I had the same issue - my giganteas seemed healthy enough but just didn’t get...giant. One friend added some lime and his took off. It didn’t work for me. Then I added some compost, about 20% I suppose, and plants that had just sat there for a couple years suddenly started taking off. (I used mushroom compost.) The front plant in this picture never even reached the edge of the pot for almost 3 years. It’s an alba so it may be a little smaller anyway, but still...
Please excuse the holes in the leaves. Slugs got into my trays!
Lime seems to work for some people, maybe it depends on what else is in the mix also. It can help nitrogen become more available, but in this case adding a little bit of gentle nutrients also seems to have helped as well.
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