Make a structure appear from "seed"

Do you want it to be bonemeal-able?

If not you can just simply do this;

1.  Create a timer that's random

2. Then use this code,

        if (!worldIn.isRemote) {
            ServerWorld worldServer = (ServerWorld) worldIn;
            PlacementSettings settings = (new PlacementSettings()).setChunk(null).setIgnoreEntities(false).setMirror(Mirror.NONE).setRotation(Rotation.NONE);

            TemplateManager manager = worldServer.getStructureTemplateManager();
            ResourceLocation location = new ResourceLocation(YourModID, YourStructureName);
            Template template = manager.getTemplateDefaulted(location);

            template.addBlocksToWorldChunk(worldIn, block.getPosition(), settings);
        }

Well, since blocks don't really have a tick method. I would suggest you create a tile entity. We would put this code in the tick tile entity method with more conditions. There is a lot that would go into that block, but if you are up for it I am happy to help.

Alternatively, we could implement IGrowable and do what a sapling does. This would allow us to use bonemeal and other functionality. I actually prefer this method, I believe it is a "better" method.

Cant you make it a plant based block that grows over time? then once it has grown enough you could probably make a detection for that "grown piece" with a replace block into a new block/what you want.

or you could possibly make it so that each bonemeal grows it by 1-3 levels instead of all of it at once