After the floods

For much of the east coast, the first half of 2022 saw constant deluge and downpour.

In my hometown of Lismore, we experienced two devastating floods back-to-back – one that topped the previous record by over two metres – then it kept raining until June. Gardens took a serious knock. So, what did we learn from the experience and how can we make our gardens more resilient to periods of extreme wetness?

Drainage is everything

Just about every plant demands good drainage, and when rainfall is relentless, it’s the one thing that will get your plants through. Roots need air to breathe and don’t function properly without it. They also rot in waterlogged soil, making them unable to absorb moisture and nutrients. You’ll see symptoms such as wilting, stunted growth, discoloured foliage, leaf drop and dieback.

Over time, trees may also ooze sap, shed bark, and develop cankers. That’s when you know they are in serious trouble. Trees are arguably the most precious of all garden plants, providing shade, structure, flowers, fruit and habitat, so it’s worth taking steps to save them.

Applying a systemic fungicide such as phosphorous acid may help control disease organisms that attack roots, but early intervention is the key, before rot sets in. Before application, feeder roots can be dug up and inspected for evidence of rot.

Another tragedy could be happening underground. Worms and microorganisms drown in waterlogged soils. These guys are critical to soil health, breaking down organic nutrients into a form that can be absorbed by plants. To restore good health, water-damaged soil can be inoculated with compost, compost teas and products that contain beneficial microbes.

In the longer term, mulching, creating raised beds and planting mounds, and installing drainage will help build plant resilience in high rainfall areas. Selecting tolerant trees such as bottlebrush, paperbark and lilly pilly is important.

Compaction is a killer

Days of heavy rainfall cause compaction. It’s even worse if flooding occurs, with water weighing down like a big lump of concrete. Walking on sodden soil adds another layer of damage. It’s best to avoid this, but if you need to move over wet ground, cover it with timber boards to disperse the impact.

Compacted soil needs aerating. Once it dries, it can be opened with a garden fork. There’s no need to cultivate; that will only damage the soil structure. Following up by layering compost and mulch on the surface will support organisms that naturally aerate the soil and build permanent structure. It doesn’t have to be dug in – they’ll come and get it!

Contamination is serious

Floodwater brings the potential for contamination from sewerage and industrial chemicals. It’s worth having soil tested by a local lab so that you know what you’re dealing with. After the floods Valuable lessons learnt following one the the wettest summers on record. To prevent possible health problems, avoid eating anything that has been exposed to floodwater. That goes for all backyard vegies, herbs and fruit. It’s heartbreaking, but it’s not worth the risk. Rainwater tanks should be drained and thoroughly cleaned.

It’s safe to get on with general gardening after a flood, as long as skin is protected and hands are washed thoroughly. Contaminated vegie beds should be emptied and refilled with clean soil. Don’t use flood-affected compost in vegie beds; use this in ornamental gardens.

Silt needs attention

Some of the most fertile soils were created with silt. Think of farmlands founded on rich river plains where many floods have deposited finely ground minerals. While floods may boost soil fertility, a silt deposit can initially set hard like concrete. It may need a fork or steel rake to break up the crust and allow air and water to penetrate the soil. Over larger areas, soil can be covered with fresh, clean mulch. As it breaks down, it incorporates with the silt layer, keeping it soft.
Silt deposits on plant foliage also need attention. Leaves cannot photosynthesise while they are covered in muck. It should be washed off as soon as possible, using a sharp spray from a hose or pressure cleaner, taking care not to damage delicate leaves.

Weeds love the wet

Weeds are the world’s greatest opportunists. Just add water and they’re off! If floodwaters pass through, you’re likely to see new arrivals that have travelled great distances.

Don’t let wandering weeds go to waste. Turn them into compost – the soil will be crying out for it.

Written by Phil Dudman, Gardening writer and educator.