Actually, heat build-up would be my main concern. Opaque pots either reflect light or absorb it and turn it to heat on the surface of the pot where most of the heat is lost to the air. Clear pots allow light to pass through the pot and be turned to heat when it strikes the soil, effectively turning the pot into an insulating wall that holds heat in. The other concern, as already mentioned, would be algae growth. I've even had root temp problems with white containers that were only partially translucent. Maybe they'd be ok for shade, or if stuck in a cache pot, but I'd keep in mind the cautions mentioned here. Al

Dose of physics: Clear containers will warm soil fastest. When using black containers, the container absorbs the suns light & turns it to heat. Some of it is conducted to the soil, but more is lost via radiant transfer to air surrounding the container walls, then dissipated by convection. Clear containers, on the other hand, allow (a portion of) the light to pass through the container where it strikes soil before turning to heat. The container walls then act more as an insulator than a radiator. The difference in temperatures can be very pronounced, varying by as much as 30* F. or more (my estimation), even under normal growing conditions. Container shape also has a big impact on solar gain. A practical application of the principle is tinted glass in a car. Tinted glass represents the darker container, & clear glass of course the clear. Are you warmer in the car with clear glass or tinted glass? Answer - clear. The tinted glass absorbs light before it can strike an object inside the car & be turned to heat. The heat is then lost to the outside air, as above, instead of heating the car. So from warm to cool: clear - black - through increasingly lighter containers until you get to white, the second coolest. Coolest would be a container with a reflective surface - mirror-like or coated with aluminum foil. You didn't hi-jack the thread. It's still on topic, even if Esme didn't realize it when asking question originally. ;o) Al

I bring in all of my potted tender plants before the first frost arrives. Some go under lights in the basement. There aren't many... a few potted Crinum, a dwarf Banana tree, a few baskets of Oxalis, Society Garlic, and a few others. Annuals in patio pots are allowed to die, pulled out and thrown on the compost pile, and the pots stored in a garage for winter. Any potted perennials or trees or shrubs are allowed to go dormant, then moved into an unheated garage for winter. In early spring, I'll begin opening the garage doors during the day so they can begin to break dormancy, and once the weather stabilizes a bit, they'll come back out into the yard. I never try to save annuals or cover anything to prolong it... I've got too many other things going on to worry about a few posies that I can replace once spring officially arrives. As for the rose and perennial beds... if it can't survive with nothing more than a few inches of mulch protection, it's not for this northern garden. I'm not babying any roses or other plants, and I don't do corms or rhizomes that require digging and storing for winter! And... what about the rest of you guys? Do you run around covering everything with sheets and such? Or do you bring in your tender plants, and let Mother Nature take care of the rest?

I am rooting in a vermiculite/perlite/humus mix in the bottom half of two-liter bottles. Since the plastic is clear and lets light thru, does the root stop growing when it reaches the side of the bottle and its exposed to light? If so, does this force more roots out from the cutting, thereby creating a more substantial root ball?