My Acer has started to wilt. I'm confused as it's well looked after. Could it be because it has been so mild for the past few weeks and now very windy and cold? It has good drainage as I put stones in the pot before we planted it.
Can I save my favourite garden plant?
The wilted part is not a good sign at all. Drainage HAS been compromised with the rocks at the bottom of the soil and above the drainage hole. The tiny pore spaces in the soil above the rocks with humongous pore spaces have to become overly saturated before water is able to begin to move into the rocks. Bad drainage.
I would repot your tree with fresh potting soil, no rocks and definitely prune that entire right truck off. When you repot, get a good look at those roots. If you and you probably will, see browned and mushy roots, pull as much as you are able gently from the entire mass. That entire branch of your maple is dying. Could be a disease but if you repot, remove some of the old over saturated soil, stay in that same pot (before putting new soil and the rest of that tree in the pot bleach and scrub and rinse very well). You might be able to save half of your tree which is just fine. If there is a disease such as...well, there is nothing to be done. Right now mitigate for root rot because of saturated soil. Rocks and gravel at the bottom of the soil is so ENTRENCHED in our thinking it is a tough one to dispel. Causes a perched water table. Find Nial's answer from another question about this very subject. Don't feel bad! Make sure your other plants have JUST potting soil, not garden soil in their pots. Raise the bottom of the pot off the ground, surface and that will enhance drainage by twice! They make 'pot feet' they sell that look like lions feet or frog feet or fish tails. All you need is quarter inch ceramic tiles or pieces of tiles to do this.
When you prune, use alcohol to clean your pruners. Clean again if you try to prune anything else with those pruners (go with bypass not anvil). If this is a disease, there are at least two and you do not want to pass this disease to any of your other plants. In fact do the repotting on newspaper and carefully collect any debris, the branch bits of soil and dispose or burn. Do not put in your compost or dump the soil in your garden. Just in case. If the rest of your tree starts to wilt, let us know and we can suss out the diseases you might be witnessing. Otherwise, got my fingers crossed!
I thought you might be in the UK from the description of the weather - yes, it has been exceptionally warm and sunny for a few weeks, warmer than Spain, then this week, temperatures dropped by over ten degrees C even here in the south, with lots of wind and, in some parts, plenty of rain. Those leaves on your Acer will have opened up during the very warm weather, and yes, they will have felt the shock of the sudden and dramatic change in temperature, particularly at night and if you're north of Watford. However, it might also be that the pot isn't large enough for the roots any more, so it's worth turning it out and just checking whether its rootbound or not - if it is, a bigger pot is called for. If it does need repotting, use John Innes No 3 instead of multi purpose to fill in round the rootball, and take the opportunity to inspect what root material is visible for signs of rot or infection. Because the weather's been so random, that's hopefully the cause of the problems you're seeing, because the other possibility of verticillium wilt is much worse, but time will tell.
If it remains cold and you get heavy frost at night, you might find the new leaves shrivel and drop off, and it might be necessary later to prune back the top to encourage new growth if it dies back, but see how it progresses. Live wood on Acers should not be pruned at this time of year because of the risk of excessive bleeding, so best to wait and see what happens - if the top stems die back, you can then cut them back to live wood.
