Californian thistle, Canada thistle
Cirsium arvense (L.) Scop. (Carduus arvensis, Serratula arvensis, Cnicus arvensis)
Creeping thistle, is the most important perennial thistle. It is native in cultivated fields, waste places, hedgerows and grassland throughout the UK and is recorded up to 2,300 ft. It is an aggressive weed that occurs on most soils but it grows more extensively on deep, well-aerated soils. Creeping thistle is relatively indifferent to soil fertility but does grow better in richer soils. It can tolerate very low temperatures but is less successful in hot dry conditions.
It is the commonest perennial weed of grassland on beef and sheep farms especially on older sward and in soils with low phosphate or high potassium levels. It is also able to survive in all but the most intensively managed arable fields. Creeping thistle is a problem weed in gardens too.
Creeping thistle colonies can expand radially by 6 to 12 m per year and will dominate large areas of vegetation if left unchecked. Patches of creeping thistle may be formed from a single clone but often contain the shoots of several individual genotypes. Leaf shape is very variable between clones. Ecotypes have also been shown to vary in spininess, flower colour, seed size and time of emergence. Populations have developed with resistance to hormone herbicides. Ecotypes may differ in the composition of epicuticular waxes that affect the rate of foliar herbicide uptake.
The seeds are an important constituent in the diet of many farmland birds. However, creeping thistle is also a food plant for several important insect pests.
Creeping thistle flowers from July to September, sometimes into October. Normally each plant bears only functionally male or female flowers. The male flowers produce abundant pollen. The fragrant female flowers are insect pollinated but the pollinators may only visit one type of flower. Plants within a patch formed from a single clone cannot self fertilise and little seed may be set in some instances. The maximum distance between male and female clones that still ensures seed set has been estimated at 50 to 390 m. Occasional plants may be found with hermaphrodite flowers and these can set seed freely.
The time from flowering to seeds becoming viable is around 8-10 days. There is some variation because seed in the outer florets matures earlier. At 8 days, around 13% of seeds are viable, at 9 days 80% and at 16 days 90%. Plants cut down in flower produce very few seeds and none are viable although they appear normal. Seeds ripen from June to September and are shed from August onwards. There may be 20 to 200 seeds in each flower head and an average of 680 seeds per stem. The seed number per plant ranges from 1,600 to 50,000.
Some seeds can germinate on dispersal other are dormant but this may vary with ecotype. Seed is shed largely in autumn and chilling over the winter leads to germination in spring but seedling emergence at other times is not precluded. Seed shed in July-August may germinate the same autumn. Seed has been found to germinate on the soil surface but the optimum depth is 5 to 15 mm, although, emergence has been reported from up to 60 mm deep. Light, nitrate and alternating temperatures can interact to promote germination. Seeds germinate best at relatively high temperatures.
Thistle seedlings are sensitive to drought and early competition for light. They are unlikely to survive in dense stands of other plants. Seedlings require soil disturbance to become established in standing vegetation. Autumn germinated seedlings may not survive if they have made insufficient root growth before the foliage is killed by frost. Spring emerging seedlings, 19 days old and with 2 true leaves, were able to regrow after removal of top-growth. At this stage the branched root system is up to 15 cm long. Seedlings develop a taproot with spreading laterals within 8-10 weeks of emergence. Adventitious buds develop at the base of these side roots. Once these are formed a seedling is able to regenerate readily if hoed off. Some of the buds grow upwards to form leafy shoots others develop as rhizomes. Seedlings undergo a juvenile vegetative phase but can emerge and flower within a year.
The deep-seated creeping root system is very brittle and easily breaks into pieces. Although the roots may penetrate several metres down, most regeneration is from roots within or just below the plough layer. It is only the thickened areas of root and the underground stems that are able to regenerate and form new plants. Regions of the lateral roots thicken due to the development of storage tissue and it is here that new shoots are initiated. Fragments shorter than 2.5 cm do not always regrow but segments 5-6 cm in length or longer regenerate readily. Regeneration is less successful in November.
