adriennemb, mortar on the bottom of the pond would hold up as long as anywhere else assuming your pond doesn't freeze solid. Cement isn't affected by cold temps, only ice. It doesn't release chemicals at least at any levels any reasonable person would be concerned about. In a freezing climate you do have to design for the top foot or so, the depth you expect ice. You have all the normal options, bare liner, dry stacked rock, etc. It you want to cover the liner my suggestion is to make cap rocks out of mortar. These can be flipped out of the pond in the fall if you like or left in the pond. Since they just hang there any ice behind them push them up a bit but no harm. The premixed mortars of today don't really fail, at least in our lifetimes. If you kept your pond in the acid range, say 6 pH or less, the acid would attack the cement and slowly dissolve it. Cement makes a poor pH buffer so you probably wouldn't be able to measure any raise in pH using normal pH tests. And in 6 pH and less the life of the mortar would be reduced to maybe the 20 to 30 year range. If the pond is in the 8 pH and above range there is very little affect on cement and you can expect it to last in the 50 to 100 year range. I got this info from studies of concrete water and sewer pipes. Here's a link to my web page on effects of cement in a pond.

Greenlydia, you can work with round rocks if you stack the second layer in further than the first, and to the extent that the rocks lean, make sure they lean into the side of the bed. In effect you are not stacking the rocks on each other, but rather embedding them into the side of the bed. I personally hate the idea of glueing and mortaring rocks together as I am prone to changing my mind about things, and the rocks can always be used for a different application if they are merely propped in place, but if they have mortar all over them they are hard to dismantle, and hard to reuse. Quite aside from which, the mortar doesn't actually make the thing any stronger. It just provides a false sense of security so that most people who use it are less careful about placing rocks so that their weight holds them in place. Both types of retaining can fail, of course, but if an unmortared rock rolls out of place... well, you roll it back, or maybe you repack the section where it came from. If a mortared rock breaks out of place, well, there goes the weekend fixing that. Regarding water, my own strategy with beds surrounded by rock retaining walls is to keep the soil level in them flat, not mounded, because I don't want water running down between/inside the rocks taking away the soil that supports them. I want the water soaking in where it lands.

I am not anti-rock. There are a lot of variables, koffman, such as the heat density of the rock the transfer qualities, size etc.. As far as pushing through the rock, that is handled by keeping the rock away from the crown of the hosta. Since you are in 7b this would be a decent zone for growing hosta. Adding rock, depending on how much sunshine the rock will get and your exact conditions, elevation, size of the rock, material of the rock, etc. you would increase your 'zone' slightly or maybe a one half zone (????). I like rock, big rocks, I don't like it as a mulch. However, personal preferences aside, Rocks are an incremental increase in your 'zone'. Will it slip you from 7b to an 8a? Perhaps if you are close to the line. In any event in your location I would look at any increase in heat as a negative for growing hosta. The more rock, the bigger the rock, the greater the heat coefficient the greater the solar heat gain. In cold climates using rock would extend the season, maybe extremely slightly, maybe noticeably; only a real life test would tell. Passive solar using stone or concrete to store solar heat in homes is very effective and was even used by the Anasazi Indians thousands of years ago who carved out their homes in cliffs facing South and such that the sun shone in to heat the dwelling when the sun was low in the winter and the dwelling was shaded when the sun was high in the summer. I wouldn't worry about pips getting through as long as you don't cover them with heavy rock. I have had hosta that,over many years, grew through landscaping fabric and squeezed the fabric into a solid block of plastic with the hosta eyes growing and thriving through the holes they poked through. Unbelievable!!! If you like a rock bed and are willing to trade off less optimum growing conditions for the hosta, then that is a personal choice that no one can say is right or wrong. In any event rocks can be removed, albeit not easily in many cases. Jon PS, a few years ago I watched as a contractor used a wet/dry vac to vacuum up pea stone behind a rock wall. The stone was for drainage behind the wall, however it kept falling into the hole as the contractor was digging it out for a deck footing. If removing the stone you may put in and eventually get tired of becomes necessary this might be a good trick to remember. OK, I have shared enough of my ramblings and potentially useless thoughts.....for now.

I personally hate fabric weed barrier for a garden. After hours of having to pull it out from old garden beds. The problem is that more aggressive weeds still grew thru when the seeds land on top, so it was all interlaced. Like a woven together weed carpet. It was such a PITA to tear out. Others might feel differently. From what I have understood is your best defense against weeds taking over, are strong healthy plants that are claiming the spot already. When we started planting again, we layered thick cardboard under mulch in between the plants. The cardboard breaks down by the time the plants grow to fill in the spots. I also go by the rule a 50 cent plant in a $5 hole is better then a $5 plant in a 50 cent hole. Work your soil a bit and take out any big chunks of weed roots you can. Be careful of spreading any seeds of the weeds you are pulling. IMO a little extra work now goes a long way later. That area is a little narrow, that is your only limitation. here is a photo to show some of the hosta and fern choices. You can combine a few of the smaller ones. They also have coral bells in the front, which are nice. Native to north america, if I remember correctly. https://www.pinterest.com/pin/111745634477225188/