soil temperature variation with depth in a bulb garden

Boyce Tankersley
Fri, 22 Nov 2013 14:14:00 PST
Hi Gastil:

To increase the probability of providing soil temperatures cool enough to
perennialize some of the Mediterranean species of tulips try planting them
on the north side of a structure and/or in the cracks between large paving
stones.  Photo's taken of Tulipa species in the wild in Central Asia
indicate a number of them grow in really rocky habitats.  I thought this
was only to reduce predation by herbivores but there is a cooling component
as well. Both microclimates (north side of a structure and use of large
stones) are amazingly efficient at reducing the temperature of the soil.
 Unfortunately when we tried cooling the soil temperatures by adding water
to the soil we ended up steaming the plants and their roots.

Best of luck,

Boyce Tankersley

On Wed, Nov 20, 2013 at 10:28 AM, Chad Schroter

> Interesting experiment, I would not assume too much about depth vs.
> location or you might miss something. I have read advice to plant tulips
> deeper to avoid excess bulb splitting FWIW.
> If bulb chill hours are similar to fruit tree chill requirements - then
> the hours at or below 40 are important. Growing at a greater depth may
> actually reduce the chilling hours since the daily lows would be averaged
> out.
> I have lived in Santa Barbara, and garden now further North in the SF bay
> area; I can grow Veltheimia (bractea) outdoors in the ground with no
> overhead protection, so I am pretty sure you should be able to also.
> Chad Schroter
> -----Original Message-----
> From: []
> On Behalf Of Gastil Gastil-Buhl
> Sent: Tuesday, November 19, 2013 6:36 PM
> To:
> Subject: [pbs] soil temperature variation with depth in a bulb garden
> I would like to determine whether bulb chilling requirements may be met by
> planting deeper when in a climate otherwise too warm. Do any of you have
> experience setting up an experiment to measure soil temperature in a bulb
> garden? I have heard from a lily grower in a cold zone in Canada with some
> good advice. My experiment is the opposite: not how warm it is deeper but
> how much cooler the soil may be deeper. Soil averages the day's highs and
> nights lows of air temperature, damping that oscillation with depth. I
> would like to know how much the soil temperature varies in my bulb garden
> during the winter growing season.
> I have 12 tiny temperature loggers, each one about the size of a stack of
> three quarters (the US 25 cent coin). I have buried them in my garden for a
> quick 3-day test to see how well this works and then plan to put them out
> for 8 months.  There are a few questions I wish to answer.
> How much is the temperature daily fluctuation muted with depth in the sand
> bed and in soil beds?
> How much is that affected by a half-inch layer of white pumice as mulch?
> Can planting a bulb more deeply simulate a colder climate?
> What is the effect of sunlight-warming on being planted near the side of a
> raised bulb box?
> And there are some practical method questions I hope to solve with this
> small initial experiment, most importantly whether these thermistors
> require more protection from moisture than their little steel cases. I plan
> to put some in those tiny ziploc baggies Dell so kindly provides with each
> bulb allotment.
> The initial test started this morning in three spots in the planting beds,
> shown here:
> Saturday I will dig them up, download their data, and check to be sure I
> understood how to program them. Then I plan to re-plant them for 8 months
> at a 3 hour sampling period. So if you see flaws I may correct in my
> experiment, please speak up. I am open to suggestions.
> Given only 12 thermistors, would you suggest it is more important to
> measure more depths or more spatial variability?
> This bulb garden is primarily full-sun, with some areas lightly shaded
> part of the day, but no significant shade.
> - Gastil
> Santa Barbara, California
> too cold for Veltheimia unprotected but too warm for most Tulipa
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