Sowthistles (Sonchus L)
Sowthistles are annual, biennial or perennial herbs, with or without rhizomes. Thet are in the genus Sonchus and also known as Milkthistles.
The distribution map of Sowthistles in Australia is courtesy of The Atlas of Living Australia.
There are three Sowthistles you often find in lawns and turf areas in Australia. These are the Annual or Common Sowthistle (Sonchus oleraceus L), the Perennial Sowthistle (Sonchus arvensis) and the Spiny Sowthistle (Sonchus asper L.).
After you finish reading this, you will be able to:
- Identify Sowthistles, or Milkthistles.
- Know the habitat of Sowthistles or Milkthistles.
- Know the best cultural and chemical options to control Sowthistle or Milkthistle.
How to Identify Sowthistle.
All Sowthistles have soft, irregularly lobed leaves that clasp the stem and, form a basal rosette.They are all upright, and hairless, and have hollow stems that occasionally branch towards the top. Their leaves are alternate, and smooth.
The easiest way to avoid confusion with Thistles is to cut the stem. Sowthistles exude a sap when cut whereas thistles do not.
Annual Milkweed and Spiny Sowthistle can be mistaken for Prickly Lettuce, due these having milky sap and similar foliage and flowers. However, Prickly lettuce leaves have raised spines along the midrib, which are absent along the midribs of Spiny and Annual Sowthistle leaves.
Category: Dicot
Photosynthetic Pathway: C3 Weed.
Flower: The flower heads of Milkweeds or Sowthistles are yellow, range in size from 12 – 25mm in diameter; and have a ray type floret with no eye to the flower.
Height: Mature Sowthistle is 30 cm to 2 m tall.
Leaf length: Leaves can be between 6 – 35 cm long.
Leaf width:
Reproduction:
Sowthistles grow quickly in a wide range of conditions and their wind-borne seeds allow them to spread rapidly. Germination is favored by light, and seedling emergence is limited to the top 1 cm of soil. The seed of Common Sowthistle is viable for up to 8 years.
Germination is good at alternating day/night temperatures of 25/10 °C, and Annual Sowthistle requires high soil moisture levels for optimum germination.
- If you cut the Annual Sowthistle when it is flowering it is still able to produce viable seeds.
- Once the perennial sowthistle establishes, it is difficult to control because its perennial root system has the ability to spread vegetatively via small root fragments, and it is also reproduces via seed.
- One Common Sowthistle plant produces between 4,700 and 8,000 seeds.
Comments: Sow thistles are common hosts for aphids. Young plants are highly frost tolerant. Plants that emerge in the Autumn can overwinter as a rosette of leaves close to the ground, and withstand temperatures down to -18 °C.
Habitat: A common weed of disturbed areas, gardens and waste places
For more information on weeds check out our weed ID Chart.
How to control Sowthistle.
You can control Sowthistle or Milkweed by cultural and chemical means, but successful management of this weed is best if you adopt an integrated approach.
Cultural control of Sowthistle:
Seed germination is favoured by light, and seedling emergence is limited to the top 1cm of soil.
As they are in the rosette stage for quite some time, Sowthistles are susceptible to shade. So grow a thick and dense turf cover and you are half way there to managing this weed!
Chemical control of Sowthistle.
Since the seed bank is relatively short-lived pre emergents are a good option. Also post emergent herbicides work best during the weed seedling stage.
Pre-emergent herbicides have been successful in the control of Milkthistle and the fact that this is a shallow germinating weed is highly likely to be the reason for this.
Currently only BASF Freehand is registered in turf grass as a pre-emergent for this weed. However, in agriculture prodiamine (Onset 10GR) , S-Metalochlor and Pendimethalin (Battalia 440) are labelled for Sowthistle control.
If Milkthistle does break through your pre-emergent barrier, the next option is to use a selective post-emergent herbicide. Luckily there are a wide range of post emergent options that you can use.
Post-emergent Selective herbicide options include: 3-D herbicide, Dicamba 500, Qualipro Crest, Pylex, 2,4-D, and MCPA
You can use Glyphosate , Numchuk Quad or Cortex Duo as a non selective options to control this weed. If you use Glyphosate and water quality is an issue then we recommend the use of ProForce Manta Ray.
