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Re: Mel and black and clear plastics

Square Foot Gardening List - http://www.flinet.com/~gallus/sqft.html

It has been quite a few years since I have used plastic as mulch.  At this
time of year, the daytime temperatures in the low desert are still warm
enough to grow a nice crop of weeds under clear plastic and also to create a
malodorous anaerobic smell with black plastic if used over a large area.
But both clear and black plastic have other uses from now through February.
As an aid to seed germination, I usually use black plastic to cover the
seeded area after planting as usual.  The difference in thermal conductivity
between clear and black is not significant (expressed in units of heat per
unit thickness, e.g., BTU/inch in my old Materials Handbook and
Thermodynamics textbook).  Most weeds require light to germinate but most
vegetables don't so that using black allows vegetables to germinate but
suppresses weeds.  But lettuce and most flower seeds require light to
germinate so it is necessary to use clear plastic.  In either case, I cover
the sqft bed with a frost blanket at night to help retain heat.
Olin - Zone 9b
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Original Message
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From: Margaret M Daniels <daniemm@mail.auburn.edu>
Date: Tue, 17 Nov 1998 09:12:27 -0600 (CST)
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>I have been wondering about Mel's statement in his book (p.119) not to use
>black plastic for temp raising. He says it's been proven that the soil
>temp under it is not raised as previously thought,that only clear plastic
>can raise soil temp,that black plastic creates a lot of heat but stores it
>in the plastic itself, and that that heat is then radiated upward at
>night,warming the plants but not their roots.Many of my organic gardening
>books (e.g. the ones published by Rodale), with later publishing dates
>than Mel's book,still claim that black plastic raises soil temp,tho not
>as high as clear plastic does, and
>is better to use for this purpose in southern climes (I'm in Alabama) so
>that the temp isn't raised excessively. Does anyone know the source of the
>"proof" that Mel refers too,or know the definiive facts of the matter?

>Second question:is there any harm in transplanting my corn salad and
>miner's lettuce (mache and claytonia), both very cold hardy, into black
>plastic now? We're still having day temps around 70 and nights in the
>50's. Soil temp about 70. I wouldn't want them to fry while sitting on the
>plastic-I don't know if it would make a difference if they're growing thru
>slits in it, or if I enlarge holes around them so they don't touch the
>plastic at all. They receive close to 3 hours of direct sun,12-3 p.m..

Third question. I've made a large version of Mel's sunbox. For warmth
later in the winter, I was thinking of putting plastic milk jugs filled
with water inside the box.. Similar to my first question, some books
recommend painting these jugs black, so I was wondering if the clear jugs
or the black jugs would raise the temp more.

Hope someone can help me!
Thanks,
Peg Daniels
daniemm@mail.auburn.edu


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