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Re: rooting temps and delta t

Leave it to an engineer type to ask the probing questions  :-)  My dad is
an engineer and there is never a simple question nor a simple answer.

In general, when we discuss the temperature of rooting media, it is the
temperature at the basal region of the cuttings as this is where the
necessary cell divisions and root initiation occurs.  Ideally your
thermostatic control should be located in this zone.  The surface will be
cooler by several degrees due to evaporative cooling taking place.  The
lower the humidity of the air over the cuttings the greater the impact of
this.  Heat is also lost by radiation, especially at night.  Though on
sunny days the dark peat will absorb a great deal of energy due to
insolation and the surface will become warmer.

The delta t (change in temperature for non chemists/engineers/
biologists/etc.) depends on the species and type of cuttings being rooted. 
For example, hardwood cuttings of Taxus cuspidata (Japanese Yew), a
temperate species, root well in a cold greenhouse with bottom heat of 70
deg F (21 deg C)  and an air temp of  50 deg F (10 deg C) thus the delta t
= 20.   This allows for rapid root formation but helps to keep the buds
from breaking dormancy too quickly.  This assumes the cuttings were taken
early in the winter.  If taken late, close to bud break in late winter or
early spring, then the tops may break dormancy and begin to grow before the
roots have formed.  This situation usually leads to the failure of the
cuttings to root and subsequent death.

For tropical species the delta t should be very small.  There is a
phenomenon called chilling injury that occurs with many tropicals where
damage to foliage, flowers and stems may occur at air temperatures only as
low as 55 deg. F (13 deg C). In general I root my tropicals at  80 -85F 
(27-30 C) soil and air temp.

To further complicate things, it is very important not to have an
excessively high night air temperature over the cuttings.  A drop in air
temperature for the tropicals down to 75F at night would probably be
beneficial.  This is done to prevent the loss of carbohydrates due to
increased cellular metabolism (respiration) that would occur during the
night.  There is intense competition for food resources in a plant and in
general it has been reported that the top (buds and foliage) win out over
the roots.  So it is best not to tip the balance more than necessary.

Because I experience days on end of gray cloudy days I use supplemental
lighting all the time with my cuttings and seedlings in the greenhouse and
am always careful not to allow the air temperature to get to get high
unless the sun pops out.  Generally keep it at 70F (21C) during the day
when it is cloudy.  Night temps are 60 F. (16C)

Best Regards,

Scott
..........................................................
Scott Gregory Vergara		
Washington  USA
USDA Zone 8 Ave Min Temp 10-20 deg F. Record low 0/hi 104 
svergara@pacific.telebyte.com,  HORTULANUS@aol.com
..........................................................

----------
> From: Duncan McAlpine <dm2477@lab3.ca.boeing.com>
> To: Scott Vergara <seeds-list-request@eskimo.com>
> Cc: seeds-list@eskimo.com
> Subject: Re: rooting temps
> Date: Thursday, January 09, 1997 10:44 AM
> 
> We have been talking about soil temperatures and I would like to
understand 
> this issue. I understand the above statement, but in the past postings I
was
> assuming the soil temp needs to be a defined temperature. This is not the

> surface soil temp but the actual soil temp. If the process is zonal or
base
> temp control, what is the delta in air temp versus base temp?
>
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