here's a question... what type of Phals? Are they Home Depot, Lowe's purchases? Where the spikes a single branching spike or multi-branched?
I'm asking since if your spike branched, it might be a Dtps.
Either way, I usually cut back to the green/brown line of the spike. If the spike continues to brown, I chop it all the way down. I guess I rely on what my plant is telling me it wants to do.
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I haven't seen any difference in bloom size or amount if I cut back totally or if I let them rebloom. <shrugs>