What i don't understand is how 2 identically positioned pots can differ: one with algae, one without? All of mine get algae if not shielded well from light , even in slight shade/ in back of shelf/ balcony.
Even with peroxide added ..but that is for potted plants on wicks with fertilizer added. May be that plays a role .
i do not add fertilizer when i water-propagate: i had once left a bucket with 2qts of water on the table : last ditch effort to save a cutting (no direct sun and tented too) and i had no algae 3 months later! (It had a film covering water too).
So, of course it had peroxide added , but also water was shielded from surrounding air.
Long time ago i learned on the forum that it helps to cover the top of container with saran film with poked holes when rooting begonias in water . It stabilizes the stems and prevents air-contamination and also they develop air-roots above the water fast as there is 100% humidity between the water level and film. There are always mildew /fungal spores floating in the air-soup , so it's important to screen them out, especially for begonias.
With this method i can leave cutting for water-rooting when i am away for very long times (4-8 weeks). I started doing it when i am not away too: no need to monitor; can do many more at once.
I am very long-winded here š; but i think that runoff water from pots (if you water on occasion from the top) can contribute to algae growth too. Or air access to water.
So when pot is tight in the water container, not watered from top and water is shielded from light there should be no algae at all (+ peroxide added, of course).
I use huge 2.5 gal water jugs filled with water for walling off my pots on the balcony (high wind, pots might topple) when on vacation in summer: no peroxide, small half inch hole on top to fill; exposed to sun for 2-3 hoursā¦and 1 month later they are still clean, and even after 2 months ..
