I have just planted two magnolias in my garden (1.1 hectare). They are about 3 feet tall each and were in plastic pots when I bought them at a local nursery. I planted one of them on my patio in a 3.5 feet x 3.5 feet square space I had provided on my concrete patio. I am aware that when (and if) it achieves full growth, it may damage the concrete slab but I doubt that I will be alive to see it. My first question is, would the concrete slab act as mulch, keeping the roots damp and moist? The slab is about ten inches thick.
The second question: The soil under the concrete slab is extremely compact and rocky. Therefore, I planted it deep providing a lot of space around the roots and added a lot of manure and compost to give the roots a healthy start. What are its chances of survival? Other factors: Northern exposure, cold winds in the winter (temperature sometimes drop below 0), plenty of rainfall in the winter and almost 3 months’ summer drought (watering in the summer is no problem as I have a well and city water).

Thank you.

You don’t specify what kind of Magnolias that you planted. There is the Southern magnolia that comes from the SE United States. It needs water all year and can’t handle extremely cold temperatures. It has many surface roots and its roots cause damage. The chinese ones are deciduous, also require summer moisture.