I have a Melix Seed in my inventory (not locked) but when I go to Subeta Gardens it says this:

You do not have any seeds to plant! Come back to me when you do!

That's one of the seeds you use from your inventory.

Click the seed, click the "Water Seed" option and you'll get a Melix Flower in your inventory.

Oh okay thank you. Is that intentional, do you think?

As far as I know, there are some seeds that you can only "grow" from your inventory while the rest are strictly grown in the gardens. It's a strange mixed bag.

I know Bubble Insta Seed and Julip Insta Seed use the same mechanics as the Melix seed. I don't know if there are any others. D: It might be a good idea to mention it in this topic so that all the inventory-grown seeds are renamed into insta-seeds.

Thank you, Frost, I appreciate the in-depth help you've offered me.

You're welcome, I'm glad I could help! :)