Sharon: since 2012 I grow tiny own-roots in pots before transferring into my clay for zone 5a winter survival. I try different fertilizers & soil mix every year (never the same, experiment is the key to learning).

These are what I learn for the past decade of growing 140+ roses in pots:

1) High nitrogen attracts chili thrips, next is high phosphorus (this is from horticultural studies).

2) Osmocote PLUS releases its phosphorus and trace elements fast for quick blooming, but the release of potassium is too slow. I get the worst health (black spots, mildew) from too much Osmocote PLUS.

3) Alfalfa tea is the best source of nitrogen, and NOT alfalfa meal mixed into potting soil (too acidic).

4) Calcium and potassium are the key for health. My most healthy roses in pots were the 2 years which I fertilized with SOLUBLE gypsum and sulfate of potash.

5) Worst health in pot was when I didn't mix in gypsum for fast drainage.

6) Next worst health was when I mixed acidic alfalfa meal into potting soil .. that induced mildew fast.

7) Another worst health (mildew) was when I used SOLUBLE fertilizer with chemical nitrogen, too salty and too acidic.

8) MG-soluble for roses NPK 18-24-16 gives fantastic blooming, but it attracts thrips due to fast nitrogen and fast phosphorus. I always get thrips in the year which I use this fertilizer. To avoid thrips, SLOW-RELEASE nitrogen (alfalfa) and SLOW-RELEASE phosphorus (in the potting soil itself) is best.

This year I try a new potting soil: Beyond Peat potting soil from Walmart (very dense with horse manure and chicken manure), so I used lots of gypsum (calcium sulfate) to break up this soil. I let a week of rain washes out the acid in the gypsum, before planting.

Since I already got gypsum in the soil, I only need to supply nitrogen via alfalfa tea and potassium (sulfate of potash). For phosphorus Beyond peat potting soil already has plenty via chicken manure and horse manure (plenty of trace elements). For more trace elements I used 1/4 cup of Osmocote per 3 gallon pot (this should last for 6 months).

For every other day watering: I put 2 TBS of alfalfa meal and 1 TBS of sulfate of potash per 1 gallon of water (soak this for 24 hours). In between watering, I dunk pea-gravel and red-lava rock into rain water for alkaline minerals (need 1 week of soak).

Within 20 days of receiving 4 bands from LongAgoRoses, I get buds on ALL 4 (Clothilde Soupert, Wise Portia, Augusta Luise, Purple Lodge), including buds on Roses Unlimited's Kiss me Kate (refusing to bloom for many people).

Below pics. were taken yesterday May 19, and the day that I planted newly bought bands & gallons was April 28. So it took only 20 days to produce these buds:

Below are multiple buds on Firefighter (from RU). Prior to watering with sulfate of potash (dissolved by the fermenting bacteria in alfalfa tea), Firefighter got bad mildew thanks to my failed experiment of mixing rock dust (pH 8) with alfalfa meal (pH 5) in the potting soil. So I had to replace the soil. Mildew went away the minute I applied sulfate of potash/alfalfa tea.

Below is band-size Wise Portia (planted April 28), now with a fat bud (plus 2 other buds), pic. taken May 19.

Note below 4 bud-terminals on Kiss me Kate (from Roses Unlimited, planted April 28). Kiss me Kate sat there doing nothing, until I fertilized this DAILY with sulfate of potash in alfalfa tea. It gets daily weak fertilizer thanks to its stingy reputation. Zero nitrogen except for alfalfa tea, and zero phosphorus except for what's in the potting soil and 1/4 cup of Osmocote.

Since Kiss me Kate has a reputation of refusing to bloom, I topped with a tiny amount of Rose Tone (has chicken manure). It responds well to Rose Tone but absolutely HATED chemical granules fertilizers (MG-shake and feed NPK 12-4-8) .. this was too high in nitrogen and it grew big instead of blooming.

Tiny Clothilde Soupert broke out in 6 bud-terminals with alfalfa tea and sulfate of potash (soaked for 24 hrs.). If I had used my alkaline tap water at pH 9, I would let it soak for 2 days so it ferment to bring down the high pH of my water.

Purple Lodge broke out in 4 bud terminal, and the buds get fatter the minute I applied Rose Tone (has chicken manure). For fast blooming, Rose Tone on top, plus sulfate of potash/alfalfa tea is a sure bet.

The above is a repeat of my 2012 experiment with using alfalfa tea to induce bloom on TINY BAND Crimson Glory. Below pic. was taken 1 month after purchase from Burlington back in spring 2012, with 3 blooms plus 1 bud. Back then I also used MG-soluble for roses NPK 18-24-16 but got thrips in that summer.

Hi @strawchicago z5,

I've been reading your comments about using higher potassium with great interest and thinking of starting to add Sulfate of Potash in my next round of weakly weekly fertilization.

However, I also have an unused bag of Masterblend 4-18-38 lying around and it seems more complete with its calmag and other trace minerals included.

I remember you mentioned Masterblend a long time ago but I don't see you mention it anymore. Did you have a bad experience with it?

I'm thinking of doing a half dose of either Masterblend or the sulfate of potash... Is one of them better? I'll admit I am a little scared of the very high numbers on both.

Thank you so much for sharing your knowledge on here, my roses have never been happier than since I started following your advice!

Below, Olivia Rose and Boufarik, almost completely recovered from this spring's storm damage:

I can't wait to see more pics of your Purple Lodge as it grows, I'm sure it's going to be spectacular!

Seaside: Your Olivia Rose and Boufarik look fantastic: so healthy with many buds.

