For most gardeners, the fiddly task of sowing seeds is eventually rewarded by seeing the fruits of their labours blossom and grow.

But for pioneering volunteers involved in a major underwater garden project, the flowering meadows of seagrass which they plan to nurture from tens of thousands of seeds, will remain hidden from sight beneath the waves.

In a few weeks’ time, dozens of people living in on the fringes of Loch Craignish in Argyll and Bute will enter its chilly water to gather bouquets of seagrass, which will be used to harvest around 100,000 seeds.

In the first project of its kind in Scotland, the seeds will then be carefully sorted and prepared by hand, before being tucked into thousands of tiny hessian bags which are then individually planted on the sea loch’s floor.

If successful, their massive labour of love will see new plants cover around a quarter of a hectare of seabed, helping to restore degraded seagrass meadows, boosting biodiversity and creating lush underwater fields capable of capturing carbon up to 35 times faster than tropical rainforests.

Although highly labour intensive and requiring the support of volunteers from local communities to plant and then monitor progress, the project will pave the way for more in coastal sites where large areas of once thriving seagrass meadows have been lost.

The seagrass planting is part of a wider ‘rewilding’ project run by the charity Sea Wilding, which will also see a million native oysters reintroduced to the sea loch’s waters.

Since its launch in October, around 60,000 junior oysters have been placed by hand on the seabed, with plans for a further 100,000 later this month and another 100,000 in October.

Loch Craignish, between Oban and the Crinan Canal, opens onto the Sound of Jura, and in Victorian times was famed from its abundant marine life, huge oysters and scallops. Its lush seagrass meadows provided an ideal spawning ground for white fish such as cod and herring.

However, a combination of bottom trawling, dredging and pollution has been blamed for the loss of oysters and destroying once huge areas of seagrass which covered its floor. Although the loch has ten small and relatively healthy seagrass meadows totalling around one hectare, they are isolated and fragmented.

As well as the impact on carbon capture, the loss of seagrass has been partly blamed for the collapse of west coast white fish stocks. While seagrass meadows are said to harbour up to 40 times more marine life than areas without grass.

Danny Renton, founder of Sea Wilding, which has received £225,000 of National Lottery Heritage Fund grant to support the oyster project, said both it and the seagrass restoration work are aimed at replacing important elements of marine life to the loch.

“We are trying to prove that it’s possible to put something back and regain something that has been lost,” he said.

“Seagrass is disappearing. But it is huge in terms of biodiversity and is seen as critical to blue carbon – we are now waking up to the amount of carbon that seagrass and native oysters sequester.

“Seagrass has dense root systems, it’s like a peat bog on the seabed. Trying to enhance it is very important.”

However, both projects are hugely labour intensive both in terms of returning plants and oysters to the seabed and additional work is required to monitor and assess the success of both the oysters and seagrass harvest.

It means local communities and schoolchildren are being drafted in to support marine scientists in helping to ensure their success.

Seagrasses are the only flowering plants able to live in seawater and pollinate while submerged. They can grow up to 2m long, creating a rich underwater meadow that provides a habitat for a wealth of species.

Globally, seagrass accounts for around 10% of annual ocean carbon storage, however, it’s estimated that the UK has lost more than 90% of the seagrass meadows that once surrounded the nation.

Teams of trained volunteers from the local community will either wade into the water of Loch Craignish or snorkel to pick individual blades of seagrass embedded with seed, without harming the original plant.

The seeds will then be harvested in a mobile process unit consisting of water tanks and pumps which will remove and filter the seawater.

After a period of a few weeks, the sprouted seeds will be transferred into small golf ball-sized hessian bags which are then manually pegged to the seabed.

Work is due to begin in late August, with planting at the end of September.

Areas of Loch Sween, Ganavan Sands near Oban, Loch Melfort, an area off Tiree and another near Arran have all identified as potential seagrass restoration spots.

The oyster project has involved juvenile native oysters known as ‘spat’ and weighing just one gramme, sourced from Morecambe Bay Hatchery and delivered to Loch Craignish.

They are then grown in a nursery of floating cages for around four months until they reach around 12 grammes and are large enough to sit on the sea floor and survive possible predation from starfish and crabs.

There they act as water filters – just one can clean almost 40 gallons (200l) of seawater every day – and create reefs which provide nursery fields for fish, help reduce erosion and act as buffers from waves.

Once well established, the oysters may be harvested at sustainable levels, helping to create jobs. Similar schemes are also being planned by the charity at Loch Aline in the Sound of Mull, which will grow 20,000 oysters over a three-year period, and at sea lochs in Wester Ross.

Mr Renton blamed bottom trawling and dredging for damaging oyster beds and seagrass meadows, which he said have left sea beds like “raked gravel” and left white fish species with nowhere to spawn.

“Dredgers have trashed so much while bottom trawlers are operating within metres of the shore. A lot of coastal communities have had enough,” he said. “In many Scottish sea lochs, there were once major oyster beds in Loch Craignish which provided complex 3D habitats for a wide biodiversity. However, they have pretty much disappeared everywhere.

“A lot of coastal communities have had enough and want a say in inshore fishery management.

“They are really interested in trying to restore what they see is really degraded inshore waters.”