Winning the War Against Sour Sobs in South Australia

Learn how to identify soursobs, why they keep returning and which control methods can reduce them in South Australian lawns and gardens.

Summary:

South Australian soursobs are persistent winter weeds that can be difficult to eliminate because they reproduce through underground bulbs and bulbils. This guide explains how to identify soursobs, distinguish them from similar-looking weeds, understand why they keep returning and choose between hand removal, mowing and herbicide treatments. It also outlines when to begin treatment, what results to expect and why consistent control over two or three winter seasons is often needed.

What are Soursobs?

Soursobs are one of South Australia’s most recognisable—and frustrating—winter weeds.

According to Herbguide, more than 1.25 million hectares of land in South Australia are affected by soursobs. Most experts agree they were introduced to Australia from South Africa in the 1840's and had become a serious agricultural pest by 1880's.

Geoff Manning, in Insight into South Australian History, humorously notes that they were “deliberately imported from Tasmania, whither the beastly things had been introduced!”

Today, soursobs are commonly found in lawns, garden beds, roadsides, paddocks and other disturbed areas throughout South Australia.

How to Identify Sour Sobs

When soursobs first emerge, they can look surprisingly similar to clover and several other common lawn weeds. This can make them harder to identify before they flower.

Soursobs have three heart-shaped leaflets arranged in a clover-like shape. Their leaves are generally bright green, featuring black spots, and are carried on long, slender stems.

Carefully pulling a plant can also help with identification. Soursobs usually have a pale, white underground taproot connected to a bulb at the end. However, the bulb may remain in the soil when the plant is pulled.

Once the plant matures, identification becomes much easier. Soursobs produce their familiar bright yellow flowers, usually held above the leaves on long stems.

Correct identification is important because weeds that resemble soursobs may respond differently to mowing, hand removal or herbicide treatment.

Why Soursobs are so hard to kill

The resilience of soursobs comes from their underground survival structures: bulbs and bulbils.

The main bulb stores energy that allows the plant to return each winter. Smaller secondary bulbs, known as bulbils, can form around the underground parts of the plant. These bulbils can detach, remain dormant in the soil and eventually grow into new plants. A single mature soursob may produce up to 20 bulbils. This means that removing or poisoning the visible plant does not necessarily remove all the underground material.

However, there is some good news.

Each bulb or bulbil can only produce one weed. Once that weed has used its stored energy and is repeatedly removed or weakened before it can replace that energy or produce more bulbils, the underground source can eventually become exhausted. This is why persistent treatment works, even though the results may not be immediate.

When Should You Treat Soursobs?

Begin controlling soursobs as soon as they emerge, which is generally during autumn or early winter in South Australia. Early treatment gives you the best opportunity to weaken or remove the plants before they mature and produce new bulbils. Do not wait for the yellow flowers to appear before taking action. Flowering indicates that the plant is becoming established and moving further through its seasonal growth cycle.

Soursobs will not always emerge at the same time. New plants can continue appearing throughout the cooler months, so regular inspection and follow-up treatment are essential.

How to Control Soursobs

There is no single treatment that will completely remove a well-established soursob infestation. The most successful approach usually combines several control methods over more than one winter.

Hand-Pulling Soursobs

It might sound crazy, but hand-pulling can be effective, particularly in garden beds, small lawn areas or where you would prefer not to use herbicides. Pull or carefully dig out the plants as soon as they appear. It's important to remove the plants before they mature and produce another generation of bulbils.

Hand-pulling may sound overwhelming if you have a large infestation, but consistent removal can significantly reduce soursob numbers over several seasons.

Spot-Spraying with a Non-Selective Herbicide

A non-selective herbicide containing glyphosate, such as some Roundup or Zero products, may be used to spot-treat isolated soursobs. These products must be applied very carefully because they can damage or kill any green plant they contact, including your lawn. Spot-spraying is generally more suitable for soursobs growing in garden beds, along fence lines, between pavers or in areas where damage to nearby grass can be avoided.

Always follow the product label and protect surrounding plants from spray drift.

Using Selective Herbicides in Lawns

Selective lawn herbicides are designed to target certain broadleaf weeds while being safer for suitable lawn varieties when used according to the label. Products such as Bow & Arrow may help suppress and weaken soursobs without killing the surrounding lawn. However, selective herbicides are not usually an instant or direct cure for an established soursob problem.

Soursobs can be painfully slow to react to herbicide treatments. The visible leaves may take time to curl, yellow or die back, and new plants may continue emerging from bulbs and bulbils already present in the soil. Repeated applications may be needed. Allow the correct interval between treatments and follow all label directions relating to application rates, lawn varieties and weather conditions. A suitable wetting agent may also improve coverage and absorption where permitted by the product label.

Before applying any herbicide, confirm that it is suitable for your lawn variety and the area being treated.

Regular Mowing

Regular mowing will not kill the underground bulbs or the soursobs themselves, but it can play an important role in controlling them. Mowing helps remove flowers and reduces the plant’s opportunity to mature and produce new bulbils. It also repeatedly removes leaf growth that the plant needs to rebuild its energy reserves.

Try to mow before the flowers become well established. Avoid mowing too low, as scalping your lawn during Winter will discolour it and it won't recover its colour till Spring. 

Patience and Persistence Pay Off

Soursobs are unlikely to disappear after one spray or one round of hand-pulling. For an established infestation, expect to continue treatment over two or three winter seasons. New plants may appear throughout the cooler months as bulbs and bulbils already in the soil begin to grow.

This does not necessarily mean your treatment has failed. It may simply be another underground bulbil producing its plant. The aim is to remove or weaken each new plant before it can mature, restore its underground energy reserves and produce more bulbils.

With regular treatment, the number of soursobs should gradually decline from one season to the next.

Need Help Controlling Soursobs?

At Paul Munns Instant Lawn, we understand how frustrating soursobs can be—especially when they seem to return after you thought they were gone.

Correct identification and choosing a treatment that is suitable for your lawn variety are important. Our team can help you identify the weed, understand your treatment options and select an appropriate lawn-care product.

Call us on (08) 8298 0555, email info@paulmunnsinstantlawn.com.au, or visit us in-store for practical advice.