hardscapes, we can use permeable materials, such as stone, gravel or pebbles, or lay the materials so that there is space for the water to run through.

Stone, pebbles, and gravel have been used to improve drainage since antiquity, so it's no surprise that they are some of the most useful materials for porous hardscaping in the garden.

Additionally, the invention of honeycomb paving, which secures the stones in place, has made it easier to lay and more secure to walk on. Naturally, we're big fans.

Where the hardscape material is not permeable, such as concrete or porcelain tiles/pavers, it's important to lay adequate space between the tiles to allow water to run off. In the example below, we cut a solid concrete patio into pavers and planted it in between. Not only does it look good, but it also allows us to add more soil and increase our plant-to-hardscape ratio in the garden.

NB: Sometimes, the job calls for a drainage specialist, especially if water run-off occurs due to neighbouring properties. In these instances, we suggest contacting a drainage specialist or engineer or chatting with our team, and we can liaise with one on your behalf.

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