Flood garden recovery

Following the floods and heavy rains that have affected much of eastern Australia, HMAA Queensland member Kate Wall (BSc) has provided this informative article to help gardeners and horticulturists follow the most effective clean up practices. Kate will also present a free webinar on the topic to HMAA members on March 24 at 1pm (AEST), 12noon in Queensland, 12.30pm in SA and 10am in Western Australia. Email HMAA to register for the webinar.


I recall crying while receiving a humanitarian award in 2011, as the horror of having lived through a natural disaster, the sort of thing that happens to other people on TV, hit me. Now we are rolling up our sleeves again and being told natural disasters will be a regular part of our new normal.

I am writing this while I wait for my garden to dry out enough to get into it. I’m in Yeronga, a Brisbane suburb, but this is a story that relates to millions of people along the East Coast from the Wide Bay to the Illawarra. So far.

I set up the Brisbane Garden Recovery Group in 2011 because while homes were the priority, the sooner the garden received some attention, the less of it was lost. The reason this resulted in a humanitarian award was that we found gardens contributed significantly to improving the mental health of ‘floodies’. When the world feels hopeless, some fresh green growth and a flower or two brings back hope. I found out firsthand the huge therapeutic value of gardens.

I was in a unique position at the time as I had been working in the water/wastewater and environmental engineering industry leading up to the flood and therefore I understood the issues around contamination and soil impacts.

Whether you are dealing with waterlogging from the extreme rain in your own garden, or trying to support flood affected communities, the right garden care can make the difference between a garden that survives and thrives or becomes a mess of dead plants.

10 steps to recovery

Here is a short summary of how to approach a waterlogged or flooded garden.

Keep off waterlogged grass as much as possible to reduce further compaction.
Keep off waterlogged grass as much as possible to reduce further compaction.

1.     Keep off

For the gardener who has suffered waterlogging, the very first thing to do is nothing at all. Do not rush in, as foot traffic on wet soil will add to the compaction of the soil. Unless you need to, try to keep off until it dries out enough to no longer give under foot.

 

 

 

 

Even after a series of heavy storms, the flood silt has not been washed off these leaves.
Even after a series of heavy storms, the flood silt has not been washed off these leaves.

2.     Pressure wash

If possible, while you are pressure washing your home, pressure wash your garden. This might seem like a very harsh way to treat your plants, but fine silt dries on and sticks to leaves like crazy. I had volunteers scrubbing leaves with scrubbing brushes and soapy water in 2011. Gardens that were pressure washed early had far less plant losses. Many plants that were not cleaned struggled to photosynthesise and died a slow death.

 

 

This murraya hedge is dropping leaves as a sign of stress. It was underwater for three days and is close to a petrol station so experienced additional contamination issues.
This murraya hedge is dropping leaves as a sign of stress. It was underwater for three days and is close to a petrol station so experienced additional contamination issues.

3.     Wilting and waterlogging

Waterlogging depletes the soil of oxygen, which kills off a lot of the soil life. The longer a garden is underwater, the more damage is done. This will apply to gardens that did not flood but have been badly impacted by the heavy rains. The garden can develop a sour smell as soil conditions become anaerobic. This now creates the perfect conditions for fungal pathogens, including phytophthora species (which cause die back and plant death).

Initial plant wilt or leaf drop will be a short-term reaction to the lack of oxygen. Many plant recover within a few days with sunshine and drying out. If the plants are still wilting after two or three days of sunshine, they could do with some help. Delicate feeder roots have begun dying off and the plant cannot take up water. Pruning can help reduce the plant’s need for water as it recovers. Overly heavy pruning will reduce the plant’s ability to photosynthesis and so slow down its recovery, so it is a balancing act.

4.     Soil microbes

What the garden really needs next is for soil microbes to be replenished. This should be the highest priority in any garden that has been waterlogged or flooded – if you do nothing else at all, add microbes! A healthy and diverse soil microbiota will provide bacteria that prey on fungal pathogens. The ongoing wet and hot weather, combined with the loss of natural predators and the increase in dead and rotting plant materials, creates the perfect conditions for fungal pathogens to thrive. These are going to be the biggest threat to the immediate recovery of most gardens.

There is a rich world of good bug versus bad bug happening on the microscopic level in our soils and by having a healthy soil micro-fauna we are more likely to have the good microbes that eat the bad disease-causing microbes. The risk of both plant and human diseases in the soil after flooding is greatly reduced by enriching the microbial life of your soil.

A note about commercial soil microbe products: There are many commercial products available that contain soil microbes including Seasol, Go Go Juice (Neutrog), Searles Compost, Eco-Seaweed from OCP and others. I personally prefer to use Garden Mate by Earthlife for a few reasons. It is easy to apply, and as it contains no nitrogen or anything in salt form, it is safe without any risk of overdosing. It contains a lot of microbes, and silica that helps to open the compaction. It also contains both selenium and calcium, which help to bind heavy metals.

5.     Fungicide

If the garden is showing signs of significant stress, apply a general organic fungicide before any other treatment. Adding any product containing live microbes should then happen a few days to a week later. The fungicide can harm some of the soil microbes that we want, so it is best done before and separately to adding microbes.

6.     Trees

While soft leaved plants show immediate signs of stress, most trees do not. By the time a tree starts showing signs of root damage due to flooding, it will be too late to save it. If you have trees, it is highly recommended that you treat them with fungicide now and then microbes in a week’s time. In 2011 we noticed large trees started dying about three months after the flood.

