Hmm, it may be too late to make a meaningful difference this season but maybe more next year. New, thin stems eventually grow into thicker, woody stems that flop less. You could opt to just keep the stems and allow them to age and thicken up (use stakes to minimize flopping) or you can prune the thinnest stems as you were planning. But consider waiting to prune. Pruning can be done either now, once dormant or as late as in early spring. Since pruning stems tends to put the shrub into "grow mode" and tends to force new stem growth, early frosts could zap any new stem growth, Some foliage appears to be yellowing; at this time of the year, that may mean that the shrub is getting ready to go dormant so it will not direct much energy towards thickening stems. The days are also getting shorter. Your blooms are also color changing, indicative of their getting closer to turn brown. All this suggests a slumber approaching so prune yes but, once the plant has gone dormant or as late as early spring just before the leaf buds typically open. Woody stems can be pruned too once dormant but to keep most of those woody stems, consider only cutting 1/3 of the stem length or less. I would not fertilize this late. You can reduce watering once the plant has dropped the leaves but water if the soil feels bone dry. Stop watering once you the soil freezes or freezing temps stick around all the time.
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Luis is amazing with hydrangeas.
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