This perennial is native to Europe and the Mediterranean, but is a garden escapee and now naturalised throughout the British Isles. It spreads by seed, blown on a small fluffy pappus. It grows on old walls, dry banks and stony wasteground - the fleshy taproots can sustain it during dry periods.
It is not the Valerian which is used as a relaxant - this is Common Valerian (Valeriana officinalis) which has pale lilac flowers and the leaves are made up of toothed leaflets.
The upper leaves of Red Valerian are toothed and do not have a stalk, the lower leaves have a stalk and have smooth edges. The stems are smooth and can reach about 80 cm. The plant dies back to the basal leaves over winter. It does not have spreading roots, but remains as a clump which enlarges and becomes woody as it ages.

The flowers are red and sometimes pink or white, appearing from June to August. They have five petals, four facing in one direction and one facing backwards, all arising from a long tube with a single stamen protruding from the centre. They are pollinated by members of the Lepidoptera which have a long proboscis to probe the flowers. When the seeds mature they have a fluffy carrier and this is when it becomes a weed as they germinate everywhere. It is sometimes grown as a garden plant and it can make a good show when in flower, but it must be deadheaded to prevent an invasion.

The seedlings are easily uprooted and the mature plant can be eased out with a fork. Most of the larger roots should be removed. Care should be taken if it is growing in an old wall to avoid dislodging the stones.
Weedkillers to use:-
Glyphosate kills the whole plant, but it may be capable of shedding viable seed so remove any flowers that are present.
A residual herbicide suppresses germination.