I say it's a case-by-case basis: the overall category can be on topic on both sites, but some such questions belong much more on one site than the other.
For the specific questions under consideration here:
Hostas - I think this is way better on gardening. It's not a common cooking ingredient, it's not sold in stores (except maybe some places in Asia?), so gardeners are way more likely to have good advice. I would probably like to migrate it, but I don't feel strongly enough to do so unilaterally.
Elderberries - would've been better on gardening, but it did okay here. It got an okay answer: pretty clear, except how do you actually judge full ripeness? Note that the answer mentions branches drooping under weight as a sign; that's something that's only applicable to picking from a plant, not selecting in a store, so it is actually a little gardening-specific.
Blueberries - again better on gardening, did okay here. It got what seems to be a good answer here. But again, it mentions sun exposure, which is gardening-specific.
Figs - significantly better on gardening. The answer relies substantially on fruit drooping on the tree, though it does also mention things that work in stores. (You have a lot more freedom to squeeze and taste if it's on your tree, though.)
Note that you don't actually find out if there is gardening-specific advice to be had until you see the answers.
So to try to generalize a tad...
For some things, you might not want to decide when to pick from your garden in the same way you'd select produce in a store. For example, berries are probably best picked completely ripe, and you can easily taste-test to get an idea. In a store, completely ripe berries probably won't survive transit well, so you have to balance ripeness with smooshedness. Or consider herbs: they're be fine to cook with at basically any time, so the best time to pick is basically a gardening question. (When and how much can you pick while keeping the plant healthy.)
On the other hand, for many things you'll pretty much be looking for the same thing whether you're in a store or in a garden. Even in these cases the OP might be best served by asking on gardening, even if the question would do okay here. Even if deciding the optimal time to harvest uses the same criteria you'd use in the store, there might well be other related things a gardener would know best about, e.g. how reasonable it is to harvest before or after the optimal time.
So on the whole, as an OP, I'd tend to go for gardening. If you're asking the question, you don't know which of those categories it's in. But you can be confident people on gardening will have good advice, while you don't know whether cooks' knowledge will be complete enough to give you a good answer. It's definitely where I'd personally ask most questions like this.
As for what we should do, this means we have to consider questions individually:
- For some, it'll basically be a cooking question, so we don't need to do anything.
- For some, it'll be in a gray area, so we should leave it where asked... but remind the OP that gardening exists and might get them better answers. (And we can certainly migrate if the OP wants.)
- For some, where we're confident it's mostly a gardening issue, we might want to just migrate.
Since I do think that gardening is the best place to ask, as we evaluate questions, I'm okay with erring on the side of migrating.