In article ,
(Steve Henning) wrote:

(kamm) wrote:
I planted a rhododendron (dwarf rhododendron "baden-baden") in the
alkaline limey soil in my garden, before realising that these plants
hate lime. It was planted about 6 weeks ago and so far it is doing
fine. It had two flowers when I planted it, which have now fallen,
and although it has had no new flowers, it has loads of new shoots
growing.

It will flower once each year on buds formed the previous fall and the
flowers last 2 to 6 weeks.

If your plant came from Europe, your Baden-Baden may be grafted onto
root stock of Cunningham White which is somewhat tolerant of alkaline
soil. If it is of American origin, it is just rooted from a cutting
and needs acidic soil. It is best to dig it up now and plant it
correctly. The most important factor in achieving vigorous growth is
an acid soil mixture high in organic content. Rhododendron and azaleas
need an acid soil with a pH of 5.0 to 6.0, well mulched with organic
material. Rhododendron thrive in a moist, well-drained, humus-filled
soil, enriched with peat moss or leaf mold. Prepare the soil by
thoroughly mixing equal parts of loam, coarse sand and ground oak
leaves or redwood before planting.

Stop at this point.

Many commercial growers root
rhododendron and azaleas in pure sphagnum peat, or in a 50-50 mixture
of sphagnum peat and coarse sand or perlite.

"Correcting" soil with peat, sphagnum, or acidifying fertilizers does more
harm than good even in the short run. Field tests done in Edinburg have
shown that attempting to amend alkaline soils to suit rhododendrons fails
because the limestone is still there & the addition of acidifying chemical
fertilizers OR peat or sphagnum, merely causes mangenese to be bound to
the soil, & neither magneseum nor iron is accessible to the rhody
thereafter. Sulfer didn't bind the manganese but this "fix" was temporary.
Many rhodies did better in UNammended alkaline soils than in soils that
had been tricked out to give a neutral to acide pH reading despite the
continued presence of limestone in the soil.

Iron & magneseum reached the shrubs better if the alkaline soil was never
ammended. "Feeding" was best done by saving all the leaf-fall & prunings,
chopping them finely, & using them as mulch. Presumedly a mulch of conifer
needles would have the same effect though the Edinburgh trials didn't
study that. It is not done to change the pH but to get an accessible form
of manganese to the shrubs; if they can get those nutrients, it turns out
many rhodies tolerate more alkalinity than previously believed.

So in essence alkaline-hating rhodies simply should not be planted at all,
the corrective measures mainly increasing the chloratic outcomes. But
rhodies on the verge of alkaline tolerance can be adapted by feeding them
their own ground-up prunings & fallen leaves. Beyond that, most
fortunately, there are many genuinely alkaline-tolerant species-rhodies,
mostly small lepidotes, but also some that get to be the size of trees.
There are also the Inkarhos grafted rhodies developed specifically for
alkaline soils, & these are the big evergreen lepidotes that would
otherwise be alkaline-hating.

Here is a list of some of the species rhodies & azaleas that either
tolerate or prefer neutral-to-alkaline soils, some of these self-selecting
alkaline ridges as their primary habitat:

R. hirsutum
R. smsii
R. roseum
R. primuliflorum
R. telemateum
R. cuneatum
R. vernicosum
R. yungnanese
R. racemosum
R. taliense
R. rubiginosum
R. micranthum
R. orthocladum var orthocladum
R. kygwai
R. makinoi
R. myrtilfolium
R. opchraceum
R. bypoglaucum

I made that list some while ago when I first encountered the Edinburg
study, & added to it from the RHS and RSF lists of alkaline-tolerant
rhodies. Armed with such a list, anyone whose soils are not naturally
acidic can still have a broad collection of rhododendrons.

The Edinburg trials (& similar trials done in Germany) really indicate
that one great reason it has appeared that even slight alkalinity injured
most rhodies was because of the false belief that soils containing lime
can be "improved" by changing pH readings, when that just increases the
chloratic effect of the soil when manganese bonding irretrievably to the
limy soil, & enforcing a pleasing pH reason is simply not the end-all.
Some rhodies really are entirely unadaptable to any alkalinity, but as
many adapt as don't, & when the alkalinity increases, the vastly more
tolerant species rhodies or the Inkarho grafted elepidotes are the best
recourse. Amending soils to be at oddds with the greater environment will
not only have poor immediate benefit because of the bonded manganese, but
in the long run the greater environment will re-alkalinze even enormous
amended stretches of garden, & continuous re-acidifying increasingly
toxifies the ground (the harmful effects of artificially acidified then
re-acidified soils on camellias is well known, but the same thing happens
to rhodies just more slowly).

Such is my understanding from reading the papers. Luckily I live in a
naturally acidic-soiled part of the world & don't have to test those
tolerances myself.

-paghat the ratgirl

A favorite mixture on the
West Coast is 1/2 sphagnum peat and 1/2 ground bark dust, but in such
mixtures, plants must be fed regularly. My favorite soil mix is a
50-50 mix of peat humus and the natural soil. Soil around the
rhododendron's shallow roots must be kept cool and moist but well
drained. If the soil is too alkaline, acidity may be increased by
adding flowers of sulfur (powdered sulfur) or iron sulfate. I add 1
tablespoon of sulfur powder around the base of any plant showing signs
of chlorosis. Do not use aluminum sulfate. Aluminum can build up in
the soil to toxic levels eventually.

Because the roots grow near the surface, a bed prepared especially for
rhododendron and azaleas need not be more than 12 inches deep; deep
planting or too much mulch in the growing season keeps the roots from
getting the air they need. In fact, it is a good idea to set
rhododendron about 1 inch higher than they grew at the nursery.

Visit my Rhododendron and Azalea web pages at:
http://www.users.fast.net/~shenning/rhody.html
Also visit the Rhododendron and Azalea Bookstore at:
http://members.aol.com/rhodyman/rhodybooks.html

Cheers, Steve Henning in Reading, PA USA

--
"Of what are you afraid, my child?" inquired the kindly teacher.
"Oh, sir! The flowers, they are wild," replied the timid creature.
-from Peter Newell's "Wild Flowers"
See the Garden of Paghat the Ratgirl: http://www.paghat.com/