Question
Is it possible to root a cutting from a very large, 12 year old flowering magnolia tree?  If so, how should it be done?

Answer
It is possible BUT will take some time.

Most propagation is done through seed, cuttings taken in the summer, or from grafting. Cutting grown plants are vastly superior to most seedlings because they begin flowering the first year or two after propagation while seedlings may take 15 to 20 years to bloom. Rooting magnolias is not easily done. Even with intermittent mist systems and rooting hormones, the percentage of successfully rooted cuttings is often very low.

For cuttings, you need bottom heat below and automatic mist spray above, although there are growing trays that are very useful for some plants and quite fun to work with. After all, even if you do strike some magnolias that will grow from cuttings, you will have to wait a very long time before your plant equals the one you can buy from your garden center. If it is a variety that will actually grow from cuttings, then the chances are that when you select it from the nursery or garden center, it will very likely be in bud, or at the very least planning to flower the following year. Your homegrown magnolia, on the other hand, will be about three years behind.

For those who are dedicated to "doing it yourself' and are not bothered about the timeframe, this is what you must do. In early mid-summer, when the new growth has hardened a little, take a cutting of new growth where it joins last year's hardwood. Remove all but three or four leaves, then reduce the remaining leaves to about half their size. This prevents excess water loss, and also enables more cuttings to fit into the tray, which will have been filled with a suitable commercial propagating mix, watered and firmed down. Dip the cutting into a rooting hormone powder and place in the tray. Keep damp. Wait.

When the cuttings are rooted, and this will take between three and six months, pot the plants and place them in a shade house or such a shelter, until they have recovered from the shock. When they start to make new growth, plant them in the vegetable garden or other cultivated space and let them grow on until large enough to be planted permanently in the garden. This will take at least a year.

The period between sowing of the seed and germination may be as long as 18 months. Magnolia Grandiflora trees grown from seed may take from 15 - 20 years to produce a blossom, while trees that are grafted bloom much sooner. Please keep in mind that trees grown from seed may not be exactly like the tree the seeds came from due to haphazard pollination.

Seeds should be sown in a fresh state, and not allowed to dry out. Remove the seeds just before the pod bursts open or immediately after.

Remove the red-orange coating. This can be done very easily if you soak them in water for a few days. If, when put in water, the seeds float on the surface, they have gotten too old.

Squeeze out the hard seed and wash them in dish washing liquid to remove the oily coating that prevents them from absorbing moisture.

Sow in a light compost of two parts peat, one part loam and one part sand. Don't let the seeds dry out. Cover with 1/2" of compost. Cover the tray or pot to maintain moisture and protect the seed. They should germinate in about four weeks. Pot them after about four weeks, making sure the roots don't dry out.  

Now after saying all that my feeling is it is better to purchase another magnolia from a nursery that way the tree will bloom soon and will be healthy. But if you really want a tree from a cutting or seed it can be done.