The upper leaves of one of my rhodies, about three feet tall, are
turning yellow getweing the veins, leaving a kind of green "skeleton"
in the center.

I judged it as a need for more acidic soil, and added a mulch of used
coffee grounds several months ago, but the condition seems to be
worsening.

Does it need something stronger, or a different kind of remedy? I'd
appreciate advice on what to do next.

Thanks. vince norris

vincent p. norris wrote:

The upper leaves of one of my rhodies, about three feet tall, are
turning yellow getweing the veins, leaving a kind of green "skeleton"
in the center.
I judged it as a need for more acidic soil, and added a mulch of used
coffee grounds several months ago, but the condition seems to be
worsening.
Does it need something stronger, or a different kind of remedy? I'd
appreciate advice on what to do next.

Hi Vince,

I don't know where you live, but Kathy Van Veen in Portland, Oregon,
uses coffee grounds in her rhododendron nursery and she finds:
1) it helps to aerate their clay soil.
2) slugs don't like to go through. (So you can see they have both mixed
in and put on top.)
3) It does help to make the soil more acidic. But it does not replace
fertilizer.

Warning, you don' t mix anything into the soil around a planted
rhododendron. They have very shallow roots. She obvioiusly mixes it in
her planting soil before she plants.

What you have, yellowing of a leaf between dark green veins, is called
chlorosis and is usually caused by an iron deficiency. Many conditions
can be responsible for an iron deficiency. Poor drainage, planting too
deeply, heavy soil with poor aeration, insect or fungus damage in the
root zone and lack of moisture all induce chlorosis. After these
conditions are eliminated as possible causes, soil testing is in order.
Chlorosis can be caused by malnutrition caused by alkalinity of the
soil, potassium deficiency, calcium deficiency, iron deficiency or
magnesium deficiency. Iron is most readily available in acidic soils
between pH 4.5-6.0. When the soil pH is above 6.5, iron may be present
in adequate amounts, but is in an unusable form, due to an excessive
amount of calcium carbonate. This can occur when plants are placed too
close to cement foundations or walkways. Soil amendments that acidify
the soil, such as iron sulfate or sulfur, are the best long term
solution. Foliar sprays of iron sulfate or chelated iron can reduce
symptoms. A combination of acidification with sulfur and iron
supplements such as chelated iron or iron sulfate will usually treat
this problem. Chlorosis caused by magnesium deficiency is initially the
same as iron, but progresses to form reddish purple blotches and
marginal leaf necrosis (browning of leaf edges). Epsom salts are a good
source of supplemental magnesium. Chlorosis can also be caused by
nitrogen toxicity (usually caused by nitrate fertilizers) or other
conditions that damage the roots such as root rot, severe cutting of the
roots, root weevils or root death caused by extreme amounts of
fertilizer.

There is a tonic that remedies some cases of chlorosis. It was devised
by Diane Pertson, of Vancouver Island, BC. Purchase a bag of Epsom
Salts crystals (magnesium sulfate) (available here in bulk at
farm-and-feed outlets), about $4.00 for a 5 lb. bag - and a bottle of
FULLY Chelated Iron & Zinc (this is a very concentrated liquid - the
chelation means it is in a form that can be readily absorbed by the
plant), about $7.00 for 1 quart; In a one gallon watering can, put in 2
Tbsp. of Epsom Salts crystals and 2 Tbsp. of Iron and Zinc liquid - fill
with warm water and stir to dissolve; Sprinkle this over the
rhododendron - by that I mean drench the leaves with the solution and
pour the remainder around the drip line of the root ball.
In 1-2 weeks, the leaves should be nice and green. You could repeat the
process at this time if the leaves aren't fully green.

She warns that in some areas where she lives the problem is too much
acidity and she must use domonitic limestone to keep the soil from being
too acidic. I have never witnessed that problem, but depending upon
where you live, you could have that also.

I hope this help. Good luck.
--
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Cheers, Steve Henning in Reading, PA USA Zone 6

Thank you, Stephen. I was hoping you'd respond to my query.
You've provided a wealth of information, as usual.

vince norris