Gallon size black plastic pots for herbs?

rainypnw(7b)November 11, 2009

I'm enlarging my herb garden substantially this year - and

moving it out of the 'in ground' or raised bed sections all together,

mostly so that each herb can be placed and dealt with individually per its needs.

So I'm going to pots in an open area with full sun.

A local landscaping company has MILLIONS (probably not an

exaggeration) of standard black plastic nursery pots, straight

sided 1-gallon 12" tall variety, pretty much free for the


My question: size wise, these are perfect for all the herbs

I grow, no question. Our summers near Seattle never get

hotter than maybe 95 for maybe 1-2 weeks all summer. Most

of the time it's 70s and 80s. The herb pots (25 of them)

will all be in full sun. Will the black plastic be

detrimental for the root systems due to heat on the roots?

I'm a frugal gardener and also love the idea of reusing these

pots that otherwise just go to a landfill.


Dave in University Place

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Due to heat? probably not in your climate. Some herbs (such as those from Mediterranean climates) will probably appreciate the extra heat early and late in the season.

    Bookmark   November 11, 2009 at 2:58PM
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tapla (mid-MI z5b-6a)

"Will the black plastic be
detrimental for the root systems due to heat on the roots?"

The answer is 'yes', but there are caveats. Yes, if the plant material doesn't have enough mass to shade the container and the container is not otherwise shaded or is exposed to direct sun when ambient temperatures are high. I just talked about this a little in my last post. If you have it available, Dr Whitcomb in his book 'Plant Production in Containers II' laments the negative effects of high root temperatures and even has tested how long it takes roots to die when black nursery containers are exposed to direct sun. During periods of high ambient temperatures, roots at the perimeter of black plastic containers can die in minutes when the container is exposed to direct sun.

In many species, growth is reduced by as much as 40% as root temperatures move from the upper 70s to the upper 80s, so you can see that you have a vested interest in trying to keep root temperatures as low as possible.

I see a marked difference in summer growth between the different container types. Unglazed clay and wood are second to containers made of screen or containers that have highly perforated sides and bottoms (like pond baskets), then comes light colored ceramic/plastic, followed by dark ceramic/plastic - the worst. I probably left out some materials like pressed wood fiber, but you get the idea. Containers made of gas permeable materials have much greater potential for producing plants with high vitality levels because of the increased gas exchange in the root zone and and because they keep roots much cooler than plastic and other non-permeable materials.

Perhaps you could double pot or shade the containers if you wish to utilize them. Is sinking them into the soil (partially) an option? That would help keep the soil cooler, too.


    Bookmark   November 11, 2009 at 3:05PM
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Hi Al... hm... I'll have to think about the shading
side of it - they will be very closely packed in most
cases (mints with mints, oregano with oregano, etc.),
with pots touching in many cases - picture a 2 step
"stair case" made of cinder blocks, spanned with 6"
aged fence boards. the pots will be arranged on a lower
and upper row, side by side. Maybe a white board at an
angle in front of each row to keep the sun off the black
of the pots....

I wonder too if they could be spray painted white?? :)
Or even latex outdoor paint might work (and be a bit
more organic). I might have to try that as well.

Afterall, the pots are free - I really have no interest
in going out and having to spend $3 - $5 to buy the
25-30 pots I'm going to be using!!


    Bookmark   November 11, 2009 at 4:36PM
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tapla (mid-MI z5b-6a)

Hi, Dave. I was typing my reply above as JaG was posting his. ;o)

I have a very precious (to me) tree that I lifted from my a raised bed this spring that I'm very excited about as a bonsai prospect. I needed a large container, and it wouldn't fit in the white Sterilite container I bought for it, so I had to wing it and put it in a black nursery container. I was very concerned about root temperatures while it was recovering from what it 'thought' was a horrendous root pruning, so I used a piece of closed-cell (white) foam, cut to the same ht as the container and wrapped/tied in place around the container. It worked like a charm, and really kept the root temps down.

I know most won't see the beauty in this tree (yet), but the bonsai practitioner looks mainly at the roots & thickness of the trunk. The rest can be built fairly easily. Doesn't this tree look like the roots are ancient?

Sorry if I strayed too far OT, I thought you might enjoy seeing a different perspective of beauty. ;o) Note the cut down nursery pot it's in. I grow all my herbs in the soil you see in the pic, too, which is another reason for posting it.


    Bookmark   November 11, 2009 at 5:49PM
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That tree is MAGNIFICENT from this view!! That's the
first thing I even noticed about your post - scrolled straight
to the tree! So yes, I do indeed appreciate it - great
Bonsai candidate no question.


    Bookmark   November 11, 2009 at 6:10PM
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dickiefickle(5B Dousman,Wi.)

