How to heat a forest garden?

I know something about common ways to heat a garden, creating sun traps, hugelkultur, ponds, rocks etc. Is there any other idea, maybe still not tested, or tested and working?

My first thought would be to place additional thermal mass. Maybe you have rocks, maybe you don't. But you could always get some old water heater for free? They are commonly used as solar collectors, painted in black and placed on flat roofs. So they could be also filled with water, sealed and left in the garden? Since they are pained in black, they would absorb sun and heat water, and release heat during the night, wouldn't they?

Second thought is to dig a well. Left it empty, in the middle of the garden. Earth is warm down there, so will the air be. Since warm air raises, it will come out this well all night? I can not estimate how significant this could be, has anyone tried? I know it is not so easy to dig a well, but if it could heat the garden, this would be permanent solution, would work when there is no sun, and it does not take any space, can be placed in least productive shade.

Of course I don't think that only one technique is enough, I would use this only as addition to proven techniques. Frosts bothers me a lot, there are a lot of plants that I can not grow because of them or they will bring fruits only after warm springs, so garden heating system would be a miracle to me

I think the painted water heaters is a good idea and should help as thermal mass. 

One type of sun trap is a south facing parabola of light colored wall (rock, brick, etc) that reflects and focuses light on a central pillar (dark rock, water filled barrels painted black, etc).  That can raise the temperature significantly in the center.  If tall deciduous vegetation is used inside the parabola, it's growth will "turn off" the suntrap once the growing season gets going and it is warm. In cold summer climates, one might want to grow shorter plants to keep the temperature higher and reflect more light on the plants.  Tall evergreen vegetation behind the parabola wall (north side) provides a wind break so that more heat is retained. This can create a nice warm pocket during leaf-off season and might add a zone or two of protection against the winter minimum temperatures, and an earlier spring.

For visualization and calculating the curve:
http://www.jc-solarhomes.com/fair/parabola20.htm

In that diagram, sunshine would be coming from the top; ie, south is up, and it is reversed compared to normal expectations.  The curve can be made steeper or shallower, depending on the design objectives.

there are a lot of ways but one way to help your fruit trees is to put them up on a mound, some of mine are up on hugel mounds and they do quite well. Also if you have some large rocks available you can place them near a more tender plant..

windbreaks are a wonderful way to increase yields from a warmer garden..as well as fences if windbreaks aren't feasible..or walls.

I have planted windbreaks in lots of areas on our property and our neighbors, bless their hearts ..have planted a windbreak for us on their property as they are west of us and winds come from the northwest here most of the time.

our main food forest garden is 20' east of their property line so the evergreens the planted inside of their property line will provide for our garden when they grow..I'm so blessed.

most of our plants right now are still in their younger stages..as we had a fire and had to relocate our food forest gardens