Friday, July 24th, 2009...12:47 am

Blooming Problems With Knock Out Roses

Gotta love those knock-outs!!! by perennialpal

Gotta love those knock-outs!!! by perennialpal

TheGardenLady received these two questions about Knock Out roses from Mary and Diane respectively.

I planted eight knock out roses in the sun last year and they bloomed. After blooming the first year I put mulch on the base of the rose bush and the ground was not frozen. This spring I cut them back slightly the first bloom was great. We went on vacation and had a bad rain storm and the roses stopped. What am I doing wrong?

Last Spring (2008) I planted 9 knock-out roses. They grew and bloomed fairly uniformly. This Spring they looked very healthy, and all burst into bloom at the same time. After the first bloom, two of them turned a very light green.  I have checked them for over/under watering. Fertilizing is not an issue as I have fertilized them all uniformly. The other 7 are beginning to bloom for the second time, but these two do not have new buds. They do not look like they are ready to die.  I can find no evidence of insect damage either at the root or leaf level. They are all planted in the same bed in one grouping for concentrated color (i.e. within feet of each other).  I am stumped as to what to do. Any ideas?

TheGardenLady imagines that you must have a fantastic show of concentrated color with those beds Knock Out Roses. TheGardenLady has just two Knock Out Roses and the floral show is amazing.

Two questions that are similar but not the same; however the treatment of the roses should be the same.

The first lady had Knock Out roses that stopped blooming after heavy rains knocked off the flowers and haven’t started reblooming.

The second lady has some of her Knock Out roses in one bed that are not blooming.

Roses need sun and lots of it. The first lady had a major storm that obliterated her roses. TheGardenLady does not know if rains continued as they have for an exceptionally long time this June in the Northeast.  Rain and gray weather are not conducive for flowering.

Before TheGardenLady gives both of you some suggestions to help your Knock Out roses, let me explain to the second woman that for plants even planted in the same bed, not all parts of the bed are created equally. Sounds silly, but parts of the soil just a few feet away from other parts might not be quite as ideal- in the GardenLady’s property there is lots of shale in the ground which can affect the water drainage. And even though you fertilize your plants equally, some plants might need more fertilizer than others. Both of you can try trimming back your Knock Out roses that aren’t showing buds. You can do this any time of the year to stimulate new growth. Trim no more than 1/3 of the shrub.

Even if you don’t trim, you must use a fertilizer that has a high phosphorus component in it to stimulate rose flowers to bloom.

When you read the fertilizer content on the label you will see 3 numbers. The container might say 10-10-10 or something like that. The three numbers on your fertilizer container represent Nitrogen (N), Phosphorus (P) and Potassium (K) in that order. The number is the % by weight of the nutrient contained. For a simple understanding of what these chemicals represent, the first number, Nitrogen is good for growing new leaves on a plant, encouraging good plant growth and the healthy green color of the plant. The second number stands for Phosphorus. Phosphorus is for encouraging more flowers, fruit and seeds and a strong root system. And the last number is for Potassium which is a catalyst for many plant functions like forming strong stems and fighting disease, etc. These are the three nutrients that are needed in greater amounts by plants. They are called the major or macronutrients.

It is recommended that to have a lot of flowers on your perennial plants like Knock Out Roses, once they are about one year old, is to feed them a fertilizer that encourages flowering. These fertilizers are referred to as bloom booster type fertilizers because they have a very high Phosphorus number on the label. TheGardenLady has used Shultz BloomPlus plant food very successfully on Knock Out Roses and other flowering perennials and annuals both when she worked on a show garden and on her own flowering plants. Shultz label says 10-54-10 so you can see how high the phosphorus % is. There are other brands that make bloom booster fertilizers- just look at that middle number. The reason TheGardenLady does not use bloom booster fertilizer on perennial plants the first year is to allow the plant to become established and acclimated to their new location in her garden before forcing them to bloom.

And be sure to read the label and FOLLOW DIRECTIONS exactly. Scultz includes a measuring spoon in the box and tells you exactly how much water to use. You do not want to guess when you give the food because the plant fertilizer or food has been scientifically formulated and the manufacturers know what will work and what will harm. You don’t want to burn or overfeed the plant. Too much of a good thing may make it a bad thing.

Another thing that some top nurseries recommend to add to all their plaints is NOT a fertilizer but is a vitamin and hormone supplement. It comes in a small bottle and is called SUPERthrive #1 plant health EXTRA LIFE. You add a few drops to the plants WITH the fertilizer. The label says it should be used all the time but some people feel that this vitamin-hormone supplement is particularly beneficial to use when your plant is under stress. Read the label when applying . You can buy it in stores like Wal Mart.

I always have a weakness for knock out roses. by kitty kitty @...

I always have a weakness for knock out roses. by kitty kitty @...

Try these suggestions and you should have your Knock Out roses reblooming this summer if there is lots of sun. Let TheGardenLady readers know if you have success. We would love to see photos of your roses.

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  • I recently came across your blog and have been reading along. I thought I would leave my first comment. I don’t know what to say except that I have enjoyed reading. Nice blog. I will keep visiting this blog very often.


  • My Knock Out Roses have looked good untill about a month ago. They were cut back in March and have grown several feet since then some are seven feet high now. We have more than a dozen of them. The pboblem I have is that the leaves are falling off and most of them are almost bare. I had the same problem last year. What can I do to not have the same problem next year? Thank you

  • To John Mahoney:

    I have a similar problem with my Knockouts losing almost all of their leaves; but they still kept on blooming. I don’t know if they are poorly drained, to dry, of an insect problem. Did you ever get a an answer?

    It is winter here in SC, and I am not sure what remedial action to take.


  • What a great web site and very helpful. After the knock out roses have bloomed how far should I cut them back?
    I live in OK where the heat and wind are constant.
    Thank you so much.

  • Is it safe for a dog to chew on echinacea leaves?

  • 1. I live in middle of CT.
    2.soil is clay
    3. I just planted knock out roses in mid to late May 2018
    4. We added bone meal to soil and then planted bushes then, put weed mat and mulch over the Matt around each bush.
    5. I just added Rose Tone fertilizer
    6. Base of plant watered 5 minutes per day , twice per day. I put my finger in to make sure the soil around each Bush was moist.
    7. Green parts look fine
    8. Not blooming. The early flowers have almost disappeared
    9.old buds falling off
    10. New buds look ok
    11. I can send a photo if allowed
    12. Overall, plants don’t look perky . Appear a bit wilted due to no flowering.

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