Can someshed some insight. I believe the general sage advice is to put down fertilizer prills right before or just as bud breaks. My potting medium drains well.....maybe too well. I have chunky perlite mixed in my bark mix. What goes in goes out.... In my mind, it might be less wasteful to put down fertilizer more when it's at least semi mid-full leaf stage, where uptake is slightly more in demand ? I'm kinda looking at this, just short of uncontrolled conditions like when it rains.....watering in Slow Release fertz. pills when there is no much foliage may just result in fertz water draining right out ? I suppose *some * PK may remain ? Unless the theory is that the roots are hungry , and whatever *fertz* does come out of a slow release prills is better during the -universal- advice of putting it down in pots when bud breaks
There are two reasons I don't like to feed chemical fertiliser early in the spring. The first is related to your concerns about the ferts washing out. There is insufficient root activity in Japanese maples in spring so fertiliser is not really needed at this time of year - much will be flushed out to waste. The second is related to frost. I don't want my JM's taking up excessive ferts (especially nitrogen) at this time of year as it might make the new growth softer and more at risk from a late frost. The ideal time to feed is when the first flush of leaves has fully expanded (or nearly so). At this time major root activity is starting and danger of frost is much less.
