Cleavers

Other names: 

beggar lice, clithe, cliver, cliders, goose-grass, goosebill, hariff, gripgrass, catchweed

Latin names: 

Galium aparine L.

Occurrence: 

Cleavers is a native annual found in fields and hedgerows. It occurs on most soils throughout the UK but prefers nutrient-rich sites. It is considered an indicator of loam. Cleavers is most frequent on soils with a pH of 5.5 to 8.0. It has been recorded up to 1,500 ft.

Cleavers is a frequent weed of winter cereals and oilseed rape where it may grow over and drag down the crop making harvesting difficult. It forms dense patches and has a more extensive root system than many weeds or crops allowing it to survive better under dry conditions.

The occurrence of morphologically distinct ecotypes has been reported but some differences are related to habitat. Hedgerow plants are thought to differ in levels of seed dormancy from cleavers growing within the crop. The effect may be to spread the period of emergence in the field to avoid a complete loss of emerged seedlings following autumn cultivations. Potential variability due to polyploidy enables cleavers to respond to environmental conditions.

The roasted seeds are said to be an excellent substitute for coffee but as cleavers also has medicinal uses as a laxative and emetic, consumption should be in moderation. Anthoquinones in the sap may cause skin irritation.

Biology: 

Cleavers flowers mainly from June to August and is self-pollinated. Seed is set from July to October. The average number of seeds per plant is 300-400 but counts of over 1,000 seeds per plant have been recorded. Populations can differ in their 1,000 seed weight.

Fresh seed has been reported to germinate readily. The optimum germination temperature varies with seed age but in the field, germination usually ceases above 15°C. Older seed germinates at higher temperatures than fresh seed. Germination is inhibited by light and seeds do not germinate unless covered with soil. The seed from hedgerow plants is less dormant than the seed from arable plants.

Seed dormancy is broken by chilling. Buried seed loses dormancy in the autumn then gradually acquires it again in spring. Seeds germinate late in the year and in mild conditions through the winter into early spring. In the field, most seedling emergence takes place from August to May with a peak in March-April. Optimal emergence is from depths of 20 to 50 mm. Maximum depth of emergence varies between 100 and 200 mm.

Seedlings that emerge in the autumn reach a height of 10-20 cm at which stage they overwinter. The seedlings are not damaged by a severe frost. Stem growth begins again in April and the first flowers appear in early June. Growth is slow to begin with but then increases rapidly. Daylength and temperature are the most important factors that determine the time from emergence to flowering. Cleavers is able to take advantage of a wet summer and plant biomass may double between June and July.

Persistence and Spread: 

Seed viability in soil is reported to be limited to 2-3 years. In studies of seed loss in cultivated soil the mean annual decline rate ranged from 58% to 66%. In separate studies the estimated time to 95% decline was 4 to 5 years and the time to 99% decline was 3.6 years. Seedling emergence represented 2 to 15% of cleavers seeds in the seedbank. Seed longevity in dry storage is 4 to 5 years.

The surface of the fruits is covered with hooked bristles that catch on clothes and animal fur. Cleavers seeds were one of the most frequent contaminants in crop seeds especially home-saved cereals. Farmyard manure produced on farm or bought in may also contain seed from contaminated cereal straw. Viability is lost after 34 days in stored manure. Seeds in a compost heap are killed when temperatures exceed 50°C for two weeks. The seeds survive passage through cattle, horses, pigs, goats, and birds. Seedlings have been raised from the excreta of various birds. The seeds will float in water but in flooded soil they were no longer viable after 20 days.

Mapping of cleavers patches in the field has shown that the areas remain relatively stable from year to year but expand on the leading edge by 1 to 3 m in the direction of harvesting and cultivation.

Management: 

Control is aided by sowing only pure crop seeds. Surface cultivations encourage cleavers seeds to germinate and harrowing or ploughing can then kill the seedlings. Around 60% of freshly shed seeds will germinate if the soil is surface cultivated. Many seeds that are more deeply buried will also germinate and then fail to emerge. Ploughing does not lead therefore to a build-up of a vast soil seedbank. Minimum tillage, however, puts the seeds at the optimum depth for germination and emergence. Mulching the soil surface reduces seedling emergence.

Cleavers is favoured by winter cropping with cereals and oilseed rape. It can reduce winter wheat yield by 64%. Even a low population can have an appreciable effect on crop yield and calculating a threshold value for applying control measures is not realistic. In cereals, harrowing with a tine weeder at an early crop stage can give a 79% reduction in density of the weed. A second harrowing at a later crop stage improves the level of control. Cleavers populations have been found to decrease following a series of spring-sown cereals.

Seed numbers in soil are reduced to less than 20% by fallowing for a year. In winter wheat, seed numbers would increase over the same period. Fallowing every 5 years over a 15-year period maintains seed numbers at around the 20% level. A continuous 4-year fallow is said to eliminate the weed.

Cleavers is often found spreading from the hedge bottom into the arable field. The growth of cleavers in headlands is suppressed by sowing a grass and wildflower mixture.

Updated November 2007.

Fully referenced review: