Artificial seed casing made from wood buries itself when wet

A seed carrier made from wood screws itself into the ground when exposed to water. It could be useful for projects to replant forests from the air using drones

By James Dinneen

A seed casing made of treated wood is stronger and more versatile than natural self-burying seeds, and could make it more effective to sow seeds from drones and planes.

Many species of plants produce seeds with structures that respond to changes in humidity to help protect the seed or bury it in soil. Plants in the genus Elodium produce a seed with a coiled tail that uncoils when it gets wet, twisting the seed pod deeper into soil. “That’s a behaviour that’s been optimised through generations of evolution,” says Lining Yao at Carnegie Mellon University in Pennsylvania.

Yao and her colleagues took inspiration from Elodium and other plants with self-burying seeds to design a seed carrier that would be able to carry different sizes of seeds and twist into soil in different environments more effectively.

After screening natural and synthetic materials, the researchers decided to use wood from white oak, which would respond to changes in moisture but also be stiff enough to drive a seed into soil, and be biodegradable.

The team developed a chemical treatment for the wood that made it possible to tightly coil it. This gave the twist a maximum bending curvature 45 times larger than other comparable wooden components, and therefore more torque when twisting a seed pod into the ground. “Thrust force depends on stiffness of the coiled body and tightness,” says Yao.

The researchers also designed a coil with three tails, rather than the single-tailed Elodium seeds. The added tails make the coil more likely to end up situated at the proper angle for the seed to be driven into the soil when it uncoils, especially on flat ground.

Seed pods made of wheat dough and a cellulose coating attach to the carrier to hold seeds and other beneficial payloads, such as fungi and nematodes that could help fertilise the seed.

In field tests on flat ground using arugula seeds placed in 136 carriers, about 66 per cent of carriers successfully anchored in soil, and 39 per cent of seeds germinated. The researchers didn’t compare this to seeds planted without the carrier, but Yao says it is a proof of concept that the carrier can work. Weather was also a factor: in one test, a period of heavy rain dislodged more than half of the anchored seeds.

A better carrier could make aerial seeding with planes or drones more effective, says Yao. Aerial seeding is useful when large areas have to be planted rapidly or areas to be seeded are difficult to access, for instance in some forest restoration projects.

Naomi Nakayama at Imperial College London says the researchers have more work to do to show the carriers can work in the field, and that they could be manufactured at the scale needed for a real planting program. But she says the new wood material opens up new possibilities for soft “wooden robots” that can respond to changes in moisture. “This morphing capability is very beyond what we could achieve before,” she says.