Why Planting Baby Corals Isn’t Enough to Save Reefs

A new study finds coral seeding boosts survival in the first year, but long-term success still depends on the neighborhood

A few years ago, my family set up a saltwater aquarium at home. What can I say? I’m a very curious biologist. 

The goal was modest: raise a few corals, keep them alive, and maybe learn something. We quickly realized that even with daily testing, stable temperatures, and near-perfect lighting, getting corals to survive — let alone grow — was no easy feat. The smallest shift in algae growth or water chemistry could set them back. 

Or cause a bleaching event.

That’s probably why a recent study on coral seeding and survival hit home.

Although I’m not a marine biologist by training, I spent my PhD years in Australia, surrounded by reef researchers and close friends who worked out of Heron Island and AIMS. I’ve taught university courses on marine ecosystems, and I’ve seen firsthand how sensitive corals are to their surroundings, especially when they’re young. 

So when I read that researchers had tested coral “seedlings” in macroalgae-dominated reefs, and actually tracked their survival beyond the first year, I was all ears.

The study, published in the Journal of Environmental Management and led by PhD candidate Hillary Smith of James Cook University and the University of New South Wales, explores whether coral seeding, an idea borrowed from land-based reforestation, could help young corals survive the risky early years of life. 

Think of it like planting coral babies into small ceramic flowerpots and dropping them into degraded reefs. Would these tiny coral recruits do better than natural settlers? Could pulling out some of the surrounding seaweed, a process the authors call “sea-weeding,” give them a better chance?

Turns out, yes. But only at first.

The idea behind coral seeding is “simple.” Scientists collect coral larvae, let them settle onto tiny plugs in a lab, and then insert these into ceramic “seeding devices” designed to protect them from predators and harsh conditions. In this experiment, the team deployed these devices across two reef sites at Yunbenun (Magnetic Island) on the Great Barrier Reef. Some areas had their seaweed manually removed, while others were left untouched.

To compare how well the seeded corals did, the team also placed standard terracotta tiles, a classic stand-in for “natural” reef settlement, and monitored everything for nearly two years.

In the first year, seeded devices worked better than natural settlement. “Just like planting a seedling in a small flowerpot and plucking weeds, the same strategies seem to benefit baby corals,” Smith explained.

On average, more corals survived on the seeding devices than on the tiles, and survival was significantly higher when the surrounding seaweed was removed. Coral spat settled on the side-facing surfaces of devices did particularly well, possibly because they were better protected from sedimentation and abrasion.

But by the second year, that early edge began to fade.

Survival rates dropped across the board. In fact, the benefits of “sea-weeding” disappeared altogether in year two. Only 0.87% of the more than 13,000 coral juveniles survived the full 21 months. While 50% of the devices still had at least one surviving coral, this translated to an average of just over one coral per device. That’s far fewer than the natural recruitment observed in weeded plots.

And then there’s the cost. Depending on survival rates, it could cost anywhere from $9 to $23 million to restore a single hectare of reef using this method. That’s a hefty price tag, especially when macroalgae removal alone yielded better results in some areas.

Here’s what I found especially compelling: this is one of the first studies to directly compare coral seeding to natural survival over more than a year. That matters because up until now, many restoration efforts have stopped tracking results after 12 months, assuming that any coral still alive has “made it.” 

But, like in our home aquarium, surviving year one doesn’t mean the coral is out of the woods. Temperature stress, competition, and slow growth still pose significant challenges.

What this study shows is that restoration isn’t just about planting and walking away. Habitat conditions, especially seaweed cover, still make or break a coral’s chances. Even the best-designed devices cannot fully offset poor environmental conditions or long-term stressors.

So, where do we go from here?

Smith’s team emphasizes that coral seeding might still be useful, especially on degraded reefs where natural recruitment is unlikely. But they’re also clear that it’s not a magic fix. “We found that sea-weeding combined with seeding can give young corals a crucial head start,” Smith said, “but it’s not a silver bullet.”

To make this approach scalable and cost-effective, we’d need better device designs, cheaper aquaculture, and perhaps even new deployment methods, like dropping the devices from boats instead of installing them manually underwater.

In the meantime, this research gives us a clearer picture of what works, what doesn’t, and where we should invest our energy. And it reminds us that even the most high-tech restoration methods can’t replace the value of protecting reefs before they collapse.

The bottom line? Coral seeding might plant the seeds of hope, but it’s the surrounding environment that decides if they grow.

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Best,
Sílvia P-M, PhD Climate Ages