Gardens are a lot of fun, and especially as you can repeatedly sacrifice your garden for lumps, plants meant purely for tweaking the garden itself are -really- fun to play with. Tidygrass and ichorpuffs are great utility plants to keep things in order when I'm at work or asleep, and Ichorpuffs especially have a lot of interesting uses with their intense aging slowdown.
I think Elderworts are currently a missed opportunity as a garden utility plant, doing the reverse of Ichorpuffs; aging plants to maturity faster. Now, I realize that's a very powerful effect; slowing down aging has strong drawbacks, but speeding it up is more easily beneficial, especially with the many plants that you just want to harvest at maturity. But I think Elderworts already balance that out by maturing veeery slowly, making it a long-term investment and preventing micromanagement woes.
You can kind of use it now, but the effect is so slight that even surrounding a juicy queenbeet with 8 elderworts only makes it mature 19% faster in fertilizer, not counting the lessened effect while growing from seed. And it's unrealistic/not-useful to crowd any other plant type in -that- many elderworts, while less provides almost no effect at all. If it's specifically problematic that people will use stronger-aging elderworts to pump Juicy Queenbeets, it can just mature even more slowly than it already does. They're immortal, so it'd still work fine with every other kind of plant.
Encouraging use of some plants to boost other plants enhances the garden game, by encouraging layouts using more than one or two plant types; like using Nursetulips and ichorpuffs. Although I unfortunately think Nursetulips are currently more trouble to use than they're worth, but that's a different subject.
Alternatively, elderworts could be kept as-is; they already provide beneficial effects with an immortality tag bonus and are completely fine without any aging bonus at all. Instead, another new plant that expands upon the aging bonus could be crossbred from them, bringing the seed count to an even 5x7 grid. The game currently seems to try very hard to arrange in rows of 7, so having seed count divisible by 7 would be a nice touch. :V