Grow bed on driveway

[Graftolstoy]

This is my first time posting, but I have been using this forum extensively as I prepare for my first full growing season in bonsai. I am a beginner, so I apologize for any stupid or obvious questions. At the moment, I only have prebonsai as well as some seedling that I will grow out, so I am planning to build a grow bed. Unfortunately, due to space limitations, the only possible location is on top of an unused part of my driveway. I am planning to make a 4x8 bed that would be about 18" deep. However, I am worried that the heat from the asphalt might burn my roots. Is 18" too shallow? Would it be better to place a layer of astroturf or some other ground covering on top of that portion of the driveway before I begin my construction, or should I raise the entire bed off the ground by a foot or two off the ground?
As a second question, I am also planning to make cuttings, and was wondering if I could actually use the asphalt to my advantage when propagating? Would creating a separate shallower section of the grow bed (possibly with a different substrate) closer to the actual asphalt allow for natural bottom heat that would help the root development of my cuttings, or would the temperature swings and excess heat outweigh any potential benefits. Thanks again.

 

[jeanluc83]

I think you are thinking about this wrong. The reason the asphalt is hot is because it is a dark color so it absorbs heat from the sun. If it is not in the sun it will not absorb heat so it will not get hot. It should not effect your roots.

Depending on the location the area around your bed will be warmer than if it was a planting bed with lawn surrounding it. This could be an advantage and a disadvantage depending on what you plan on growing. You will need to be careful about not letting it dry out since the asphalt blocks any moisture from the ground.

I don't see why it wouldn't work.

 

[Anthony]

Here is what we call down here a ---plant trough.
It is about 10 feet long x under 2 feet deep x 1 foot wide.
Made of concrete blocks, plastered and painted..

Sits on a concrete slab and has drainage pipes at the back.

Soil mix was a basic loam and has been enhanced to a Bonsai
soil mix, with 8 mm silica based gravel. For better drainage.
Once in a while a thin layer of compost is applied to the
surface.

It is in the driveway, next to the asphalt and faces west.
It gets hot as explained by Jean-Luc.
Easy to weed, just sit and clean.
Now testing J.B.pines in colanders in the plant trough.
Image below,
Good Day
Anthony

 

[Graftolstoy]

Thank you both for the helpful replies. I was a little a worried about the heat from the surrounding areas of the driveway, but now I am less stressed about it posing a serious problem. I might make bed 2 feet deep rather than 18" (like in your plant trough, Anthony). Given the depth, I am also planning on planting most of my trees on wood boards/tiles.

Jean Luc, you mentioned that the slightly warmer soil temperatures might be a problem for certain types of trees. Are there any in particular you had in mind? I have found quite a bit of information about the preferred temperatures and horticultural zones of trees, but haven't seen as much research regarding optimal soil temperatures for the roots of particular species of trees.