[Arid_gardener] Question from Home-Hort WWW page

[Dick]

Vergie Thompson
> 85205
> VDAKPARTY at aol.com
>
> I have several hisbisus bushes; the ones on the south side of my house
>
> have some yellow leaves on them.  What would cause that?  Thank you so 
> much for your help.

--------------------------------------------------------------------------------------------------------------------------------
Generally speaking, Vergie, assuming adequate deep irrigation, that yellow 
leaves are from the lack of nitrogen. Following the directions on the bag, I 
preferably use 16-20-0 or rarely 21-0-0. Some yellow leaves on any plant are 
as normal as our skin shedding dry skin cells. It is a normal physiological 
process. This shedding can, however, vary widely in intensity depending on 
the health of the plant and weather, of course, hot, dry moonsoons, common 
in mid-summer, take a toll. If most of the leaves are green and the plant 
otherwise appears healthy with normal growth, rake up the leaves for your 
compost bin or consider them mulch and keep on doing what you are doing. .

Since our water and desert soil has a high salt content, repeated shallow 
irrigation cycles will build up the salt content in the root zone. The salt 
travels up the vascular system to the leaf where it packs in cells from the 
tip traveling up the leaf until to falls. Water will transpire into the 
atmosphere cooling the plant surface as it does, the salt cannot. The packed 
cells are brown (dead tissue), not yellow. The dead tissue cannot take part 
in photosynthesis producing carbohydrates to to produce food to build new 
cells thus the plant literally starves itself to death, in a sense. Tip burn 
by this process is quite normal and not terribly damaging. The only 
preventative issue is deep watering flushing salt below the feeder-root 
zone. Plants can, look at all the healthy vegetation around you, tolerate 
fairly well the natural existance of salt in our water but evaporation 
concentrates the level in the root zone until it becomes toxic to the plant. 
Salt will not evaporate.  It can only be washed below the feeder roots.

The concentration of disolved salt in our water cannot exist in lower levels 
than that coming from your garden hose. It is the evaporation from the 
surface that continuously vacuums water to the surface where only water 
evaporates leaving every atom of salt to be washed back into the root zone 
again when you rewater. Repeated cycles pumping salt to the leaves is 
eventually fatal.

A typical problem with desert gardeners is chronic shallow watering, a 
regime under which most plants we cultivate suffer. Plants adapted to our 
climate with no irrigation tend to do better. Deep irrigation cycles 
maintain a plant for much longer cycles. Alayer of mulch on the surface 
reduces avapotation significantly.

If this creats more questions than answers, Vergie, don't hesitate to ask 
for clarification. With 8.5 wt. % salt in our water, it is a miracle we can 
grow anything in what we tend to forget is a miserable, ungodly, hot desert 
fit only for horney toads and cactus. Other desert gardeners are welcome to 
weigh in.

Dick Gross, Master Gardener Volunteer.

U of A Maricopa County
Cooperative Extension


















0

Dick Gross, Master Gardener Volunteer
Maricopa County Cooperative Extension

----- Original Message ----- 
From: <VDAKPARTY at aol.com>
To: <arid_gardener at Ag.arizona.edu>
Sent: Thursday, March 13, 2008 9:02 AM
Subject: [Arid_gardener] Question from Home-Hort WWW page


> Vergie Thompson
> 85205
> VDAKPARTY at aol.com
>
> I have several hisbisus bushes; the ones on the south side of my house
>
> have some yellow leaves on them.  What would cause that?  Thank you so 
> much for your help.
>
>
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