When to trim back knockout roses and how much!!

jess2132000(PA)February 10, 2008

This is my third year summer with my pink and red knockouts and last year I liked them to stay as tall as the photo I have included here. The problem was by August they too tall and hate to cut them back as they were in full bloom in August and did not want to cut any flowers off at that point. Should I cut them harder in the spring and how low to the ground so they don't get any higher then the ones in the photo? Does anyone fertalize theirs in the spring as I did not last year and they stopped blooming the month of July then bloomed again all August.. Would feeding them make them bloom all summer not take a break in between??

Here is a link that might be useful:

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diane_nj 6b/7a

So, you want them to be about 3' tall? I'll tell you that Knock Out grows over 5' tall here, and that is with a hard (down to 18 inches tall) pruning. If you want to keep them at that height, you'll have to cut them down during the growing season. They rebloom quickly, so that shouldn't be a problem. But, they won't stay at 3' tall, no matter how low the initial pruning in the spring. Blushing Knock Out seems to be smaller, around 4' tall.

Prune when the forsythia is in bloom. I would say early to mid-April, but really keep an eye on the forsythia. It is a pretty reliable tool in these parts.

Many roses stop blooming in July here because of the heat. Some people have started to water more frequently, and they have reported that it helps with mid-summer rebloom. Otherwise, we just wait until the heat lets up. Fertilizing in the spring doesn't matter in this case, it is really related to the summer heat.

    Bookmark   February 10, 2008 at 3:46PM
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Terry Crawford

Hi Jess - I only prune my Knockouts to clean out dead wood in the spring. I don't cut mine down at all because I want the height and bushiness.

As far as fertilizing, roses are heavy feeders, so you should start and maintain a good feeding program. My fertilizer of choice is Rose Tone, which I apply starting when I see new growth in the spring and I apply monthly until about 6 weeks before the first frost. During the extremely hot summer months (July-August), it is normal for roses to take a breather and then start up again when the weather cools, so I don't think your roses are performing any differently than most.

    Bookmark   February 10, 2008 at 3:53PM
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barb_r(Z8 western Wa)

I have to prune my roses back in August so I can see when I back out of my driveway.

    Bookmark   February 10, 2008 at 4:30PM
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seil zone 6b MI

First off, what ever you're doing keep doing it because your roses look fabulous!

Now, did you dead head them after the first bloom in June? If so don't just take off the spent blooms do a good pruning to reduce their size then. If not then you need to dead head as I suggested after that first flush of blooms are spent. The thing to remember is any time you cut them back you stimulate new growth and therefore new bloom. Roses can take between 4 to 8 weeks to recycle a flush of bloom. I'm not sure how fast Knockout reblooms but pruning them will stimulate them to grow and bloom. As for fertilizer don't stint. They're hungry devils. I use Rose Tone monthly too but I also put a slow release fertilizer down in the spring as well, Miracle Gro, Bayer or whatever I have or is on sale the week I need it. I also use Messanger and Superthrive through out the season.

    Bookmark   February 10, 2008 at 8:49PM
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karl_bapst_rosenut(5a, NW Indiana)

Jess's photo

    Bookmark   February 10, 2008 at 9:06PM
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karl_bapst_rosenut(5a, NW Indiana)

I prune mine to clear dead wood, then give them a little overall trim with a pair of hedge trimmers. I fertilize in spring and mine bloom all summer without a break. I do deadhead though, using the hedge trimmers. Too many blooms to do it with pruners.

    Bookmark   February 10, 2008 at 9:10PM
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jean_ar(z8 Ar)

wow. they are knockouts alright.They are gorgeous.Mine are about 4 yrs old and I have never pruned them,but I intend to do it this year,as I want mine to grow as big as it wants.Mine is in a huge pot and its about 4ft tall and 4 ft wide.I fertilize mine once a month, too.


    Bookmark   February 11, 2008 at 1:00AM
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hoovb zone 9 sunset 23

Wow those are stunning!!

What about if you were to give them a second, light pruning when japanese beetle season really kicks in? (Do you get those terrible pests?) Since you would be losing flowers anyway, a light pruning wouldn't matter so much. Here we prune in January, but sometimes give the roses a second light pruning in August to keep them reasonably sized--it doesn't seem to harm them at all. Just a thought.

