The wood sorrel, which belongs to the botanical genus Oxalis, is a real nuisance in the garden, because the species, which is as robust as it is prolific, likes to colonize garden beds, lawns and pavement joints. Once established, you won’t get rid of Oxalis corniculata anytime soon. The weed thrives almost everywhere and spreads again and again through its numerous seeds as well as its deep roots. With these means you finally fight wood sorrel for good.
You can easily recognize the horned wood sorrel by its dark green to reddish brown leaves, which are perfectly adapted to strong sunlight due to their dark coloring. Between June and October, the bright yellow flowers appear incessantly, which soon turn into capsule fruits up to two centimeters long. These, in turn, contain numerous small, brown seeds.
In principle, wood sorrel grows on almost any substrate, so it is quite undemanding in terms of soil composition and location. However, the weed feels most comfortable on soils with these characteristics:
- slightly acidic to neutral pH
- little moist to dry
In the garden, the plant is mainly found in lawns, where it threatens to quickly crowd out the less robust grasses. But perennial beds, plant pots and stone joints are not immune to colonization either.
Note: Many weeds in the garden can be used in the kitchen and practically “eaten away”. Not so the horned sorrel: This contains high levels of toxic oxalic acid and is therefore not suitable for consumption.
The weed is extremely persistent and difficult to eradicate, especially since the species reproduces extensively by root runners as well as seeds. The narrow capsule fruits burst open at the slightest touch, thus hurling the seeds inside several meters away – for this reason the species is also called “spring clover”. Small animals such as ants do the rest and distribute the seeds in the wider area. More problematic than the seeds, however, are the runners that sprout from the roots anchored deep in the soil. Since these are almost impossible to remove, especially in loamy soils, the herb will always come back if treated only superficially.
Fighting wood sorrel
If you want to successfully control and finally destroy horned sorrel, you need to start as early as possible: The longer you let the weed run rampant in your garden, the harder it will be to get rid of it. Moreover, without physical effort you have lost right away, because only through regular and deep weeding you will prevail over time.
Since Oxalis corniculata sprouts again and again from even the smallest root fragments, the frequently practiced flaming of overgrown stone joints is only a short-term solution. It is better to use a joint scraper to deeply remove the weeds, including as many root fragments as possible, from the joints and then seal them. Seal the gaps between slabs and stones as follows, then you will have no more trouble with stubborn weeds so soon:
- Scrape the joints thoroughly empty
- Fill in new, clean sand
- sweep the area and distribute sand evenly in open spaces
- cover stones if necessary
- Spray on joint sealer
- optionally use liquid resin
On the other hand, it is easier to prevent the weeds from growing in the first place. For this purpose, you should already position a weed fleece under the stones when laying the terrace or paved path. At least the roots of the horned sorrel will not be able to penetrate through it.
In the lawn
It is much more complicated to control wood sorrel in the lawn. The use of herbicides is not recommended here, as the agents do not differentiate between the desired grass and the weed being controlled. Instead, switch to other methods that are essentially designed to make life difficult for wood sorrel. Be patient, because the plants cannot be removed completely by weeding once. Therefore, start as early as possible in the year – ideally as early as March or April – to pull out the clover and check the lawn at short, regular intervals for new shoots, which you also remove immediately. It is important not to allow the plants to flower in the first place.
After all, what does not bloom, can not form seeds. For this reason, mow the lawn regularly to keep the horned sorrel from growing. However, since the species grows very shallowly on the ground, set the lawn mower as low as possible. Go over the entire lawn about twice a week between April and October, continuously keeping it as short as possible. Frequently cutting off the above-ground parts of the plant will weaken the weeds in the long run, causing them to gradually decline.
Liming the lawn regularly can also help against horny sorrel – but it doesn’t have to. After all, the weed still thrives on slightly calcareous substrates and cannot be driven out with mere liming. Nevertheless, this measure is positive because it strengthens the growth and defenses of the grasses – and Oxalis finds fewer opportunities to spread unhindered.
Tip: However, do not simply start liming, but first check by means of a soil analysis whether there is a corresponding need at all. Contrary to some claims, horned sorrel is by no means an indicator plant for too acidic soil.
The right fertilization often works better against wood sorrel than liming the lawn: Since the weed prefers to grow on nutrient-poor subsoils and has only a low nitrogen requirement, you can prevent it from spreading or even destroy it with regular lawn fertilization. Perfect for this purpose is horn meal, which is a purely organic fertilizer. Horn meal only adds nitrogen to the soil, but decomposes very slowly and thus counteracts overfertilization.
Tip: Be careful, however, with fertilizers that contain high levels of phosphate. The horned sorrel needs phosphorus above all for its growth and reacts accordingly joyfully to an appropriate supply.
A mass spread of wood sorrel on the lawn can only be effectively combated by one measure. Destroy the weed by a deep renewal of the entire lawn. The best way to do this is as follows:
- Fertilize the lawn vigorously
- Let it grow for 14 days, do not mow it
- then cut off as deep as possible
- scarify the area
- remove areas densely overgrown with horned clover
- apply fresh topsoil
- reseed the lawn
Water the area thoroughly
Only partial heavy growth of the weeds can be removed in the same way, except that in this case you do not need to treat the entire lawn. Remove the wood sorrel with the help of a digging fork. Remove the top layers of soil (and thus remove any root residues) and sow new turf or cover the bare spots with sod.
Vinegar and pelargonic acid
Instead of toxic herbicides, you can also combat a growth of wood sorrel by spraying undiluted vinegar on leaves appearing between stone joints. However, the treatment has only a superficial effect and the plants will always sprout from the roots. On the other hand, vinegar is not suitable for lawns, as the agent also attacks the grasses. The natural fatty acid pelargonic acid, on the other hand, is completely unsuitable for weed control because it breaks down very slowly and also easily enters waterways.
Prevent a horned clover invasion with both careful and regular lawn care: Mowing, dethatching, liming, fertilizing and watering during the summer months will help ensure that the green area remains healthy and that only the desired grasses actually grow in it. Also, before any planting within the lawn, thoroughly inspect newly purchased container plants for any oxalis and other weeds that may be present and remove them if necessary. Weeds are often introduced from outside, but you can avoid this with proper control.