Steve
I have always cut Yarrow when the center of the flower cluster has developed / aged. For the first time in the four years I've been growing it, I have seen pollen for the first time. Good time to look is in the morning, also the very best time to cut it. You'll see little bright yellow bits sticking up out of the center of each little flower in the cluster. I'm now letting more of the flower clusters develop on the stem than I used to when I sold to florists. Only downside to this is that the first flower cluster looks a little worse for wear.
This is one of those flowers that you need to experiment with and see what works for you. Let some flowers age and then cut and cut some immature and see how they hold up. There's nothing worse than cutting this one and within an hour all the flower heads bob their heads and nothing you do, revives it. Good Luck
Cheers Kim
Billabong Fresh Flower Farm

Thanks Kim, I appreciate the advice. I'll look closely in the morning. I've had the drooping problem in the past when cutting yarrow for myself. Now that I'm cutting it to sell this season, I really need to avoid that problem. I'm mostly cutting Moonshine right now which is nice and stiff but I planted a big patch of the Colorado mix which is a different species (millefolium) and more prone to drooping.

Try putting them in the fridge for a couple of hours and the stems will stiffen right up. I do this for ammi (False Queen Ann's lace)and black or brown eyed susans. Also, it helps to cut in the A.M.

Thanks Patty, I'll give that a try tomorrow morning.

Well I tried Patty's suggestion and it did seem to help stiffen the stems up. But I also noticed that this white yarrow picked up a pink tinge after being in the preservative in the fridge all day. I'm using Jeanne's recipe: 2 tblsp. sugar, 2 tblsp. vinegar and 1 tsp. bleach per gallon of water. Maybe I don't need to use so much vinegar since I'm on city water? Do you think my preservative mix is too acidic? Would this cause the color shift toward pink?