Mind Your pH!

Do you know the right pH for growing roses?  And do you know the pH of your soil right now?  If not, you're flying blind and you're eventually going to have problems; just like I did a few years ago, when virtually all of my roses in two beds stopped growing after their initial spring bloom.   I knew something was wrong when pouring on high-nitrogen Miracle-Gro had no effect. The roses acted like they didn't even know it was there; and that's about right because they couldn't "feel" its effect.  I found out later that the pH in those beds was way too high (about 7.5) and that I made it that way through an unfortunate series of organic gardening mistakes, the biggest of which was using way too much horse and cow manure; a condition I'm still trying to fully correct.  More about that later.

The measurement of relative acidity or alkalinity of soil is its pH, where neutral is a pH of 7.0, which is the pH of distilled water.  A pH below 7.0 means that soil is acidic and above 7.0 means it's alkaline.  Roses need a fairly acidic condition and if they don't have it they simply stop growing (just like mine did), because they can't absorb nutrients; plus they develop an iron deficiency, which I could definitely see in mine.

Consulting my collection of rose books on the subject of pH, they all say about the same thing:  that the ideal pH range for roses is between 6.0 and 6.8 (a mid-point of 6.4).  However, exchanging e-mails with Dr. Peter Bierman, retired Professor of Soil, Water and Climate at the University of Minnesota, after an excellent presentation he made to the Twin Cities Rose Club (TCRC) in February 2011, he concluded: "Because many roses are susceptible to iron deficiency at high pH, I think I'll stick with the range of 5.5 to 6.5." That's a mid-point of 6.0 and I like Dr. Bierman's slightly more acidic conclusion, based on my experience.  So,that's all you need to know about the right pH for growing roses; just memorize it and head out to recess!

Now, how do you find out the pH of your soil?  Well, one way to do it is to take a soil sample and send it to a soil measurement laboratory.  Around here, that would be the University of Minnesota Extension Service. The problem with that, however, is that you will only find out the pH of one or two spots in your garden.  What you really need to know is the pH of your soil in many spots in your garden, preferably at the bases of several roses in each bed.  The only way to do that is to own an inexpensive pH meter and make it one of the most important tools in your garden.  (Forget about using litmus paper pH measuring strips; they're not accurate enough and hard to use).   Rosemania.com offers several pH meters; I use their most inexpensive model and it works fine. Here's the address of that page on Rosemania’s website:

 http://rosemania.com/shopsite_sc/store/html/page17.html .

When using a pH meter you must first wet the soil you are measuring with distilled water, which has a neutral pH.  This is important, because you don't want your measurement of your soil to be influenced by the pH of the tap water.

Can you tell from the scratches on my Kelway pH meter, above, that it gets used a lot?

So, how do you control pH in your garden to keep it in that 6.0 range for your roses?  Well, the first thing is not to put alkaline composts, like animal manures, on your garden unless you supplement them with something acidic, like peat.  As mentioned above, I learned this the hard way.  I grew up in a rose garden and, whenever we visited my uncle's dairy farm, my dad always brought home a couple of 5 gallon buckets of rotted cow manure for his rose beds.  In retrospect, that small amount of manure would not have elevated his pH, but I thought that if my dad had beautiful roses using manure, more would be better for me.  So I set out to get trailer loads of horse and cow manure every year and I spread all that poop liberally on my beds, especially when I mounded my roses for winter.  Well, that, of course, was a mistake, as I mentioned above, when my pH went up to an unsustainable level of 7.5.  I subsequently learned from a very knowledgeable dairy farmer in Wisconsin, who sells composted cow manure (Cowsmo), that his pH is 7.3 and he advised me not to use it without peat to bring its pH down.  That's when the light went on.  My organic fertilizing was doing more harm than good!

In that regard, we recently had several organic fertilizer vendors come to a TCRC meeting to talk about their wares.  One of them was selling little bags of "Alpaca Pearls", i.e. alpaca poop, from their farm.  Knowing that animal fertilizers have high pH, I asked if they knew the pH of their product.  They did not but, to their credit, they subsequently had it measured and reported back that it is 8.3!  So Alpaca Pearls turns out to be the wrong thing to put on your roses, along with other animal manures.

I have learned from all this that green, organic manure, like coffee grounds, is the right thing to use on your roses.  Coffee has a perfect pH for roses of 6.2, has a decent nitrogen content, and is a perfect medium for nurturing worms and their castings (poop) around your roses.  See my two recent posts about using coffee grounds in the garden: http://jack-rosarian.blogspot.com/2013/03/coffee-grounds-and-roses.html and http://jack-rosarian.blogspot.com/2013/05/gettin-wormy.html

So what should you do if your roses won't grow and you find out, like I did, that your pH is too high?  Well, first, get started and be consistent with the coffee grounds, but that won't be enough to make a quick correction.  Here's a quote from a University of Minnesota paper on the subject by Dr. Bierman:  "Elemental sulfur is the most commonly used material to lower soil pH.... Iron sulfate also lowers soil pH and it acts much more rapidly than elemental sulfur (2 to 3 weeks vs. 3 to 4 months)."  Dr.  Bierman also points out that elevated pH drains roses of iron, so using iron sulfate to lower your pH also provides a needed dose of iron.

I use both elemental sulfur and iron sulfate in my garden.  Garden sulfur is available around here, inexpensively, in 25 pound bags, at Mills Fleet Farm, and iron sulfate is available in 50 pound bags at Waconia Farm Supply.  We have lots of farm stores in Minnesota, which is nice for serious gardeners, but you may have to look around a bit more in other areas.  Suburban garden centers sell these products in small containers at ridiculous prices, so I advise finding a farm store and doing the necessary driving to get there.

I have also found that using acidic Miracle-Gro or Schultz fertilizers, labeled for azaleas and rhododendrons, helps with the pH lowering process, in that, while you are treating the underlying problem with sulfur or iron sulfate, your roses will start absorbing nitrogen administered with a dose of acidity. As a matter of fact, unless your pH is under 6.0 already, a dose of acidic liquid fertilizer on your roses in the spring would be a good idea.  It's what I do with my beds.

I haven't mentioned the potential need to increase soil pH because it generally is not an issue with roses.  However, if for some reason your pH is below 5.5, repeated doses of limestone (lime) will increase pH back into the 6.0 range, over time.  But, be careful you don’t elevate it above 6.5 in the process.

Just before publishing this post, I measured the pH at the base of several plants in my Earth Song bed, which still had elevated pH last summer.  I had treated it with iron sulfate last year and put a good dose of elemental sulfur around each plant when I winter-protected the plants last fall.  To my pleasant surprise, the pH has dropped to 6.0 through the whole bed! And, of course, the roses are performing better than they have in quite some time.

Minding your pH is one of the most important things you can do to raise healthy roses.  I suggest that rose clubs should own a couple of pH meters and loan them out to members.  In addition, consulting rosarians should consider providing pH measurements to members, either with the club’s meters or their own.

Jack Falker

June 20, 2013