soil temperature variation with depth in a bulb garden

Gastil Gastil-Buhl
Wed, 20 Nov 2013 18:28:08 PST
This mail list is such a great resource. These considerations are the feedback I had hoped for. Thank you Chad, David, Leo, Alberto, Iain and Richard for looking over my tentative experiment plan. Also thank you to those of you who responded off-list. 

Chad mentions the planting of tulips deeper to avoid excess bulb splitting. I have seen this with Narcissus tazetta and Scilla peruviana, which do not have chill requirements beyond what is natural in my climate. So possibly the effect of too-shallow planting is the variation in temperature more than the mean temperature. (Or it could be moisture, or light, or...) It is a good example of depth affecting bulb growth though.

Several mentioned fruit tree chill requirements and this I find a useful concept. Some of the terminology used in that field may help me research that concept applied to bulbs. I could apply the same kind of integration to the temperature data I collect as the fruit tree growers apply to their air temperature data. 

Microclimate is not just another word for 'zone denial'. Chad mentions he grew Veltheimia bracteata in the same city I live and I do not doubt this. Just 2 miles from my house, toward the beach, there is never frost and there is a true Sunset zone 24 climate. Near the beach they can also grow bananas. Not so in my neighborhood. When I used to try to grow V. bracteata in the ground it frosted to mush even when I covered it with little tents made of pillow cases. The bulbs survived, but never bloomed until I moved them to pots brought indoors on frosty nights.  My neighborhood is a cooler microclimate. 

Alberto makes an excellent point that the Dutch bulb industry surely has already answered these questions. I obtained a textbook from the library which I am hoping reveals some of that research. I have not read it yet.

Title:  Ornamental geophytes : from basic science to sustainable production
Author: Kamenetsky, Rina
Publisher: CRC Press/Boca Raton  FL
Date: 2012

As to covering the soil in summer with shadecloth, I agree with Alberto. Not only does it cool the soil enough that I can grow lettuce in August, but also it makes digging up the winter-growing, summer-dormant bulbs a far more pleasant task in the shade. However, shading the bulbs in winter would deprive them of the sunlight their leaves need.

Iain asks if my question is about chilling bulbs in a Mediterranean climate. Yes, but it is not just a question of what does a particular species need. It is more a question of what is the climate in my soil here as a function of depth. Then, once that is known, I hope to select species that match that climate. I do expect not just different species to have different needs but also different populations of the same species, as Richard points out with Lilium columbinum. I also am guessing that for some species it is the integration of hours below some threshold coldness, such as fruit trees need, while for other species the requirement may be to remain below some maximum tolerable daily high temperature of the soil. 

Iain's "How long is a piece of string" is a succinct way of saying unless you are clear what you are asking, you cannot get a clear answer. I will not be able to answer how well any given species grows as a function of depth from just the temperature data alone. I realize this is just a first step, really just an exploration of the method. 

Richard mentions finding the best season for germination temperatures, for his lily seeds. Note that my experiment does not provide real-time data. These inexpensive iButton thermistors do not broadcast to wi-fi. Rather, I do not see the data until I dig them up and download their data. I suggest you might want real-time data loggers for your purpose. 

Thank you David for sharing your data from thermistors planted in pots. Pots not plunged in sand do fluctuate a lot in day/night temperature. Possibly this is one factor which makes some bulbs grow better in the ground. It was David's pots data that convinced me to use a shorter period between recordings to capture daily variation.

Leo points out that soil moisture affects heat conductance and heat capacity and any real experiment of this type would include soil moisture, as I had originally planned. However, soil moisture sensors cost more so they're postponed until at least next year. Also soil moisture is a more complex measurement, needing to be related to the soil type.  I am keenly curious about soil moisture and omit it only due to practical limitations. I agree moisture is a critical variable. 

- Gastil
Santa Barbara, California
34.416 degrees latitude, 91 ft above sea level

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