    Bookmark   February 11, 2008 at 2:06AM
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Thank you for the compliments on these roses. It really is exciting to me to wake up and come down to breakfast and look in the back yard at these guys in bloom!! They are really great roses!! I will fertilize this year as I have never but they seem to slow down in July not sure if it is the heat and maybe more water might make them rebloom faster or if it was because I snapped most of the flowers off at the same day in June and maybe I should only do a few at a time not all in the same day and they might bloom more in July.. I just love them anyway and can not wait till May here to see them again!!!

    Bookmark   February 11, 2008 at 4:08PM
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pkapeckopickldpepprz(z9 a/b FL)

Wow those are pretty. You definitely have a green thumb as they look wonderful!

    Bookmark   February 11, 2008 at 6:56PM
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karl_bapst_rosenut(5a, NW Indiana)

Yes, if you stagger the deadheading you can have blooms most of the time.
Intense heat can cause the bloom cycle to lengthen. Making flowers takes a lot out of a rose bush, more so when they are under heat stress.
When I visited the rose garden in Tyler, Texas one year in mid July, their roses looked terrible. I was told they don't do anything to them in very hot summer weather. They prune them in August so they have a really nice fall bloom.

    Bookmark   February 11, 2008 at 9:18PM
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how big were they when you planted them? they're the best..

    Bookmark   February 18, 2008 at 5:47PM
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The red one's came from Lowes in one gallon containers. The pink were bought at a local garden center but also in one gallon containers and they grow very quickly by the end of summer they were as tall as these in the picture. These are now 3 years old but get almost as high as the fence if I let them grow that tall..

    Bookmark   February 18, 2008 at 6:02PM
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thats amazing. i think i'll line my backyard fence the same way. hard to believe they were in one gallon containers. Did you do anything special when you planted them?

    Bookmark   February 18, 2008 at 6:46PM
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What a beautiful display. I think I have talked my husband into helping me plant a bunch along my fence now that I showed him your pictures. They look amazing!

    Bookmark   February 22, 2008 at 11:32PM
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Don't deadhead mine but do prune them quite a bit, down to about 18 inches twice per year and they are back up to 6 feet in no time. They would take over if we did not prune hard.

    Bookmark   February 23, 2008 at 6:41PM
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sld_sandy(z7 TN)

Rosenut, your KnowOut Roses are a such a stunning display.......After seeing yours, that's what I would like mine to look like.....

Mine were just planted a couple of months ago. The only trimming I have done so far is just cut the spent roses.

I'm in the Tennessee, so it's pretty hot here. I think 95 today. So after reading all these remarks, I should not be thinking about trimming......

One question......Does anyone do cuttings of these roses for vases, etc. and if so, does it matter where on the stem they are cut?

Any advice on Knockout Roses will be greatly appreciated.......

    Bookmark   July 19, 2008 at 9:50PM
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weedlady(Central OH 6)

Jess, your roses are spectacular and this conversation thread very helpful. But when I clicked on the link to your picture and saw "KOCKOUT" in the caption I could not help but think, "look how many people have seen at this caption since the post was made in 2008 and it is still misspelled." :-)

Hope you are still enjoying your roses!

    Bookmark   July 13, 2012 at 6:37AM
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This is the third season I've had knockout roses and I've never pruned them down to a foot, it just scares me. Now it's October and soon the snow will be coming along with freezing temperatures. Should I cut them down now or leave them alone.

    Bookmark   October 9, 2013 at 11:34AM
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dublinbay z6 (KS)

Myself, I'd let my roses grow to whatever height they naturally want to grow to. However, if for some reason I wanted to go through the hassle of continually pruning them to keep them shorter, I'd coordinate with their natural blooming cycles.

Like most roses, KOs will have 3 or 4 bloom cycles per season (depending on your geography and how long your growing season is). They naturally rest in between--and to tell the truth, they look awful during their rest period if they are not deadheaded (and most KO owners do NOT deadhead--KOs are sold as not needing to be dead-headed!).

So given all those factors, I would trim them back (with the hedgesheers) about 1/3 the way back in early spring (late March or early April) (and take out any dead canes). Then, AFTER each bloom cycle ends, I'd prune them back lightly (or up to 1/3 the way back, if necessary) with the hedgesheers again--that way you would be accomplishing deadheading (getting rid of the ugly dead blossoms) and controlling for height during a time period when the rose is already resting. That would give you about 4 "haircuts" or so during the growing season.

