If there’s no root, then I think all you can do is put it in a large vase and bring it indoors - then at least you can enjoy the colours for a bit longer.

try taking lots of soft wood cuttings. These are the stems that have not yet started to become slightly woody.
the RHS suggest thi method and I use it myself.

◾Using a sharp knife trim below a node to make a cutting about 5-10cm (2-4in) long
◾Remove the lower leaves, pinch out the soft tip and dip the base of the cutting in hormone rooting powder or liquid
◾Make a hole for the cutting in a a container of cuttings compost using a dibber (a clean blunt stick) and insert the base of the cutting with the first pair of leaves just above the level of the compost
◾Label the pot and water it from above to settle the compost
◾Place the pot in a closed propagator case with bottom heat of 18-24C (64-75F). Covering with a plastic bag and placing somewhere warm will suffice if no other equipment is available, but remove the bag to ventilate the cutting at least twice a week for 10 minutes.
◾Cuttings should be placed in good light but not direct, scorching sunlight. Covering with fleece will help diffuse bright sunlight
◾Ensure the compost is moist until the cuttings are well-rooted which takes about 6 to 10 weeks
◾Once rooted, harden off the cuttings for about two weeks and pot them on individually. Covering with fleece or gradually increasing the ventilation of plastic bags or propagators will allow the soft leaves to develop a robust water-proof cuticle so that they can survive in lower humidity environments
◾Remove any dead, rotting, dying or diseased material at least weekly

You may be lucky that some of them will root.
If there are any dormant buds left at soil level they may also grow come the spring.