watering the sand between pots in a plunge bed

Rodger Whitlock totototo@telus.net
Fri, 01 Mar 2013 12:06:54 PST
On 28 Feb 2013, at 18:05, Gastil Gastil-Buhl wrote:

> Hi PBS folks with plunge beds,
> Should I only water the pots or should I also water the sand between the
> pots in a plunge bed? 

When I had plunge beds, I just turned the hose on them, using a good rose, so 
everything got watered, bed, pots, everything. [Nelson appears to no longer 
make the watering roses I prefer. They have a baffle in them to break the force 
of the spray.]

> The between-pots sand is as coarse as sand gets before it gets called
> gravel. 

That may be a mistake. As far as moisture management is concerned, plunge beds 
operate via capillary attraction, and fine sand has a greater moisture holding 
capacity. It's also important that the sand be in direct contact with the soil. 
Thus if a pot is overwatered, the excess is wicked away (via capillarity) into 
the sand bed and thence into the surrounding soil. If a pot begins to dry out, 
the sand and surrounding soil act as a source of moisture. The overall effect 
of a plunge bed is to greatly even out conditions in the pots, which would 
otherwise vary between flooded and desiccated.

When I had plunge bed, I always used red terra cotta pots in them with no 
drainage material in the bottom, again in order to maintain that all-important 
capillary contact between potting compost, sand bed, and surrounding soil.

Note that if you don't have plunge beds, terra cotta may be counter-productive. 
I remember Alberto at the 1993 Western Study Weekend in San Mateo, California, 
remarking that he grew his bulbs in (unplunged) plastic pots; terra cotta would 
have been subject to evaporative cooling, which would have kept the soil too 
cool for the bulbs he was growing. (Alberto may have since changed his 
practices, and I hope he will speak up if my account misrepresents his point of 

Rodger Whitlock
Victoria, British Columbia, Canada
Z. 7-8, cool Mediterranean climate

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