I've inherited this avocado-in-progress from a friend moving house and I'm wondering if it's still healthy and worth keeping, and if so how best to care for it?
So far it has lived in a cup of water in quite a dark apartment. It's currently about 70cm tall (although can't "stand up" on its own).
It now lives in Barcelona with a balcony facing north-west so there are some hours of sunlight, although winter is coming.
Avocado trees have an incredibly strong initial "burst" of growth. That is a benefit of having such a big and rich seed.
Typically (wild) avocado seeds find themselves in a canopied forest with high competition for sun light. So the seedlings have evolved to "jump" up into the sunlight as best they can.
That's the current state of your avocado tree.
As for whether it's worth saving or not, I have never seen an avocado which grew large enough to produce fruit while living in a pot. I have seen some avocados which grew quite nicely in massive containers (where the tree roots actually end up growing into the ground).
So, life as a "house plant" will be limited. You may be able to keep it alive in a pot for many years to come, but it won't be easy. The leaves will tend to wilt and turn brown and as the tree becomes more and more "pot bound" (as the roots grow larger and larger in the limited size of the pot). The more "pot bound", the higher the potential for problems.
Not sure how the winter temps are in your area of Barcelona, but it appears to have the same "hardiness zone" as Mexico (where the avocado grows phenomenally well):
So, if you have space outside, then you might want to try it in the ground. Typically avocado trees take about 3 years to produce fruit if they are happy.
Note: an avocado tree can grown quite large but takes well to pruning (if it is in the ground, outside).
