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exploding seed pod weed

Call it what you will – hairy bittercress, winter bittercress, hairy cress, popping cress – Cardamine hirsuta – is a weed that tries the most forgiving gardener’s patience. Growing worldwide (except in the Antarctic, this genus of the Brassicaceae family numbers more than 150 species, both annual and perennial. The plant is self-pollinating and in bloom throughout the year. It loves moist soil and grows aggressively under those conditions.

As the snow melts, tiny white, pink, or lavender flowers begin to appear. Yes, flowers. This tenacious weed is short-lived, which is good, you say. A life cycle of 6 weeks doesn’t seem like such a big deal. Think again – how many 6-week cycles are there in a year?

One of the biggest problems with bittercress is that, by the time you discover you have a problem, it’s almost too late to do anything about it. The first flowers appear in late February or early March, quickly form seed pods, and mature. If you touch those trigger-happy seed pods, i t’s all over – the pods explode, distributing seeds over an area up to 36 inches around each plant. Those seeds will germinate and begin sprouting with a few days and the cycle begins again, only over a larger area. Small to medium size plants produce about 600 seeds, and larger plants can yield up to 1,000 seeds.

Hairy bittercress is not invasive enough to warrant using herbicides. As soon as new plants appear in February or March, begin pulling them; these are the offspring of the previous fall’s seed crop. Through the season, always pull the seedlings when you see them; they have shallow roots and come away quite easily; however, bits of root left behind are capable of re-rooting under optimum conditions. The key is to get the plants before they set seed, which happens quickly after blooming. Eradicating this weed from large areas is almost impossible, unless you can hoe and remove. Keeping bittercress out of the flower beds is a little easier, but requires diligent hand-weeding to stay ahead of the seed formation. The leaves release a pungent aroma when bruised.

(This article was originally published on March 29, 2010. Your comments are welcome but please be aware that authors of previously published articles may not be able to respond to your questions or comments.)

Spring is in the air, little green things are popping up all over, and we all heave a sigh of relief that the blanket of white stuff is finally gone. But beneath the snow that stopped everything in its tracks lurks a hardy, robust little puff of tiny green leaves that virtually grows before your eyes.

Hairy bittercress is a problem in greenhouses and nurseries, so be sure to clear off the top 2 to 3 inches of soil before planting anything you purchase. Scoop the soil into a plastic bag and dis card. Keep a close watch on newly planted containers, especially those that are positioned near flower beds. The propulsion factor of bittercress seeds can sneak new plants into your containers while you aren’t looking. Hairy bittercress is a real problem near flagstone patios or walks, brick work, or any hard-scaping that has space between the pieces. This weed does not need much to set down roots – even a small amount of sand between two bricks is plenty.

The authors used a computational model of three-dimensional plant cells to show that when these cells are pressurized, they expand in depth while contracting in length – much like an air mattress does when inflated.

Previously, scientists had claimed that tension was generated by differential contraction of the inner and outer layers of the seed pod as it dried. So what puzzled the authors of the Cell paper was how popping cress pods explode while green and hydrated, rather than brown and dry. Their surprising discovery was that hydrated cells in the outer layer of the seed pod actually use their internal pressure in order to contract and generate tension.

The scientists, led by Angela Hay, a plant geneticist at the Max Planck Institute for Plant Breeding Research, discovered that the secret to explosive acceleration in popping cress is the evolutionary innovation of a fruit wall that can store elastic energy through growth and expansion and can rapidly release this energy at the right stage of development.

Plants use many strategies to disperse their seeds, but among the most fascinating are exploding seed pods. Scientists had assumed that the energy to power these explosions was generated through the seed pods deforming as they dried out, but in the case of ‘popping cress’ (Cardamine hirsuta – a common garden weed) this turns out not to be the case. A new paper by an international group of researchers, published in the journal Cell, offers new insights into the biology and mechanics behind this process.

Another unexpected finding was an evolutionary novelty explaining how this energy is released. The authors found that the fruit wall seeks to coil along its length to release tension but is prevented from doing so by its curved cross-section. Professor Moulton said: ‘This geometric constraint is also found in a toy called a slap bracelet. In both the toy and the seed pod, the cross-section first has to flatten before the tension is suddenly released by coiling.’ Unexpectedly, this mechanism relies on a unique cell wall geometry in the seed pod. Professor Moulton added: ‘This wall is shaped like a hinge, which can open, causing the fruit wall to flatten in cross-section and explosively coil.’

Several teams of scientists spanning different disciplines and countries, including Oxford mathematicians Alain Goriely and Derek Moulton, along with colleagues from Oxford’s departments of Plant Sciences, Zoology and Engineering, worked together to discover how the seed pods of popping cress explode. A rapid movement like this is rare among plants: since plants do not have muscles, most movements in the plant kingdom are extremely slow. However, the explosive shatter of popping cress 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.

Emphasising the multidisciplinary, collaborative approach to this paper, Professor Goriely said: ‘This approach was only made possible by combining state-of-the-art modelling techniques with biophysical measurements and biological experiments.’