growing lavender

[moutsie56]

I am somewhat new to hydroponics, but I have had few problems with the
"normal", water loving plants I have grown. I recently purchased
several Goodwin Creek and Spike Lavender plants from my local
greenhouse, but I am lost as to how frequent a flood/drain cycle should
be used for them. I normally flood for 15 minutes every 2 hours, but I
think this will drown my lavender. I have them planted in hydroton. Any
suggestions would be helpfull, thanks.

[Flygirl]

moutsie56,
I am not familiar with the idea of drowning hydroponic plants, I do
know that with some plants such as orchids you need to watch your light
cycle as they do better with up to 18 hours of dark. You can under
flood your plants & this is usually evident with a wilted look& try
roots. I know that I had a flow chart at one time for veggies that
stated recommended flood & drain preferences, but not for
flowers/herbs. My tomatoes are flooded every 4 hours for 30 minutes &
they are doing great, I am about to increase it to every 3 hours for 30
minutes. I know there has got to be a list of suggested times for most
common items grown somewhere out there, does any body know about this
list or know where to look for one?

[Ian Smith]

I am assuming that "drowning plants" or "over watering" or "waterlogged
roots" is referring to a lack of dissolved or atmospheric oxygen in the
rhisoshpere, not to over watering per say. As the nutrient solution
temperature rises, cellular metabolism & respiration increases in the
rhisosphere (i.e. higher growth rates). However, there is an upper limit
because D.O. is temperature dependent(as is air or any gas/vapor dissolved
in a liquid) so as the temperature of the nutrient solution rises, D.O.
decreases. At sea level(D.O. is also pressure dependent) water at 1 deg, 20
deg, & 25 deg C (34,68 & 77 deg F) will hold 14, 9.0 & 8.2 ppm of D.O.(at
3000' & 6000' these numbers are reduced by 10% & 20% respectively).
Therefore, the optimal root zone temperature for most plants is in the 20 to
23 deg C range(68-75 deg F) assuming D.O. saturation. Higher D.O. levels
allow for a warmer nutrient solution which in turn increases yield. The
nutrient solution in my greenhouse hydroponic system is held at 74 F & is
continually aerated to 12 ppm. Although considered a micro nutrient due to
its low levels in solution, D.O. is assimilated at a higher rate than any
other nutrient, & thus needs to be continually replaced.
The atmosphere is approximately 20% oxygen(200,000 ppm). As such, the
nutrient solution is only a minor source (1-5 %)of rhisospheric oxygen. The
main reason to aerate the solution is to kill off anaerobic pathogens,
pythium for example. This is especially important in closed (recirculating)
hydro systems. Moreover, oxygen affects the electrical charges (ionic
balance)in the water & nutrients to lower the energy needed for
assimilation.
Some plants can grow very well with their roots completely submerged in
water if the dissolved oxygen is adequate. Rice is grown in this manner, but
it's genetics have evolved to grow well in low dissolved oxygen. Trying to
grow tomato or cucumber in this manner would result in poor yields because
to achieve high growth rates with such voracious plants, the roots must
receive far higher levels of oxygen than available in solution alone, even
if supersaturated with O2 mechanically via aerators or chemically via
hydrogen peroxide.
This is where aeroponics, or hydroponics using a high field capacity
medium (high air space) excel. Aeroponics bath the roots directly in air. In
hydroponics, rock wool or perlite for example, will hold at least 20% & 30%
air respectively (unless submerged in water) even if irrigated continuously.
These percentages can be increased to as high as desired by using a
combination of moisture & E.C. meters to control the irrigation sequence,
but increasing media air content will significantly raise the E.C.(ppm) of
the nutrient solution so one must approach this method very carefully! A
less accurate but simpler & less expensive method is to adjust the "off"
portion of a cycle timer sequence by weighing the media between irrigation
cycles; again beware of rising E.C.
So, the moral of this story is: feed the roots as much oxygen as
possible to avoid " drowning " your plants.
Happy gardening.

Ian

[moutsie56]

Ian, thank you for your reply, it has been most informative and
extreamly helpfull. I will lower the water levels during flooding, so
that the roots are not completely submerged, take steps to increase my
dissolved oxygen, and adjust my off cycle by weighing my media. Thank
you, very much. Spring is on the way! :^)

Kevin