Q: For the first four years I had a very nice weeping cherry: all the branches wept downward and all of the flowers were pink.
In the spring of 2000, after flowering all pink again, new branches began to grow upward. They grew very rapidly and I tried to make them weep by hooking bird feeders and other heavy objects on them. It didn’t work. I tried to cut the top out but the new growth still grew up. This spring when it flowered, the weeping branches were pink, and the erect branches produced white flowers.
A: It helps to understand how a weeping cherry is produced in order to see what should be done now.
Your weeping cherry is actually a grafted plant. Twigs from a weeping form of cherry (probably ^Prunus subhirtella ‘Pendula’) was grafted onto the nice straight trunk of a non-weeping cherry (probably Mazzard cherry). The bottom stem is called the rootstock. The weeping limbs grew longer under the care of a wholesale nursery and made a very attractive plant. You purchased and planted the tree and everything looked normal for four years.
However, a graft disrupts the flow of nutrients back and forth from the rootstock to the leaves above. The interference makes nearby dormant buds hyper-sensitive to hormones that initiate sprouting. One or more of the dormant buds of the rootstock cherry sprouted last year underneath the weeping part that you like so much. Those sprouts became vigorous erect limbs and headed for the sky through the weeping limbs.
The difference between the rootstock cherry and the pendulous cherry explains the contrast in flower color. The cure for your problem is simple: remove the upright limbs cleanly at their base, leaving only the weeping limbs.