I don't use MasterBlend NPK 4-18-38 for roses in pots since high phosphorus defeats growing roses for zone 5a winter survival. High phosphorus result in more branching, thus more blooms, but roots stay thin cluster, which can't survive -20 F below zero in my zone 5a.

In contrast, high potassium results in THICKER & WOODIER and LONGER roots, which helps with winter survival. For established roses in the ground, if I want a rounder bush (more branching), I still use MasterBlend NPK 4-18-38 in hot & dry summer when I want roses to be short & round bush.

MasterBlend has potassium chloride (salt index of 116) and chemical nitrogen (salt index of 104), both are higher in salt than sulfate of potash (salt index of 42) plus using alfalfa (zero salt) as a source of nitrogen.

MasterBlend is useful if one uses alkaline tap water since potassium chloride dissolves easily in alkaline tap water, and chemical nitrogen is acidic.

Sulfate of potash is more useful if one has tons of acidic rain .. it's hard to dissolve sulfate of potash (a natural mineral) in alkaline tap water.

straw I think that one time you had mentioned that alfalfa could cause canes to become black..... I was thinking that is what happened a couple of times to me..... I think I actually killed a few bushes in late summer when I fertilized them with alfalfa in hopes I would get basal cane breaks..... but it rained after that and I lost several.... I had thought they were diseased but now I wonder.....

I am getting ready for rainy season and I just want to confirm about alfalfa tea before I put it down...... I should put this down WITH the sulfate of potash correct. Do I want the tea to get stinky or just mix and pour.... If I am correct the longer it sits the more it changes the ph,,,,, I know you have said, but I am old and can't remember and now houzz has taken away my ability to search.

Do you put the sulfate of potash on the ground and water it in, or mix it first. Of do you let it sit in your tea.

since I have sand and not clay should I still use gympson?

Thanks

Sharon

Sharon: since you have sand and not rock hard & alkaline clay like mine, NO to gypsum (too acidic with 18% sulfur). For calcium in sandy soil: Garden Lime powder (with 10% magnesium and 20% calcium) is best. It's a yellowish powder "agricultural lime powder", and it glues soil together thanks to its sticky magnesium. When my soil is too dry and loose (from too much gypsum), I fix it with organic matter plus Garden Lime.

Spreading sulfate of potash around the bush doesn't work since potassium binds up with soil, thus best IN A SOLUTION. Soaking 2 TBS of alfalfa meal with 1 TBS of sulfate of potash per gallon of water helps potassium to be SOLUBLE. An overnight soak won't change the pH, but if you soak it for 24 hrs. and it starts to "smell fermented", then the pH has dropped (too acidic).

OK to soak for 24 hrs if your tap water is alkaline like mine at pH 9. Since I use acidic rain water, a few hours of soak is enough to convert potassium from a solid into a SOLUBLE form for plants to use.

WARNING ABOUT Masterblend NPK 4-18-38: it's extremely acidic and dissolve easily in my alkaline tap water at pH 9. Today I was foolish to put too much MasterBlend in a 2 gallon-bucket of acidic rain-water: instead of 1 teaspoon I used 1 Tablespoon and my 6 tomatoes WILT IMMEDIATELY from its acidity. The other 12 tomatoes with just regular rain-water DID NOT WILT.

In contrast, sulfate of potash always make leaves firm & perky and put a shiny gloss on leaves. I reversed blackspot on Golden Celebration with sulfate of potash years ago, and this year I reversed mildew on both Summer Sun (disgusting white mildew like a white paint), and Firefighter (drying & leaves curl up mildew). It took 1 week of watering with sulfate of potash (dissolved by the bacteria in alfalfa meal) and mildew went away for good. Will post pics. of shiny leaves later.

Thanks so much for your comment @strawchicago z5 !

So much interesting information. I hadn't realized Masterblend has such a high salt content. I will NOT use it on my rugosa hybrids!

I will mix a gallon of it at half strength and check the resulting pH to make sure the mix isn't too acidic. If it is, I will give it to my acid-loving plants!

Regarding the effect on roots, since I garden in pots only, I prefer to encourage branching and lots of fine feeder roots rather than a long thick root that would have nowhere to go in a pot. So maybe the relatively high phosphorus could be good in the context of roses that permanently live in pots?

@sharon2079

I'm not sure if there is a difference between the MG for roses sold in the US versus here in Europe but the Liquid MG for Roses & Shrubs here has much lower numbers than those mentioned in this thread. I use it every now and then as part of weakly weekly. If it's available, it might be gentle enough for your purposes?

seasiderooftop Thank you for the pics. of your fertilizer NPK 7-3-5 with potassium oxide. From Wikipedia: "potassium oxide is about 83% potassium by weight, while potassium chloride is only 52%". Your fertilizer NPK 7-3-5 is more gentle than our MG-soluble for roses at NPK 18-24-16 (super high in fast-release phosphorus).

Agree with what you wrote: "since I garden in pots only, I prefer to encourage branching and lots of fine feeder roots rather than a long thick root that would have nowhere to go in a pot. So maybe the relatively high phosphorus could be good in the context of roses that permanently live in pots?"

Phosphorus induces branching (more blooms) but too much cause weaker necks on Austin roses. I tested SOLUBLE for roses NPK 18-24-16 on Evelyn and it gave more blooms, but all with weaker necks and poor quality, lasting 2 days in the vase rather than 4 days.

Phosphorus is fantastic for floribunda roses (more branching for a round shrub), or for cluster blooming Marie Pavie ... but NOT best for my zone 5a winter survival.

Last year I used high phosphorus for my pots, and roots became thin cluster rather than long & woody for zone 5a winter survival. This year I focus on high potassium and roses become taller in pots with longer roots.