A number of our local fig trees have been dropping leaves. This is usually a drought strategy and even now it is a sign of lack of water. In this case the roots are damaged and cannot take up the water the tree needs. While some of these trees did not get affected by riverine flooding there has been so much overland flow from rain around them that the ground is completely waterlogged. These trees are now at risk of further damage from fungal pathogens.

Caption: Volunteers working in 2011, approximately six months after the flood, to clear the garden of weeds and dead plant matter for an elderly couple. By this time the silt had baked rock hard on top of ground compacted by the flood. Breaking the soil to plant new plants here was challenging!
Caption: Volunteers working in 2011, approximately six months after the flood, to clear the garden of weeds and dead plant matter for an elderly couple. By this time the silt had baked rock hard on top of ground compacted by the flood. Breaking the soil to plant new plants here was challenging!

7.     Compaction

Heavy rain causes some soil compaction, but if it is not compounded by foot traffic it might not need much remediation. The weight of floodwater causes more severe compaction. The deeper the water and the longer the soil is in under water, the more compaction will occur. Without help through the addition of microbes and organic matter (homemade compost plus mulch is ideal, if you can get enough of it), you might return to soil that feels more like a concrete slab than a patch of dirt.

8.     Silt

Silt is super fine soil particles. It is often quite nutrient rich and can be very good for building great soil. Some of the world’s richest soils are floodplains. It is however difficult to get off leaves, and if allowed to dry on the ground, it sets like concrete.

If we can get life back into the soil, the silt can be incorporated into the soil well. To do this, add microbes and then fresh mulch. Nature can then do her thing. In 2011 I was able to organise truckloads of woodchip mulch from council to be delivered to flood affected streets, and then spent backbreaking hours spreading mulch in garden after garden with a team of volunteers.

 

This beloved garden was completely lost in the flood. If you look at the lime tree in the back corner of the garden, you can see the flood level was at the top of the trellis. The roses in the front garden look similar but we found in 2011, roses bounced back incredibly well.
This beloved garden was completely lost in the flood. If you look at the lime tree in the back corner of the garden, you can see the flood level was at the top of the trellis. The roses in the front garden look similar but we found in 2011, roses bounced back incredibly well.

9.     Contamination

The most likely risk of contamination comes from sewerage. Faecal bacteria can only survive in soil for approximately 24 hours. They can survive in water indefinitely. Where you have waterlogging, you have free water in the soil for these faecal bacteria to live in. Once the soil dries out to a normal level (it does not have to been bone dry, just not waterlogged), the danger of illness from faecal coliforms passes. There may well be other nasties however in sewerage. It can contain heavy metals, although not usually in high enough amounts to be considered a risk, and it can have other bacteria and viruses. UV sterilisation from the sun is the best remedy for these, followed by soil microbes. Yes, the soil microbes are really that important!

If you are entering the garden for the first time after floodwaters recede, be aware of possible rubbish, including protruding nails and broken glass. Make sure you are dressed appropriately and have had your tetanus shot.

These agaves were under water for three days, but are also dealing with the suffocating effect of oil contamination.
These agaves were under water for three days, but are also dealing with the suffocating effect of oil contamination.

If you live near an industrial area, or petrol station, contamination risks are much higher. Heavy metals could be a problem. If you suspect heavy metal contamination, it is best to avoid growing any edibles until you can get your soil tested. All Australians can send up to five soil samples from their garden to be tested for heavy metals for free via the VegeSafe program at Macquarie University (see www.360dustanalysis.com).

If your garden has been contaminated with oil, you are in for a messy recovery. A gardener we helped in 2011 spent days scrubbing oil off his precious orchid collection. Unfortunately he did not get to them until a few days after the water had gone down and was only able to save around a third of them. In the case of oil, the sooner you get it off the better. It can burn leaves when the sun comes out. And yet the sun is also your friend with oil. UV light breaks down the complex hydrocarbons in the oil into smaller, less complex molecules that can be further broken down by soil microbes. The garden I mentioned was given an initial scrub and treatment of fungicide followed by microbes. This was by far the most polluted of gardens we deal with. It was then left to the sun for six weeks. Fungicide was reapplied, followed by more microbes and mulched heavily. Volunteers added donated plants to fill the now many bare spots. Within a year this garden was flourishing again, but it was not used for growing edibles.

10.  Fertilising

This is definitely NOT the time to fertilise. Stressed plants cannot take up fertiliser, so it is likely that any fertiliser you apply now will only wash away in the coming rain (sadly there is more forecast for us all), only to add to the pollutant load in our waterways. If you are applying inorganic fertilisers, the high salts can cause additional loss of soil life.

Heavy rain will leach soil fertility, which can be an issue if you are on a slope. For everyone with water sitting in gardens, it is sufficiently enriched with nitrogen from the air for the immediate term.  In a month or so an application of organic fertiliser can be beneficial, but as the weather will be cooling down then, an application of compost would be a better option.

 

Webinar details

Kate Wall will present a free webinar for HMAA members on ‘Recovering flood-affected gardens’. The Zoom webinar will run from 1pm (AEDT – NSW, Victoria, Tasmania), 12 noon (AEST – Queensland), 12.30pm (South Australia) and 10am (Western Australia).

To register for the webinar and receive a Zoom link, email Jennifer Stackhouse (jstackho@bigpond.net.au).

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