Use the free pots until you have a problem ,If its hot you will be watering any way or you could spray the containers themselves to cool them, be frugal,reuse

    Bookmark   November 11, 2009 at 8:12PM
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calistoga_al ca 15 usda 9

If your pots are packed together it will probably help the root temperature OK. I worked at a commercial nursery in a hot weather area and it worked OK. The reason I think it worked was because the plants did not stay there long enough to cause the plant foliage to get too crowded. I think this will be a problem for you with your herbs. Al

    Bookmark   November 11, 2009 at 9:53PM
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"If your pots are packed together it will probably help the root temperature OK. I worked at a commercial nursery in a hot weather area and it worked OK. The reason I think it worked was because the plants did not stay there long enough to cause the plant foliage to get too crowded. I think this will be a problem for you with your herbs."

Hi again Al - of course, they will be separated as the
foliage speads - we're talking gallon pots, with new
(fresh)transplants next spring (just a few mature
transplants, already in much larger pots!). Will keep
them comfortably apart - it's just the 'black thing'
I'm concerned about... but I think shading with a white
board along the 'front row' should do the trick.

    Bookmark   November 11, 2009 at 11:08PM
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Since I also live in the Seattle area I know what the many cloudless days of midsummer are like. Based on my own personal experience, if I were you I would do something to mitigate the heating effect of the sun on the black pots. In potting up my dahlia cuttings I noted that the soil temp was excessively high, the roots were failing to approach the pot walls, and the leaves were peculiarly rolled. My solution was to spray paint the pots white with a Krylon product called "Fusion". It was very effective. The walls of the pots (all 72 of them) remained cool even in midafternoon, and the next generation of leaves were normal in appearance.

    Bookmark   November 12, 2009 at 12:37AM
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I guess I'm not sure what all the fuss is :-) I've lived in the Seattle area for all my life and since I work in the nursery industry, tend to accumulate a lot of plants, all of which originally arrive in a black plastic nursery pot. Some of them remain in black plastic nursery pots indefinitely, just getting bumped up to another size as necessary. I have never experienced any issues with excessive heating. I'd have to think this was much more of a concern with areas that experience a normal, hot summer.

In fact there are some plants I wouldn't consider growing in anything other than a black plastic container - tomatoes, dahlias, cannas - anything that benefits from early, warm soil temperatures and as much ambient heat as we can manage here in the relatively cool PNW. Garden soils here don't warm sufficiently until late May, sometimes early June, to support the growth of these heat lovers.

If this was a such an issue and not mitigated by the use of a proper soil mix and attention to watering, how would those wholesale growers who specialize in containered plants manage to get their stuff to market safely? Or retail nurseries with tables and beds full of black plastic potted plants keep them looking good and thriving through the season?

My primary concern with the OP's proposal is that a 1G pot would not be sufficiently large to keep the herbs happy for very long. A standard 1G nursery pot is approximately 6 1/2" by 7 1/4" and only holds 3 quarts of soil. I'd opt for something larger.

    Bookmark   November 12, 2009 at 9:53AM
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tapla (mid-MI z5b-6a)

High root temperatures is (are?) actually a pretty big concern to the nursery trade. Nurserymen often take steps to ameliorate the negative effects of high root temperatures by grouping plants together, orienting rows of containers to take advantage of the shading offered by the plant material in the containers, and by strategic placement of shade cloth to reduce direct sun on containers when the containers would normally be exposed were it not for the cloth.

Some surprising statistics that I just left on a post at another forum site: Most people don't realize that for most plants, just a few degrees difference in soil temperatures can have a very big impact on growth. I was just reading up on peach trees & discovered that best growth occurs at soil temps between 65-70*, with growth reduced by about 40% at 75* and 97% at 80*. Roses showed a similar pattern, but actually at temperatures even lower than the peach trees. Most of the plants we grow in containers would probably do best with soil temperatures around 70* or a touch lower. It's not unusual for black nursery containers to have soil temperatures higher than 100*, even when ambient temperatures are in the 70* range, when they are exposed to direct sun and the effects of passive solar gain. This is too high for optimum vitality.

In my own containerized plants, I notice that growth stalls every summer, even in plants that exhibited lots of new growth in the spring, in those plants I've purchased and am growing on in black plastic unless I take steps to shade the container. I'm certain that is due to high root temperatures. Which brings up another point.

In some plants, sap easily moves laterally as well as vertically (think of a barber pole). In others, sap moves vertically, almost exclusively. In this later group of plants, it is as if certain roots 'feed' individual branches directly above where the root joins the main stem or trunk. It's not unusual to see container plants that are extremely one-sided, or that have dead branches on the sunny side of the plant. Often we attribute the lopsided appearance or dead branches to too much sun on the foliage, even when the plant is entirely tolerant of full sun, when in reality, the root cause (is that a pun?) ;o) was more likely overheated roots on that side of the plant.

I think that for best vitality, we should try always to keep container soil temperatures below 80* whenever we can, because we can be sure that these high root temperatures are affecting growth and/or killing roots, even if it's only on the side of the container with direct exposure, and even if the damage is not immediately observable by cursory inspection of the top of the plant.

Finally, there is always the consideration that dead and rotting roots present a convenient pathway into the plant for things fungalugly. You may see problems developing with root infections in fall that have their origins in the high root temperatures of summer.



    Bookmark   November 12, 2009 at 6:11PM
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