Yarrow
Other names
Arrow-root, bloodwort, greenarrow, milfoil, nosebleed, sneezewort, thousand-leaf, yallow
Latin names
Achillea millefolium L., (A. lanulosa).
Weed Type
Perennial Broad-leaved Weeds
Occurrence
Yarrow is a perennial, native in meadows and pasture, banks, hedgerows and waysides, very common in Britain. It is recorded up to 3,980 ft. Yarrow is a persistent weed of grassy banks and roadsides but not waterlogged areas. It is found on soils between pH 4.7 and 8.0. Yarrow has roots that penetrate to 20 cm deep. It can withstand drought and thrives even on a poor dry soil. Yarrow is considered to be an indicator of loam. It prefers an open habitat and is a natural component of chalk grassland but is absent from woodland.
There is considerable variation in plant morphology both within and between populations. Plants also exhibit considerable plasticity in response to habitat conditions. Races have been recorded that are able to tolerate heavy metal contamination in soil.
The plant is rich in magnesium, calcium and phosphate. Sheep eat the plant when it is young and for this reason it is sometimes included in grass seed mixtures. It is said to control diarrhoea in sheep. Yarrow has many herbal uses but can cause skin irritation and rashes.
Biology
Yarrow flowers from June to August, sometimes into October. The flowers are insect pollinated and self-incompatible. Not all plants flower and yarrow may need to reach a minimum size before flowering is initiated. Seeds are set from July onwards. A plant may produce up to 6,000 seeds. The average seed number per stem ranges from 210 to 1,660.
Fresh seed requires a period of after-ripening. Seed germination increases after a period of dry-storage. Germination is greater in the light and in alternating temperatures. Chilling and high nitrate levels promote germination in the dark. Seedlings require an open site in which to become established. Seedlings emerge from January to October but the main period of emergence is from March to April.
Yarrow has a well-developed fibrous root system and prostrate stems that root at the nodes and become far creeping stolons. The plant has branched rhizomes that generally remain in the top 10 cm of soil. There is a high degree of apical dominance in the rhizomes unless fragmented by cultivation. When fragmentation occurs more buds develop on the smaller fragments than on longer pieces where apical dominance is quickly re-imposed and bud development inhibited.
Yarrow overwinters as leaf rosettes that give rise to leafy flowering shoots in spring.
Persistence and Spread
Yarrow is not thought to form a persistent seedbank although seed remained viable for at least 5 years in cultivated soil. In undisturbed soil, the depth of burial is an important factor in seed persistence. Seed viability in dry storage was around 30% after 10 years.
Yarrow seeds may be dispersed by wind. Seed has been found in horse droppings, and seedlings have been raised from the excreta of various birds.
In pasture the shallow rhizomes simply spread out from the parent plant at 7-20 cm per year. On cultivated land the rhizomes are easily broken up by tillage operations and the fragments spread around. Regeneration can occur from single node fragments but the shorter the fragments the greater the mortality with depth of burial. Fragments 4 cm long fail to emerge from below 15 cm.
Management
As a natural component of chalk grassland it has been included in seed mixtures to restore arable land back to grassland. It germinates best when the vegetation is cleared, somewhat less well if the vegetation is cut, and poorly if the vegetation is left uncut. Germination was high when the seed was sown on chalk grassland grazed by rabbits.
When it has been sown in mixtures with grasses it can sometimes spread too rapidly. Pastures should be close grazed with sheep in spring and early summer, although excessive early grazing can favour the weed. Sheep eat the plant when the leaves are young. Applications of nitrogenous manures will reduce yarrow growth as will liming. In roadside verges, increased cutting frequency has no effect or slightly increases the frequency of yarrow.
Deep burial may be effective if rhizomes are sufficiently fragmented. Otherwise shoot emergence will only be delayed relative to crop emergence. In dry conditions rhizome fragments left on the soil surface desiccate rapidly. Yarrow does not survive regular cultivation.