They're called the "energy willows."
Fast-growing willow trees are being planted at old coal pits in Bosnia and Serbia.
They're able grow up to one inch a day,
and can be burned alongside coal in thermo-power plants.
To help the Balkan countries transition to cleaner energy.
We are in Tuzla, a northern town in Bosnia where the country's power utility Elektroprivreda has its largest coal mine.
Only three of its original 12 pits are still in operation.
At the decommissioned pits, around 50 former miners planted the first willow trees in May.
Jasmin Brkic is the technical lead on the willow-planting project:
"In this area, a pilot project of thermal willow or Swedish willow plantations was made on one hectare of land with the goal of improving the air quality or reducing the emission of harmful gases emitted from the thermal power plant which is using coal, lignite."
"14,000 seedlings have been planted on one hectare, so for this part that we will plant now in autumn, we need about 140,000 seedlings, which we will import from Sweden."
The first harvest is expected in three years.
The trees are known as "energy willows" because their heat value is greater than brown coal or lignite - typical energy sources in Bosnia and neighboring Serbia.
The two Balkan countries do not plan to exit coal until 2050.
Serbia currently still generates two thirds of its electricity in ageing coal-fired power plants.
And some experts say the willow tree project is not a sustainable model to transition from coal to green energy.
Denis Zisko is the program coordinator at Tuzla center for ecology and energy.
"The problem I have with what is happening at the moment is that it is an attempt by the power industry to continue burning coal as an energy source, using the justification that it will mix renewable energy sources with that coal and thus contribute to reducing CO2 emissions. It is about prolonging the use of coal as an energy source, which is still not a sustainable model of energy transition."
"We need to stop looking for excuses to continue burning coal and work on serious strategies for the transition to renewable and sustainable energy sources."