Saturday, September 20, 2014

A Potassium Feast for your Roses

For those of us in the cold zones, i.e. USDA zones 3-6 (and maybe zone 7, given last year's polar vortex incursion), now is the time to begin feeding your roses a six week diet of potassium.  Here is a quote from my posting on this subject last year at this time:

"In the six weeks before the first hard freeze (i.e., down to about 25 F. at night), give your roses a weekly "potassium feast" in each of those six weeks.  Potassium blocks the growth-promoting effects of nitrogen and phosphorous, thereby hardening the canes in time for winter.  I've been doing this for more than 20 years and I honestly can't remember the last time I lost a rose to winter weather here in Minnesota. Of course, I do other things to protect my roses from the Minnesota winter, as well.  Here is my 2013 article on winter protecting your roses: .

I learned this little trick in one of my first rose books: Burpee’s American Gardening Series, "Roses", by Suzanne Frutig Bales.  Here's a quote from Suzanne's chapter on winter protection:

“Potassium is an important mineral for sturdy stems and foliage.  Weekly feeds of a gallon of liquid potassium (1 tablespoon of muriate of potash (0-0-62), dissolved in 3 gallons of water) per bush, or a granulated feeding of potash magnesium (0-0-22), during the six weeks before the bushes go dormant, will give the bushes an additional boost for winter, extending their hardiness into another hardiness zone, perhaps two.  Excess potassium, when available in greater amounts than nitrogen and phosphorus, is known as the ‘potassium feast’.  It will block the growth-promoting effects of nitrogen and phosphorus, hardening the canes in time for winter.” 

I did a little independent research on this, a couple of years ago, by talking to Dr. Peter Bierman, retired University of Minnesota Professor of Soil, Water and Climate.  Peter told me that "... winter hardiness is one of the most important functions of potassium" and that the amounts recommended above..."would be a reasonable amount to apply for winter hardiness insurance and wouldn't be an excessive amount in terms of adding high salts."  That squares with my 20 years-plus experience in administering the potassium feast to my roses each fall.  

To clarify:  The proportions are: 1 TBP muriate of potash per 3 gallons of water (or 1 TSP per gallon).  So mixing in a 30 gallon trash container, you would use 10 TBP.  Apply one gallon of this mixture on each rose every week.  That’s not very much, but remember you’re repeating it six times. I also don't think the exact amount is critical and I usually err a bit on the side of a little more rather than less.

Several folks have asked me if they could "cheat" and do only three or four applications, using proportionally higher doses of potassium.  My answer is always: "I don't know, but it's probably better than not doing it at all."  However, my observations are that the roses harden off slowly, as the potassium applications continue over the six weeks and the weather gets colder.  By the end of six weeks, the canes have turned a lovely shade of red and look ready for the winter ahead.  With the canes thus hardened-off, they are less susceptible to the freeze-drying winter winds and naturally suffer less die back than if they their tissues were still soft.  I don't know if the roses will harden off as well with fewer, larger applications of potassium, but I suspect not.  However, don't let that discourage you if you get started late.  A couple of applications will be better than nothing.  Just try to get started earlier next year!

Another question is: where one can get potassium immediately to get started?  The answer to that is farm stores that sell fertilizers to farmers, who use potassium (potash) as an agricultural fertilizer.  Another source is suppliers to commercial growers and greenhouses.  In the Twin Cities, the primary sources for me have been Waconia UFC Farm Supply and BFG Supply in St. Paul (formerly J.R. Johnson Supply). It's sold in 50 pound bags for about $.40/lb.

It was pointed out to me by a reader in England that a very good alternative to muriate of potash would be potassium sulfate (0-0-50), which is 50% potassium and 18% sulfate.  This is interesting because adding sulfur to your roses in the fall has the effect of lowering the pH of your soil, which is desirable for most of us. A slightly acid pH around 6.0 (plus or minus) is best for roses.  (See my posting "Mind your pH":

Since potassium sulfate has a bit less potassium, i.e. 50%, compared with muriate of postash at 60%, you would use a little more potassium sulfate, perhaps 1.25 or 1.5 tsp per gallon.  Again, I don't think the amount is super critical, so I would use 1.5 tsp/gallon to simplify things.

In any event, whatever form of potassium you use, I think this first step in the winterizing process is very important and I'm always amazed that many rosarians aren't aware of it.  Mid to late September is the right time for most of us to get started, so find some potassium and begin your roses' feast very soon (I made my first application this week).  I think you will be as pleasantly surprised by the results as I have been over the years.

Here are two other articles on the potassium feast that I have previously published:

Jack Falker