Do not prune in late fall. Start the pruning in early spring and end about in mid-fall, or even early fall.

And yes, if you have really hot summers, that will slow down the blooming cycles and prolong the midsummer rest period. Not much you can do about that except wait.

Your photo of the roses is lovely!


    Bookmark   October 9, 2013 at 1:14PM
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lynnpan(z6 PA)

I'm wondering if I just killed my rose bush ? I have been cutting it back for the last 2 weeks and really pruned it down this weekend to about 4 feet ( save for a few branches that are still blooming ) . As the roses were dying off, i was cutting those branches back. I did so because we planted this bush about 5 years ago in front of our front porch and has become so large - to almost 6 feet tall ! ( we got it in a gallon pot at kmart and never thought it would get so large it would almost overtake that part of the landscape area. ) With trick or treating coming up I didn't want kids to get stuck by the roses. So as I cut back the front part, I just did it all around. Was this really a bad time to do it ?
As I was cutting it back too, I was taking some of the branches that were still in bloom and had buds and put them in a vase, but they didn't last too long and the buds didn't bloom either.

This post was edited by lynnpan on Mon, Oct 14, 13 at 17:30

    Bookmark   October 14, 2013 at 4:57PM
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jim1961 Zone 6a Central Pa.

It should be ok lynnpan... But I'd wait until Spring to do anymore heavy pruning...

    Bookmark   October 14, 2013 at 7:58PM
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lynnpan(z6 PA)

Thanks jim1961 ! Will do and will wait til spring at this point.

    Bookmark   October 16, 2013 at 11:11AM
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OK, so I planted a number of KO roses in full sun in my backyard and that was pretty much the end of it. They have continued to bloom each year and have grown tall & full ... they are now 6 ft tall and looking like they need tending to. (see attached photo). And they also seem to have some kind of fungus or something? (see photo) I'm in Cincinnati - can I cut them back now or will that harm them through the winter? If so, by how much? I'm reading here they should be fertilized monthly? Oops. =/ And what do I do about this fungus, please ... or tell me what it is if you know. Someone please tell me what to do. Your green-thumb insights are most welcome. Thx

    Bookmark   November 2, 2013 at 3:59PM
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OK, so the close-up of the "ick" on the leaves didn't attach ... or maybe we can only attach one photo? Anyway, here it is ... any advise?

    Bookmark   November 2, 2013 at 4:01PM
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seil zone 6b MI

They have some black spot which is totally normal for this time of year. It's fall, cool and damp, and the roses are slowing down to go dormant. You can spray a fungicide if you must but I wouldn't worry about it too much.

I would not recommend pruning them now. Roses store energy in the canes to grow with in the spring before the leaves can sprout to produce new food. So you want to keep as much of that stored energy as possible. You will probably lose some of it to winter die back so you don't want to take off good cane that you don't need to now.

Wait until the early spring when you can see that the nubs on the sides of the canes (leaf nodes) are beginning to fatten up or tiny leaves are starting to sprout. That's the time when the roses are in their best growth mode and will bounce right back from pruning. At that point you'll be able to tell what is still alive and growing and what is dead wood. First take out all the dead wood. The easiest way to tell if you have good cane is to clip off a couple of inches from the top and look at the center. If it's creamy/greenish white it's alive. If it's tan or brown it's dead. Keep clipping down a few inches at a time until you see a clear center. Be careful. You can not assume a cane is dead just because the outside looks brown and barky. As canes age they will become thicker, browner and barky looking but they're just more mature not dead. Once that's done you can see what you have and then prune down for the shape and size you want. Remember though, pruning will stimulate growth and they'll probably climb right back up there by the end of the season, lol!

    Bookmark   November 2, 2013 at 8:14PM
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Jess, how do you edge your beds. They look so tidy. Thanks

    Bookmark   November 2, 2013 at 9:26PM
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Here is a picture of mine the day I planted them. I deadhead 2 out of 3 a few weeks later just to keep everything looking clean .

    Bookmark   May 21, 2014 at 11:07PM
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