A surprisingly simple and creative innovation could help restoration projects in the tropics, according to a recent study. Researchers from the Forest Restoration Research Unit at Thailand’s Chiang Mai University (FORRU-CMU) found that cultivating saplings inside repurposed bottle crates substantially improves the survival and growth rate of nursery-grown native saplings for reforestation.

Climate change, the biodiversity crisis and initiatives like the U.N. Decade on Restoration have spurred forest restoration efforts across the globe, including in the tropics. Many restoration projects start with native tree seedlings cultivated in community-run nurseries.

In Thailand, these seedlings are typically grown in black polyurethane bags placed directly on the earth. But the system is far from ideal.

As the sapling grows, the roots tend to spiral in the bottom of the bag, leading to poor root formation. The developing roots may also break right through the bag into the ground, which also causes problems, says Stephen Elliott, associate professor and research director at FORRU-CMU.

“When you lift the plant ready for planting, half the root system is left in the soil, so you’re immediately reducing the capacity of the plant to absorb water on the day that it’s being planted into a harsh, dry, deforested environment, where it’s going to compete against the weeds,” Elliott says. “You’re putting it at a disadvantage.”

One recognized method of dealing with poor root development is a technique called air pruning. In air pruning, seedlings are grown in a container designed to expose the root tips to the air. This exposure causes the tips to whither and sends a hormonal signal to the plant that kick-starts branching from the main root stem, resulting in a well-developed root system that lifts easily out of the growing medium.

Elliott and colleagues had seen examples of air pruning systems while touring nurseries in Australia, and previous studies have shown that air pruning can be beneficial for a number of tropical tree species. However, in Thailand, specialized air pruning equipment has to be imported and is prohibitively expensive.

So instead, the researchers cast around for a local solution. The answer was in plain sight.

“If you go to a bar in the countryside, you know, a little restaurant, out the back there’ll be a rubbish pile full of disused bottle crates,” Elliott says. “You can [also] go down to the recycling shop and buy them really cheaply.”

Each bottle crate fits about 12 bags with saplings, and the structure creates a gap of 2 centimeters (nearly an inch) at the bottom. The researchers wondered if this gap would be enough to create air circulation for air pruning.

To test the idea, they raised saplings of five native tree species using three different methods: in crates placed directly on the ground; in crates placed on wire benches (for improved air flow); and the conventional method of a polyurethane bag on the ground. They then planted the saplings in a deforested site within Pra-Bang Khram Wildlife Sanctuary in southern Thailand.

By the end of the first growing season, the crate saplings were taller and had developed a wider crown than those grown using the conventional method. It didn’t make much difference if the crates had been placed on the ground or on wire benches. This extra growth allows saplings to shade out competing weeds, an important milestone in restoration.

Crating also made caring for and transporting the saplings easier, and it lowered the labor costs as the plants were easier to move in crates. The savings in labor more than offset the cost of purchasing the bottle crates; the researchers recorded savings of about 4 U.S. cents per tree, if the crates were placed on the ground instead of on benches, making this the recommended method. The plastic crates tend to disintegrate after about three to five years, and can then be put back in the recycling stream, Elliott says. “In the meantime, you’ve grown a few dozen trees with that crate, so it makes logical sense,” he says.

Promoting these types of low-cost but effective improvements can make a difference to community nurseries, says study lead author Preeyaphat Chaiklang, then a master’s degree student at FORRU-CMU and now a Ph.D. candidate at Dresden University of Technology (TUD) in Germany. These innovations can help communities earn more money while improving the chances that restoration projects will succeed, she says

“Capacity building for local people is very important,” she says.

Banner image of a sapling by Pema Gyamtsho via Unsplash (Public domain).

Citations:

Chaiklang, P., Elliott, S., Chairuangsri, S., & Tiansawat, P. (2025). Effects of a low-cost crate-based tree-propagation system on the field performance of framework species when initiating lowland tropical forest restoration. Trees, Forests and People, 20, 100862. doi:10.1016/j.tfp.2025.100862

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Jeremy Hance Editor

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ConservationConservation TechnologyEnvironmentForestryForestsReforestationResearchTechnology And ConservationTropical ForestsThailandAsiaSoutheast Asia

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