Rosebay willowherb

Chamerion angustifolium

Rosebay willowherb, Chamerion (Epilobium in older books) angustifolium is one of the more statuesque plants of late summer. Popular names include Bombweed, Fireweed and Ranting Widow.
Rosebay willowherb has not always been common and was formerly considered a rare plant. Now, however, it is one the most successful colonisers of wasteland, car parks and railway verges. It may have spread more widely with the opening up of the railways and come late summer you can see why; bursting from the seed capsules are numerous feathery plumed seeds that are easily spread in the slipstream of trains.
Like purple loosestrife, rosebay willowherb forms spires of pink/purple flowers and from a distance, they might even look similar but get up close and there should be no confusing the two.

1. Stand of rosebay willowherb.

Taxonomy

The willowherbs belong to the Onagraceae family. They have 4 petals and 4 sepals arranged with radial symmetry. The corolla is usually pink or purple in colour though evening primrose (Oenothera biennis) is an exception being yellow in colour. The leaves are usually opposite though rosebay willowherb is an exception in that the leaves are spirally arranged. Indeed, in most keys this an early step that enables you to key out Rosebay Willowherb ahead of other members of the Willowherb family. Another useful feature is that it is almost hairless.

Habit

Rosebay willowherb forms upright spires of flowers that can reach a height of 1.5 metres. The stem is round, unbranched and without hairs. You see it in magnificent stands of flowers and the individual plants are connected by subterranean runners.

Leaves

The leaves are stalkless (sessile) and are arranged in a spiral fashion up the stem (picture 1). The blades are lanceolate with almost entire margins (no teeth).In fact, if you look carefully at the leaf margin you will see very faint teeth at the margin and at the tip a more prominent spot- these are hydathodes. Hydathodes are glands that can secret water. The undersurface of the leaf is a green/grey colour.

Another useful identifying feature is a prominent submarginal vein. This is a vein that runs parallel with the edge of the leaf and is connected to the central vein by secondary veins. See picture 2 below.

More than you want to know but the leaves do repay closer study.

2. Leaf after rainfall.

Inflorescence and flowers

The inflorescence is a long and magnificently flowered raceme. That means as the stem grows new flowers are added at the top until some limit to growth is reached. Consequently, you find the oldest flowers lower down the stem and if you want to study the maturation of flowers you can do so by ‘time travelling’ down the inflorescence.

The corolla is broadly symmetric, rose-purple in colour and  2-4 cms in diameter.  If you look closely you can see that the 4 free petals are not spread evenly around the circumference displaced away from the 6 o'clock position. Perhaps this is to leave space to fold the immature style and stigma out of the way (see below). Also in a departure form complete radial symmetry, the two upper petals are slightly broader than the lower two. The 4 sepals alternate with the petals and are darker purple in colour. 

There are 8 stamens that mature before the female parts of the flower.

There is a compound inferior ovary that ultimately forms the elongated seed capsule that is so characteristic of the family and that gave rise the old genus name of Epilobium. The appearance of the style and stigma depends on when you look at the flowers. According to the Pocket Guide to Wildflower Families by Faith Anstey (which is a great little pocket guide and an adjunct to larger keys) the stigma may be club-shaped or cross-shaped -  this, however, depends on when you look. The flowers are protandrous which is is a form of sequential hermaphroditism in which both male and female parts are represented but they do not mature at the same time.  The  8 stamens mature first and the style and immature club-shaped stigma is bent out of the way- this is the male phase. Subsequently, when the anthers have discharged their pollen the stamens droop and the style becomes erect and the stigma assumes a cross shape and is receptive to pollen from other plants - the female phase.

3. Rosebay Willowherb flower from Structural Botany by Asa Gray. At the early stage (left) the style is bent downward and stigma club-shaped. Subsequently, when the anthers are spent and have discharged their pollen the style straightens and elongates and the stigma becomes cross-shaped and receptive to pollen

Because the older flowers are found towards the bottom of the inflorescence and the younger towards the top it is not unusual to see club-shaped and cross-shaped stigmas on the same plant. Club-shaped towards the top where the younger flowers are and cross-shaped lower down.

4. Rosebay willowherb 'male phase' showing the club-shaped stigma

5. Female phase. The stamens are now drooping out of the way and the cross-shaped stigma 
can be seen in the centre.

As a mechanism for ensuring cross-pollination rather than self-pollination, this is only going to work if bees tend to visit in the right sequence because self-pollination of an individual plant is still possible if bees go from male phase flowers towards the top to female phase flowers towards the bottom of a single stem. Bees would have to start with the older female flowers at the lower part of the inflorescence work up to the newer male flowers at the upper part of the inflorescence then cross down to the female flowers at the next inflorescence thus transferring pollen from one plant to another. The other way round will result in self-pollination.

6. To avoid self-pollination bees need to start with the lower flowers work upwards then transfer to lower flowers on switching to a new plant. A possible sequence is illustrated above.

This is a nice story but is it true? Well, I can't be sure. I watched a few bees but not enough to form a sample that would impress a statistician. However three out of the four bees I watch started with lower flowers first. The fourth bee clearly hadn't thought the matter through.

If you look at the lowermost flowers you can see that the when the male and female phases are complete the corolla closes up and brings the stamens into contact the stigmas. You can see this in figure 6 above, in the bottom right flower and also in photograph 5 showing the female phase. The bottom left flower (marked with an asterisk) has closed upon itself bringing the male and female parts into contact with each other. This can be thought of as a kind of insurance policy; if cross-pollination has not been achieved there is the 'back up' of self-pollination - a process called autogamy.

Where to find rosebay willowherb in Leazes Park

You can find it almost everywhere so I am not going to single out any one area. But you should take some time to study the flowers closely compare the ones lower down the stem with those at the top and watch the seed capsules as they mature and burst.