I currently have an Albizia julibrissin 'Summer Chocolate' growing in a pot on my patio. It grew well last year, and burst into life this year. Now however, I'm noticing many of the new side shoots have fallen off and the leaves are curling and falling. I don't see any discolouration, insects, or cracked stem/bark. I don't believe it's too dry between waterings, if anything too wet maybe? Does anyone know what might be wrong? I do also have a campanula planted in the bottom of the pot. Any insights would be much appreciated!
Too much of a good thing? They grow so fast, that I doubt that you can get away with re-potting less than every year. Worse, they do not want a companion plant for the same reason. There are those that will say, "put it in a bigger pot so you don't have to re-pot that often." I say, it will fill the pot with roots anyway, muy pronto, and tree will get as big as it can. That's its job. You need to bonsai it. That means reducing the roots and the canopy to keep the top the size you want. I would re-pot every mid-June after the first flush of growth is mature. Tip the pot over and knock the dirt off the roots. Remove ~50% of the larger diameter roots, which are anchor roots, and retain as much as possible of the feeder roots, -the tiny, hair-like roots. Refresh the soil with "top soil" from any big box store, commonly known as dirt. So-called potting soil has a high percentage of ground up recycled newspaper which retains water too long. Too dry is much better than too wet for Mimosa. It usually has some additives, ferts & other growing helpers, but the last thing you want is more growth. If all you do is re-pot in new dirt every year, it will grow in a controlled and controllable way. You should trim the top kinda on a continuous basis. It wants to grow tall. You should imagine a shape of the whole canopy and the individual pads or clouds. Anything that is going to stick outside that image in your mind's eye should be trimmed off back to a least a bud on that stem/twig/branch. You can also limit, somewhat, the growth of a cluster by removing the tip of the new growth. For example, as the cluster is unfurling, remove the two end leaflets of the terminal leaf, leaving all the remaining leaflets and leafs on that stem. (one leaf = ~16 to ~20 leaflets; with ~9 leafs on one stem) This all sounds more difficult than it really is. You've chosen to have a difficult subject; beautiful and high-maintenance. You can do it! After it has lost all its leaves each autumn, trim the whole tree back (always to at least one bud on a stem) such that, after it unfurls next spring, it will be within the imaginary canopy in your mind's eye. You, can do it!
Thank you so much for such a detailed response! I'll be sure to move the campanula to it's own pot when I repot and do the root trim in phase one of the recovery.
Probably winter damage, the purple forms are less hardy. But check that the potting soil hasn't become decomposed and soggy, something that the melting of all that snow we had earlier would not have helped.
