I agree with Kim.

Black plastic pots. Love 'em.

Not pretty, but very useful.

JEri

Oh wow! Thank you so much for the responses. I would never have thought of the clay retaining heat for so much longer. Of course it makes sense after reading the explanation. I had always thought of clay as being cooler because water evaporates through the clay. Again, though, that depends a lot on keeping the clay pot well watered during hot days which can be a challenge.

Also, clay is cooler in cool weather for the same reason. So if I want my plants to be warm at night, too, then black plastic is the way to go because they'll be better insulated.

I agree with Kim. I added 26 roses this year, and I put them in pots first: the styrofoam & insulated pretty ones didn't do as well as the cheap plastic pots. The ones in plastic grew twice as fast as the styrofoam, so I was able to put their bigger rootballs in the ground 4 months earlier.

The plastic pots don't cook the rootballs unless they are sitting on hot cement patio, full sun, above 80's for weeks .... by that time it's obvious with the fast-drying, watering everyday ..... to signal it's time to put in the ground.

well I also use black plastic but I have various ways of doing this. I grow many bell peppers in large plastic pots in the greenhouse but, because the plastic absorbs heat so quickly (which bell peppers do not like around their roots), I use a smaller pot inside a larger pot with layers of newspaper between the two pots. I also grow citrus and peach in large plastic pots....which are thern placed into bigger terracotta pots. The plastic stops the huge water loss from clay while the ceramic creates a kinder environment (here in chilly damp England) as well as looking much nicer with lemon trees and dwarf peaches.

I currently have a pot ghetto of about 75 in black platic pots and they'll be in those over winter. I've even gone so far as to buy them used on Ebay!

Tammy

You also can "plant" the pots in soil temporarily for better insulation.

Bellegallica: I hoard plastic pots, and friends hoard them for me -- not only the black ones, but the dark green ones, brown ones and the maroonish ones. They're cheap (as in free, usually), easy to store, durable (some more so than others) and almost infinitely re-useable. Who DOESN'T love 'em?

However, I don't agree that plastic is superior in all ways to clay. When I've potted-up multiples of a single variety -- some in clay, some in plastic -- I've found that the ones in clay generally develop more quickly, both above and below the soil. Try a test of your own to see if you observe the same difference.

If, during the course of a summer day, one were to go to the trouble of monitoring interior soil temperatures in the two types of pots, my strong hunch is that the soil in clay would consistently be cooler than the soil in a plastic pot of comparable size. Plants in clay do dry out faster -- not only because of heat, but due to the porosity of the clay and the attendant higher rate of transpiration. The more moist the soil, the more heat it will retain under summer conditions.

IMO, the fact that plants in clay dry out faster is a good thing. Clay makes for better aeration of the soil which, in turn, promotes good root growth. Throughout the summer months I have to water everything in my pot ghetto frequently anyway, so the fact that the plants in clay get thirsty more quickly makes very little difference, time-wise or water-wise.

There are a couple of independent specialist nurseries near here that whitewash the exteriors of black plastic pots to reflect heat. I've considered attempting that, but haven't (and probably won't). You're probably correct that black plastic will absorb more heat from winter sun than clay. How long that heat is retained, though, likely depends on the gauge of plastic used in fabricating the pot.

This is giving me a lot to think about and to experiment with.

The pots will be sitting on the ground, so I don't have to worry about heat coming up from there.

I've slipped plastic pots inside larger clay ones, too.

I've heard that it's almost hard to over water a plant in a clay pot because it transpires. On the other hand it makes it harder to keep it watered during the hottest times of the year. (I've also seem to remember reading that roses, as plants go, transpire a lot themselves.)

What windeaux wrote echoes what I've read about plants in clay pots during the winter--that you should consider them being exposed to conditions a zone or two lower than the zone you're rated for, since the clay will breathe and cool the roots even more.

What I'm getting from this is for my purposes over this winter (or what passes for winter here) is that I should use the black plastic. The clay would retain heat longer, but once it's gone, the roots will be cooler than if I had them in plastic.

I'm definitely going to experiment, though.

This is also making me think that when I'm trying to grow a plant that would like to be a zone or two cooler, it would help to put it in clay--perhaps a size or two larger than normal--keep it well-watered, but also shield the pot from the direct rays of the sun by putting other pots or plants in front of it or around it??? That way it wouldn't heat up, and would keep the roots cool through transpiration?

I don't know. Just thinking out loud here.

Bellegallica -- I think your decision to experiment is the very best course of action. We've all learned thru trial and error that what works like a charm for one gardener might spell disaster for another. Still, we'll be interested in knowing how your experiments turn out, so please remember to report back, okay?

Sure thing, windeaux. I'm not sure how successful an experiment it will be. It will be different roses, so it would be difficult to tell if differences in growth were due to culture or to the variety.

What made me think more about the cooling properties of clay transpiration was that I have 'La Reine' on order, and my reading tells me that one might appreciate being kept cooler, if possible.