Geo fabrics on steep grades

Geo fabrics are excellent when used correctly. Jonathan Garner considers another practice which should be discussed before completing the planting stages of a landscape asset.

Planting to retain soil on steep slopes and riverbanks is sometimes tricky. It can be an economical solution to managing sensitive grades or possibly a costly experience if the soil erodes or, worse still, slips.

The sooner we get vegetation covering and root systems holding the soil together the better.

Safety first

Given we’re all experiencing significant rainfall events where we may receive over 30mm of rain within a few hours, or maybe a month’s worth of rain over a couple of days, relying on plants alone to retain a slope can be insufficient for a successful outcome. How the freshly planted or established bank copes with rain events generally depends on our initial preparation and the use of some variety of either biodegradable or permanent geo textile material.

Initial success will rely on how water running onto the slope is managed, soil preparation, and how the fabric is installed. Long-term success will rely on minimal erosion and how well the plants have established.

Remember steep slopes can be dangerous. It may require additional measures beyond geo textiles. Always assess the project thoroughly, and if there’s any doubt about stability and safety, involve a suitable specialist, as this could become a very expensive exercise.

Plant choice

You might be working anywhere from the tropics in the north to the cool temperatures of the south, so I’ll leave appropriate plant selections to your own research. But regardless of the location, the quest is to have the somewhat unattractive fabric and hillside hidden with healthy plants as soon as possible. Keep in mind the steep bank you’re facing might have limited or very little soil. It might be an expanse of exposed shales or compacted clay. You may need to create retained planting structures for soil to grow woody groundcovers that can grow far wider to cover more ground.

Again, I’ll leave you to determine exactly what plants to use, but consider varieties that have dense root systems, are low growing, fast to establish and are drought tolerant. Dense root systems will hold the soil better. Low growers are less likely to blow over or fall apart in strong winds, and the qualities of rapid establishment with drought tolerance are both self-explanatory.

Take control

Now that we’ve been awarded the project, time is of the essence to have it established. Analyse the soil and make plans to have the machines, the labour and the materials ready to go so there are no delays or intervals during the project. The longer the bank is left exposed, the greater chances of trouble occurring. If you’re busy on other jobs and the bank is already cleared or thinly covered with soft wooded weeds then cover it with a temporary fabric or the product you’re planning to use – even though reusing the fabric will be a real pain and installing a temporary fabric is an added cost to the environment and the project.

The costs and hassles of remediating a water-scoured bank are significant. If the bank is more or less covered with woody weeds such as Lantana, or worse still, Blackberry, then leave the bank until you’re ready to tackle it without pause or breaks in the program. Regardless of whether the bank is vegetated or not, create a temporary system at the top of the bank that can either divert or slow down the movement of water running onto the works area. Systems such as straw bales, silt fencing, coir logs, or dish drains with sediment traps will all help to temporarily reduce challenges associated with water running onto the bank, and with the same water running off with your precious topsoil.

If faced with Lantana it may be better to physically remove the vegetation and crowns by hand, or engage a forest mower to slash the area down to a smooth finish while leaving most of the root systems in the ground where they’ll help retain the soil. Image: logoboom/


Approaching the works with a machine or by hand is more or less the same.

One of the most important things to do is to keep your soil disturbance consistent. In certain situations it may be better to leave large stumps or boulders in the ground rather than tearing them out. Creating soft depressions or backfilling after removing large obstacles creates areas that will be prone to erosion or slumping after heavy rains. Naturally, retaining or removing these kinds of obstacles will be specific to the site. This is where advice from specialists will be of great help.

If I’m faced with a steep bank of a woody weed such as Lantana or worse still, Blackberry, I’ll be inclined to physically remove the vegetation and crowns by hand, or engage a forest mower to slash the area down to a smooth finish while leaving most of the root systems in the ground where they’ll help retain the soil. It’s easier to manage regrowth chemically than it is to keep a hillside intact.

