Posted: 05/07/2015 at 20:52
Rather more elaborate approach, but it works for me:
After flowering, cut off dead flowers and any stem die-back.
Get a bucket of water.
Pick a piece of rose that's about 12" long. If you leave a couple of bud points behind you'll get new branches there next year. If you leave a half-inch stub, you get one new branch replacing the severed one. Cut off the selected piece and immediately put it in the water so the cut end is submerged.
Fill a large pot with compost. You can mix in those mycorrhizal fungal spores if you want. Poke a hole in it with a stick. Drop a little rooting hormone into the hole if you want.
Take all the leaves and thorns off the bottom part of your cutting, lift it out of the water and put it into the hole in the compost. Pack the compost down around it. Water it, to wash out air bubbles.
Now you've got something like what Bob describes above.
At this point, your cutting is losing water from its leaves but at least it's not sucking air bubbles up its stem in the process and it should have a good amont of water in its stem. It's got a problem: limited resources and limited ability to get more. It's living on what's stored in the stem, losing water from its leaves, using stored resources to produce roots with which to get more resources and relying on whatever leaves it has to give it energy. If it dries out, it dies. If the soil is too wet, it rots and dies. It can't take water up as fast as it's losing it. The race is on!
Here's where it gets a bit elaborate, trying to make the race easier. Put a little dish of water (sink a 1l supermarket soup pot into the compost, for example) in the pot. Stand short canes around the edge. Put a clear plastic bag over it (the RSPB Buggy Nibbles come in a sack ideal for this job) and tape it to the edges of the pot. The canes hold the bag off the cutting, so it won't rot from touching it. The bag keeps humidity in and the pot of water keeps humidity up, so the cutting's not losing water as fast. Being in a pot not the compost means that water isn't going to make the cuttings rot.
I second the warning not to expect 100% success ... but I've had 100% success with autumn cuttings that spent winter in these individual improvised greenhouses against the south end of the house here.
If you want to go OTT with the support, you can cut a tiny hole in the bag, put a drinking straw through it and blow warm, moist, CO2-laden air into the bag every morning. Just point the bottom of the straw at the water pot so you won't drool on the cutting.
Yeah, that ... is probably going too far. Honestly, I'd like to know whether it helps, but to test that we'd need a lot of windowsills and a lot of pots and a lot of compost and a few HUGE roses from which to take the cuttings (because it's no good testing it on cuttings from 50 different roses, is it?)