Creeping thistle food reserves are minimal between May and July. After flowering, assimilates pass down into the underground organs from July to October to build up reserves for the following year. The plant dies down to just below soil level in the late autumn. Some of the underground organs may also rot away leading to fragmentation of the parent colony into separate units of swollen roots. In spring, shoots that developed on the storage roots the previous autumn grow to the surface and develop into the new aerial shoots. Adventitious roots develop on the shoots and some swell to form the perennating organs for the following year.
Field and glasshouse studies suggest that residues of creeping thistle are phytotoxic to the growth of other plants including some crops. Water extracts from the roots and foliage have been shown to inhibit plant growth.
Creeping thistle persists and spreads chiefly by means of the horizontal underground creeping roots that can exceed 5 m long. Fragments of root from plants in the field margins can be carried into the field and spread during cultivation. The collection and burial of root pieces by rodents has resulted in the unexpected appearance of the weed in previously clean areas. There is also evidence that undisturbed pieces of swollen root can remain dormant in the soil for several years until disturbed by cultivation.
Creeping thistle seed can remain viable buried in undisturbed soil for over 20 years. However, seed may persist for a much shorter period in cultivated soil or near the soil surface. In disturbed soil, most seeds germinate in the first year after shedding but odd seedlings will continue to emerge over the next 4 or 5 years. Seed may be poorly represented in the soil seedbank even though the plant is common in the vegetation cover.
If seeding is not prevented, seed from thistles in the headland can help to maintain an existing field population. Potentially, the seeds could be transported a considerable distance by a strong breeze but in general the seed is firmly held in the seed head while the parachute is readily detached, and most seeds land within a short distance of the parent. Thistle-down is usually devoid of seed or has an undeveloped seed attached.
The combine harvesting of cereals leaves more thistle seeds on the soil surface than earlier systems where the crop and weeds were cut with a binder, stacked to dry and then threshed elsewhere. In the past too, creeping thistle seeds were dispersed as contaminants in various crop seeds. A study in the USA found seeds of creeping thistle in manure samples from both heifer and dairy herds. Some seeds of creeping thistle fed to chaffinches have germinated when recovered from the droppings but these represented only a small proportion of the seeds consumed. Seeds can withstand submergence in water for up to 2 years and viable seeds have been recovered from irrigation water.
Methods of cultural control include the use of clean crop seeds, ploughing, harrowing, mowing, grazing, mulching, the use of competitive crops and the cleaning of farm equipment. These approaches applied regularly, weaken the plant and its capacity to produce new shoots, they prevent seed production and reduce further spread. However, with the exception of biological control, many of the methods currently available are little different from those employed over 150 years ago.
In grassland, infestations of creeping thistle may be the result of poor management. Under utilization when thistles actively growing combined with overgrazing in winter and spring produces an open sward that offers little competition to emerging thistle shoots. Close stocking or cutting the thistles at a young stage should reduce an infestation but can damage the sward allowing the weed to establish further. Sheep and ponies will eat young thistle shoots readily but mature stems are not palatable to most stock. Goats, donkeys and llamas are said to eat creeping thistle even at the flowering stage. Cattle grazing reduces thistle numbers more than sheep grazing. Sheep and rabbits keep the vegetation short apart from the thistle shoots that takes advantage of the reduced competition.
Where topping is carried out for thistle control the cutters need to be set low enough to remove all of the thistle leaves. Topping must be repeated at least twice during the growing season over several years to have a permanent effect. Thistles are thought to have the lowest food reserves just before the flower buds open. Thistle shoots should pulled when at the flower bud stage. Pulling is more effective than cutting and avoids new shoots simply developing from buds at the base of the cut stem. When the shoots are pulled new shoots have to develop from the underground roots.
Updated: November 2007
Leaflet available below: Creeping thistle management strategies in organic systems