I started my avocado plant from seed. It does well for 6 months or so; my last one got to about a foot long. Then, out of nowhere, they just start dying. I am not changing anything with care. Why do they die out of nowhere?
The only thing I can think of with a sudden die-off is that avos are /extremely/ salt sensitive. Having it in a container without buffer from the surrounding soil could contribute (if that’s where yours is at). So if you’re fertilizing it too heavy in those conditions, you get burning in the roots, translates to systemic root failure etc. And then the very feast-famine nature of container moisture might be giving them wet feet that they don’t like. (Not a lot of varience available for the plant to root in to keep oxygen coming in, not a lot of places or depth to spread into etc).
My understanding with avos is that they do good with low irrigation, lower nutrients, and do well with like a sand-clay almost? They’re weird.
Without seeing anything else in-person I’d guess too much nutrients and also root rot. See how much you can dial in the soil type, back off on fert etc. Might grow slower because of that, but, that might just have to be what it is IG. Remember to water only when the plant shows wilting from needing it, make sure you got sharp drainage.
Not an expert but I work with plants! Hopefully any of that helps!!!
My advocado tree, which I also grew from seed, has rotting at the very tip of the top, and it won’t go away.
When I cut the rot off an inch below the very bottom of the original rot, that new tip started to rot too. :( How do I stop the very top from rotting? There’s no visible rot anywhere else on it.
How often do you water? i think avocados like dry soil by virtue of their native climate, and overwatering is far too easy to do if you’re worried about your plants!
I say this as if we’re experts, we’re not. Though our avo has so far gotten to about 2 years just from sitting in various windowsills…
I would say I underwater my plants in general because I forget. Maybe every 10-14 days.