When I’m planting pots of up to 200mm and I have the luxury of an excavator to prepare the soil, I’ll set out the planting scheme according to the plan by locating each planting spot with a 50mm thick dressing of compost for the excavator to tickle in. The exposed roots can be removed with loppers or an axe, and the soil raked to a smooth finish.

Without a machine I’ll have to rely on muscle.

Aiming to level these areas will assist with planting and further identifying the spots a little later. If compost isn’t required, then stakes or irrigation flags work just fine. Naturally, the extent of soil preparation will depend on the ground conditions and the pot sizes of the plants going in. I’ll leave the extent to your judgement.

As mentioned earlier, 140mm-200mm plants are my preference. They’re young, vigorous and much cheaper to replace than 300mm models. Given the adverse ecology we are working with, plant losses are possible. Once the planting spots are suitably prepared and before the fabric is installed, ensure surface roots are removed and the surrounding firmer areas are raked to consistent grades. This is vital to ensure minimal voids between the fabric and soil. Voids flap in the wind and allow the soil beneath to migrate.

Approaching the works with a machine or by hand is more or less the same. Either way it’s important to keep the soil disturbance consistent. Image: ThamKC/

Keeping it pinned

The next step is to dig a 200mm deep trench along the top of the bank to rest the fabric in place by burying.

Most fabrics are available in two-metre widths, which makes the installation manageable for two or three people. Although the temptation to let the geo-tex roll down the hill on its own will be high, doing it in a controlled manner will pay dividends. By gradually rolling the fabric down the hill you’ll be able to accurately mark the planting spots while pinning the material into solid ground so the fabric hugs the contours of the soil beneath. Depending on planting densities, budgeting for five pins or staples per metre with a lump hammer on hand to knock them in will see you sorted. Providing a 200mm overlap between neighbouring panels of fabric is also a must. You’ll soon discover that both the specified 100mm overlap and the three pins per square metre are insufficient. Mild steel pins are cheaper and work better than galvanised. The rust developing on the pins increases the grip they have on the soil.

Plant and irrigate

Once the steep bank is wrapped up nice and snug, it’s time to get the green things in.

Laying extension ladders on the bank is a useful and safe way to work without disrupting the fabric, and having the planting spots marked already will also enable you to walk on the firmer areas while carefully cutting the fabric open to plant the treasures in their previously prepared locations.

Naturally, you’ll be using the excess soil from planting to create the soil berm to retain water around the root ball.

If you plan to irrigate, this is the time to install the system, be it a spray or a more efficient drip system. Pinning the drip lines above the fabric will also help to keep the material hugging the ground. Locating the system uphill and pinning the emitter directly to the soil will ensure capillary action is at play and the precious water isn’t running along the hose and away from the targeted rootzone. Inline vacuum breaking valves will prevent emitters from blocking with soil particles.

If you’re using a coir or jute-based fabric, consider broadcasting pre-emergent herbicide granules before mulching with a thicker graded mulch such as forest blend. The inconsistent particle sizes help to keep it intact. Consistent grades such as pine barks tend to wash downhill.

If you’re using a non-woven synthetic fabric aim for black and apply the pre-emergents around the plants. Black fabric blocks the necessary light to germinate weed seed and the regrowth from existing roots. White fabric will give you sunburn when installing and also stands out if your mulch shifts. Over time layers of organic debris gradually bury the fabric, but it’s there for good and will keep the hill intact.

If you need to replant, simply cut it open to access the soil.

Avoid weed mat. Mulch won’t stick to the slippery polypropylene and gaseous exchange through the soil is greatly reduced.

Although there’s loads of information about the varieties of materials online. There’s not so much advice on how to use it. I hope this advice helps.

Avoid weed mat. Mulch won’t stick to the slippery polypropylene and gaseous exchange through the soil is greatly reduced. Image: Tomasz Zajda/
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