Bittercress: A Small but Mighty Weed

As I write at the end of March, we are all restricted to staying home, so those of us with gardens can be grateful that we have an opportunity to be outdoors as well as something to keep us busy. Growing your own food is even more important now.

Here in the Highlands, we have had some beautiful sunny days but they come with frosty nights (it was -6°C last week) so I am not planting anything outdoors just yet.  However, the weeds don’t seem to mind the temperatures so I thought I would focus on a small weed that pops up everywhere in my garden, and probably in yours as well – Hairy Bittercress (Cardamine hirsuta).

Hairy Bittercress

Hairy Bittercress leaves with tiny hairs

What does it look like? It starts as a small rosette of leaves and then throws up a stalk a few centimetres high with tiny white flowers on top. It is an annual and forms seeds in narrow pods which explode to scatter the seeds far and wide. This explains its nickname of Popping Cress. It is part of the Brassica family of plants so is related to the cabbages and broccoli. All of this family have flowers with 4 petals arranged in a cross, hence the other family name of Cruciferae.

You may never have heard of it but it has been studied intensively. Its genome has been sequenced to help understand which genes control leaf formation . The popping seed pods have been researched as well. The explosive shatter of the pods is so fast – an acceleration from 0 to 10 metres per second in about half a millisecond – that advanced high-speed cameras are required to see it. To put that acceleration in context, if it continued for a whole second, the seeds would be travelling at 10 km/s or 3600 km per hour! A mighty effort from a tiny plant. Here is the video. 

You can understand why it can seed itself all over the garden. The secret to control is to remove the plants before the pods have a chance to split – and they don’t have to be dry to explode, so the sooner the better.  However, maybe you might find the plant interesting enough to let a few grow to maturity and see it for yourself!

There is another very similar plant called Wavy Bittercress (Cardamine flexuosa). It is difficult to tell them apart until they flower, and even then you have to look very closely (a hand lens helps) to tell them apart. Hairy Bittercress has 4 stamens whereas Wavy Bittercress has 6 stamens. Wavy Bittercress has a wavy stem and prefers damper ground.

Wavy Bittercress

Wavy Bittercress (left) and Hairy Bittercress (right)

(A bit of botany: Stamens are the male part of the flower which produce the pollen.  The larger structure in the middle of the stamens is the stigma.  This is the female part which has ova (eggs) at the base.  When the flower is pollinated, pollen sticks to the stigma then burrows into the base of the stigma and fertilises the ova and seeds are formed.)

Links

Bittercress genome research (Max Planck Institute)

Scientists discover how a common garden weed expels its seeds at record speeds

Update - found a video on identifying Bittercresses, including some I did not know!