Listen To Bindii…And Say Farewell To Khaki Weed

Whether you call it bindii, bindi weed or bindi-eye, this prickly-seeded little weed is currently causing big problems in Pine River Shire, Brisbane.

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People are keen to control it, so they can sit or walk barefoot on their lawns. Fair enough, but people seem less willing to hear what bindii means in terms of good gardening.

Bindii, Soliva sessilis, is a common lawn weed. It’s an annual flat weed, that is, a low-growing, ground hugging plant. They germinate as seed during the start of the cool seasons, flower in late winter to spring, and set lots of small, burred seed. Most people are picking them out of their feet when they realise it’s been growing in their garden for ages…

Bindii says “Thanks!”
If bindii is in your lawn, it’s telling you that:

  • Your soil is compacted, and needs aerating;
  • Your soil is acidic, and needs sweetening;
  • Your turf is starved, and needs feeding;
  • You’re mowing your lawn too low;
  • Get your lawn into shape

Space out these tasks – never, ever feed and weed turf at the same time – it’s counterproductive, no matter how alluring chemical company advertising is.

When is the ideal time to start controlling bindii? In Queensland you can start between May and August; in NSW start in early September.

Weekend 1
Raise the height of cut. Most mowers can be adjusted to cut grass on at least a high or low setting. Shaved turf develops bald patches, inviting weed invasion. Turf cut high naturally smothers many lawn weeds.

Mix 2 tablespoonfuls of iron sulphate, (commonly sold at hardware stores & garden centres) into 4.5 litres of water. Use a watering can and rose to sprinkle the solution over the bindii, ensuring it’s foliage is wetted. Promptly wash any splashes off paths, paving or plants with clean water.

Iron sulphate burns foliage and exhausts plants. It is most effective on broad-leaved annual weeds (aka flatweeds). Since they do not have large energy store (like a taproot or bulb) they can be killed in one or two applications.

My grandfather used lawn sand, a different home made herbicide, to control perennial flat-leaved weeds like dandelion and cat’s ear. That is discussed in a different blog.

After the iron sulphate solution has dried, it’s time to aerate your lawn. Use a garden fork, or hire an aerator. Aeration helps rain and air to get into the soil, helping your turf to develop a stronger, healthier root system.

Weekend 2
Sprinkle one handful of dolomite per square metre. This sweetens acidic soil and replenishes its calcium and magnesium, invigorating turf and helping it to smother weeds. Both minerals are water soluble and they are leached from soils by rainfall and irrigation. Calcium generally strengthens plant tissue and enhances metabolic processes, both of which counter disease. Magnesium is used to manufacture chlorophyll which helps to convert sunlight into energy.

Weekend 3
Sprinkle one handful of certified organic poultry manure or, better still, blended manure such as Rooster Booster, per square metre. This feeds soil micro-organisms, and helps replenish nutrient deficiencies whilst stimulating robust turf growth.

Mark W. of Gympie read this advice and replied:

Khaki weed, Alternanthera pungens. Image by Mark Wilson

Khaki weed, Alternanthera pungens. Image by Mark Wilson

“Thanks for the article Jerry, I have attached a picture of some prickles we get at Gympie (Qld) and I was wondering if they are in the same class of weeds. They come up every year and I spend hours digging them out but feel I am fighting a losing battle”.

Answer: Khaki weed (Alternanthera pungens) is a perennial weed from Central and South America. Actively growing in warm, moist conditions, this weed is spreading across northern Australia and southwards along the east coast.

It is adapted to rapidly colonise disturbed ground and prefers a sunny habitat, places like footpaths, roadsides, pasture and lawns (technically this is a ‘ruderal’ plant). In drought, a juicy tap root sustains the plant while the shoots die back, and they resprout from this root after rain. Seeds are prickly and are spread by animals, on shoes and by clothing.

I wouldn’t use iron sulphate. This compound works best on broad-leaved annuals (flatweeds), not perennials. Such annuals lack a good store of energy, so killing their foliage can have a big impact. Khaki weed leaves are broad, but the root stores so much energy they can recover and continue growing. You’d need to repeatedly apply iron sulphate to exhaust it, and that would acidify soil, which would then favour most other lawn weeds over turf, and the cost spent treating weeds rises.

Instead, I would focus on cultural controls. They take longer, but you engineer things in favour of weed-free turf. Khaki weed prefers good sunshine, so the higher you cut your lawn, the more it gets shaded out.

Adjust the soil pH and feed your turf and you’ll find the vigorous growth will suppress weeds. Clover, another perennial weed, is weakened by nitrogen. In my experience at the Royal Botanic Gardens, Sydney, by improving our horticultural care of turf we eliminated many pernicious weeds without using even organic herbicides. Turf also became more resistant to wear and tear and this significantly reduced the cost of returfing.

Cultural controls lead to success
Long term control of bindii means consistently improving your lawn care. So repeat this three week programme in autumn and spring.

Most people just want to know about the quick fix bindii control. My grandad’s iron-based remedy, over a hundred years old, kills bindii in 24 hours, and most people leave it at that. But bindii seed remains viable in the soil for many years to germinate in bare patches. Repeated applications of iron sulphate in itself isn’t the answer, because it acidifies the soil, and a low soil pH favours bindii.

The worst mistake is to buy the popular ‘weed & feed’ chemicals. The herbicides they use are dangerous and they’re unpredictable. Developed for use in a cool temperate climate, these highly volatile poisons rapidly evaporate when temperatures exceed 21˚C. Poisonous vapour is easily wafted by breezes to damage plants – often in your neighbour’s garden. And residues continue evaporating from sprayed areas, poisoning for longer than you’d expect – lawns become toxic hazards for kids, pets and wildlife.

Iron sulphate solution is not volatile. As it breaks down it releases iron, a tonic for plants. It is not as cheap as it was one hundred years ago, so don’t use this remedy for a park or a paddock: this remedy is best for a domestic lawn.

If you have a park, use the other cultural controls: feed the soil, raise the height of cut and manage soil pH. That’s what the turfculture team at the Royal Botanic Gardens, Sydney did when I was managing the gardens. It significantly cleaned turf of weeds, renewed the wear and tear of the turf, and avoided spraying persistent pesticides.

I expect that gardeners and councils all around SE Qld are spraying their abused turfed areas with pesticides right now.

What are your weeds telling you?

Jerry Coleby-Williams
June 2012