Filled beer bottle wall for thermal mass?

Hi all, first time poster!

I'm looking for some advice on solar building in the garden to help me keep an Arbequina olive tree alive in my zone 7b mini-farm. By all accounts they are the hardiest olive IF you can keep their roots from freezing.

I have a nook on a steep south-facing slope that's right next to a shed that blocks the North winds. I was going to build up the bottom of the slope to make a pocket planter for improved drainage; inspired by Earthships I decided to reuse our old brown beer bottles to build it. I thought the dark color would collect solar energy, and the air held in the bottles would insulate the trees' root mass from the cold.

Then I wondered about filling the bottles with water first. Wouldn't that effectively increase the solar absorption, like big water tanks in greenhouses?

However, since the beer bottles would be horizontally stacked I would have to cap them (we homebrew, so we have caps & equipment). Would this last, or would they eventually leak?
My husband also brought up the potential for the freezing water shattering the bottles in winter. What if I filled them only halfway full, is that enough room for the water to safely expand as it freezes?

Anyway, hoping someone more experienced in passive solar building can tell me whether filling the bottles is at all worth the extra risks and labor.
Thanks guys!!

Yes, more mass can absorb more heat. But this won't be the only physical phenomenon at play. There is also the conductivity of materials to consider. Air is a order of magnitudes better at insulating than water (and ice), so having that water nearly exposed to the winter (though a thin layer of glass) would actually act as a thermal bridge to planter and root mass, allowing the heat stored in the planter to be conducted back into the winter air much more quickly than if there was an air pocket.

The planter wall will already be interfacing with the mass of the earth, so the additional mass off water in bottles would be inconsequential, what you are really after is insulating the planter from the winter. If you feel you need solar gain, consider making the front of the planter a Trombe wall.

Of course, that makes perfect sense! Hey, it's easier too. Thanks so much for your reply - sorry I didn't get back to you til now!