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If they're growing up really big, they probably don't need fertilizer. I've found that looking at most fruit trees, bushes, and plants will tell you if they need fertilizer. If the vigor is satisfactory, they don't usually need fertilizer. But a little mixed fert such as 10-10-10 won't likely hurt. A soil test might be some help.

The Fruitnut

Fertilizing while they are fruiting will make the fruit softer.

Assuming you are going to renovate nad carry into next year, you should fertilize when you renovate and again in the fall.