Seems many have missed/lost/deleted the first post out there,
so here is part one again...
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This explanation is rather detailed, and based on my experience growing
roses in Portland OR, San Francisco Bay Area, CA and San Diego, CA.
This account makes some generalizations in soil science and chemistry
that I wrote for clarity sake. Note that I am somewhat biased toward
chemical fertilizers, but I use and discuss both chemical and organic
As for my backgournd, I am an electrical design engineer by trade,
but have a vocational green thumb in gardening. I had a landscaping
business in Monterey, CA with my brother for five years. I had many
commercial and domestic landscape maintenence accounts and I have had
many large (and small) rose gardens over the years. I am also an avid
cymbidium orchid grower, and I have won awards (albeit modest ones)
for rose and orchid blooms presented in various western ARS and AOS
Opinions vary, as will your rose blooms...
(c) 1998 by Scott G.***inson, All rights reserved.
Permission given to the Honolulu Rose Society to reprint by
the author, 3/6/98.
Rose Fertilizers Defined, part I
It seems that the stores have lots of types of fertilizers on the shelves,
and they vary greatly in price. So which one do you buy? Do you buy the
"Special" Rose formula, or the "Super Duper" Rose formula? Should you get
the green slow-release pellets in the box? Or the blue soluble stuff in
the bags? Should you buy the Miracle-Gro[TM] formula or the off-brand
next to it for less? Should you go organic or chemical? Or combine both?
OK, first things first. The primary compounds in fertilizer are printed
right on the label of every bag or bottle of fertilizer. They have to be
there by law; the by-weight percentage breakdown of nitrogen, phosphate
and potash. They are always listed in that order. Note that I will use
the elemental variation of these: nitorgen, phosphorous and potassium
(the N-P-K listing) here in this article. This is because most texts
and many lables list them that way. Also plants use these elements as
fertilizer, and not always in the form that they come in.
Anyway, regardless of what the package says it is for, the so-called
"big three" formula (nitorgen, phosphorous and potassium) must be on
it. The percent breakdown is labeled in a series of numbers, such as
10-15-10, or 30-10-10. The 10-15-10 on the label means that it has 10%
nitrogen, 15% phosphorous, and 10% potassium. This is the formula for
Miracle-Gro[TM] fertilizer. 30-10-10 means that it has 30% nitorgen,
10% phosphorous, and 10% potassium. This is a high-nitrogen fertilizer.
It is also the composition of Miracid[TM], another popular water-soluble
fertilizer available from the makers of Miracle-Gro (was Sterns, now
Other trace elements such as manganese, chelated iron, sulfur, etc.
are also listed on fertilizers, usually in fine print. These trace
elements are also varyingly important, but not used as much as
nitrogen, phosphorous and potassium. There are all sorts of %-%-%
combinations, and I have seen just about avery combination imaginable
claim that it is "specially formulated" for roses. I have also seen
the same formulas in bottles at places like Long's *** with the exact
same %-%-%, but labeled for different plants and priced accordingly!
The "general" house plant food was in a green bottle for $2.29. The
same formula in a yellow bottle was labeled for "flowering plants" for
$2.59. And the violet bottle was labeled for "African Violets & Orchids"
and listed for $2.89!!! And they all had the exact same %-%-% formula!
A similar thing happens with brand marketing. Miracle-Gro is sold at
K-Mart right next to their K-Gro brand. They have the same composition
(even the trace elements) and in all likelyhood are made in the same
factory. K-Gro is usually $2 a box less though. The point in these
examples is that the numbers on the packages tell you what the fertilizer
is, and not the brand name, splashy advert, or what the label says it
is supposed to be for.
So you might be wondering what to feed your roses then? It really depends
on soil type (acidic or alkaline), what climate you live in, and the time
of year. I also time fertilizer application depending on the period in
the rose bloom cycle. Soil pH creates a problem chemically in soils and
what is true for acidic soil is quite different in alkaline soil. Here's
In acidic soils, phosphorous is not as available to plant roots, and gets
locked up by elements in the soil. Something like 1% of the phosphorous
in the soil is available to plant roots. So in acid soil you will need
to add more phosphorous than you do in alkaline soil to get the same
effect. Here along the coast of California, the soil is usually alkaline,
and thus phosphorous is not needed as much. Likewise, in Oregon's
Willamette Valley where the soil is acidic, phosphorous is needed more.
A similar, but opposite problem exists for iron. In alkaline soils, iron
is not available to plants, and thus we need to add chelated iron here. In
acidic soil iron is available, and thus not really a problem (nor as much
needed in your roses' diet).
As for feeding your roses, in San Diego, CA they will require a lot more
nitrogen and heavy feeding due to larger growth than they will in say,
Vancouver, BC, Canada where they will have a shorter growing/blooming
period. Roses tend to be heavy feeders, and as someone said in a post to
the rose grower's newsgroup on the internet, "Roses are like pigs, they
will eat anything." This is really quite true, as UC Davis studies show that
fertilizer elements of all types and origins will be taken up by plants
and put to use in the same way (not just roses). Roses do not distinguish
between an ~exclusive~ fertilizer in a fancy box or a one you borrowed
from a neighbor in a plastic bag. Regradless of how much you paid or what
the fertilizer box says, if the composition is the same then it will have
the same effect. However, the type of fertilizer used, the type of soil
you have, and the time of year you add them will be factors in how well
your roses can feed on the fertilizers you apply. The type of fertilizer
application also can be a factor (ie., if they are water soluble or dry
formulas, fast or slow release, or if you are leaf feeding or root feeding).
We will get to these points in later segments.
So lets review: All fertilizers have the 3 main components listed on
them in the order of: nitrogen, phosphorous (in phosphate) and potassium
(in potash) in % breakdown by weight. Trace elements are also listed. The
elements needed vary depending on the soil type and pH that you grow
your roses in. Phosphorous is needed more in acidic soils, and (chelated)
iron is needed more in alkaline soils. And finally roses are gluttons;
think of them as ravenous pigs.