I use Osmocote in all of my planters every summer. I mix some in when I plant them in spring along with some earthworm castings, mid-May to June 1st. When planters are watered daily though, none of the time released fertilizers work as long as they claim (note the words "feeds up to 6 months"), so I usually give them all a 2nd dose of both Osmocote and earthworm castings by late-July.
I also feed the planters daily through the drip system on a low dose of a cocktail of fertilizers I found out about at Pesche's (Miracle Gro, SuperThrive and Monty's PlantVantage), and this gives me the healthiest plants with a ton of flowers that last well into fall. I grow a lot of flowers that are heavy feeders, like petunias.
But I like Osmocote and will continue using it, it works out very well for me. I've never had it burn or harm plants at all, it's very safe and reliable to use.
