The researchers identified 10 tree-planting drone companies as well as university research in India and government reforestation efforts in New Zealand and Madagascar. In Myanmar, Thailand, and the United Arab Emirates, drones have been used to help plant mangrove trees, a potentially impactful development, since trees planted near the equator capture more carbon than those planted elsewhere.

But the researchers said few companies have shared success rates or research into how seeds fare after they’re dropped by a drone. They called on those involved in drone seeding to be more open about their results. They label pledges to grow a billion trees a year “propaganda.”

Mikey Mohan is a PhD candidate at UC Berkeley and lead author of the paper. He thinks commitments to grow a billion trees are largely promotional tactics by companies looking to raise funds from investors. He said half of the social media posts he saw pertaining to drones planting trees had to do with promises to plant a billion trees.

What actually matters is the number of seeds that grow into trees after two or three years, he said, not the number of seeds you can drop on the ground in a day.

The researchers cited a 2020 study by DroneSeed that found survival rates for some conifer tree seeds range between zero and 20 percent, similar to prior efforts to drop seeds from planes or helicopters in the US in the 1950s and 1960s. Like other companies in the field, DroneSeed declined to say how many trees it has planted to date. The company would not disclose the names of customers but says it is working with three of the five largest timber companies in the US, as well as nonprofit conservation groups such as the Nature Conservancy.

Last month, five-year-old DroneSeed acquired SilvaSeed, a 130-year-old company that’s one of the largest private forest seed providers on the US West Coast. For context, SilvaSeed grows more seedlings annually than the Cal Fire Reforestation Center. The acquisition was driven, DroneSeed CEO Grant Canary told WIRED, by the fact that Climate Action Reserve, which tracks the environmental benefits of emission-reduction projects, now includes benefits from reforestation.

“What we see with reforestation and carbon credits is now we’re able to take land that’s been burned and make sure there’s a source of capital to reforest it,” Canary says.

In attempts to make seeds dropped by drones more viable, companies apply machine learning and imaging technology to pick optimal places to plant trees and guide drone flight paths. They encase seeds in pellets made with ingredients like clay and soil and sometimes shoot them into the ground. Each seed capsule is designed to contain the moisture and nutrients a seed needs to get started.

DroneSeed, for example, includes hot pepper to deter squirrels or other wildlife from eating its vessels, which are about the size of a hockey puck. How these carrying cases for seeds are made varies. Some contain a single seed, but Dendra Systems says it can pack up to 50 kinds of seeds for trees, shrubs, and native grass in a single capsule.

Asked to comment about the propaganda claim, Flash Forest CEO Bryce Jones said the company still plans to plant 1 billion trees by 2028.

Dendra Systems, previously known as Biocarbon Engineering, is one of the oldest and best-known companies using drones to plant trees. CEO Susan Graham said the company was created with the belief that a key reason humanity has yet to slow the decline in tree populations is that we aren’t using enough technology.

“You can solve the biodiversity challenge, you can solve the livelihoods challenge, and you can solve the carbon challenge all in one, if you can do it at scale,” she said.

She declines to say how many trees the company has planted. Ecologists are employed to verify results, she says, and the results of their work are shared privately with customers. She says Dendra now focuses more on the total area it can restore rather than the number of trees planted.

Former Dendra CEO Lauren Fletcher says he came up with the idea of using drones for planting trees in 2008, and he was one of the first CEOs to make the billion-tree pledge. He doesn’t think any drone-planting company has yet hit that target, but he thinks it remains worthwhile as an example of the big thinking needed to tackle global ecosystem restoration problems.

“The fact is people understand trees. They can see them, they can touch them, they can feel them, and it’s a hell of a lot easier to sell,” he said. “Try selling soil microbes.”

Fletcher is currently working with Dendra Systems cofounder Irina Fedorenko on another company aiming to plant trees with small drones, particularly for small landowners. Through a partnership with WeRobotics, Flying Forests wants to plant trees with drones in 30 countries. It is exploring projects in Kenya, Panama, and Uganda.