How to plant and care for knockout roses
April 28, 2020
Once upon a time, roses were considered difficult to grow. Many were disease-prone, or heat intolerant, or cold sensitive, or bug magnets. I admit to killing my share of rose bushes. Then, twenty years ago (yes, it’s been that long, even though it’s still often called “that new variety”), a hotshot hit the nurseries. It was time to plant Knockout Roses.
It’s an ideal flowering bush. It blooms abundantly several times throughout the spring and summer, it’s disease resistant (not disease proof, that would be too much to ask!), it’s very tolerant of heat, and it’s hardy in most winter locations. You can even grow it in a container (though I can’t imagine stopping at one container — I love an expanse of them!)
Oh, and it’s gorgeous. There’s even a variety (the Double Knockout) that delivers twice the number of petals on each flower!
Like all plants, the Knockout Rose needs some specific care. Not too much, in this case. Here’s all you need to know to plant Knockout Roses and take great care of them:
• Choose a place where your rose will get full sun for at least six hours a day. Eight is even better. If you deny it light, it’ll deny you blooms!
• Plant Knockout Roses in balanced soil, with a pH of about 6. If you don’t test your soil pH, no worries. A soil pH of 7 is neutral — right between acidic and alkaline. Below that is more acidic, and above it more alkaline. Coffee grounds are good at bringing the soil from neutral to slightly acidic. Just add them to the soil or top dress the soil with them (that is, scatter them on top of the soil, around the bush).
• Make sure you pick a spot that has good drainage; you don’t want the roots staying wet.
• Plant your Knockout Roses in the spring (after danger of frost) or in the fall.
• Space each bush about 3 feet apart, from center to center.
• Dig a hole twice the width of your plant and about as deep as your plant’s pot. Loosen the soil in the bottom of the hole. Loosen the roots a bit at the bottom of the plant, too. Tuck the plant into the hole and fill in with good soil. The top of the soil on your plant when it came out of the pot should be about even with the ground level when it’s planted.
• Mulch, but keep the mulch about six inches away from the base of the bush to prevent it from rotting.
• Give your newly planted rose a good, soaking drink of water.
• If you need to transplant your rose bush (I’m forever moving plants!), do it in very early spring before it starts growing.
If you plant Knockout Roses in containers, make sure the pots are big enough and you keep them well watered. (Choose pots with drainage holes!) Unlike rose bushes planted in the ground, roses in a pot will need to be brought indoors (a shed or garage that doesn’t freeze will work) for winter in most locations. Give them a little drink now and then throughout the winter to keep them from completely drying out.
• Don’t fertilize your Knockout Rose until it’s established. The Knockout Rose breeders suggest waiting until it goes through one bloom cycle.
• Choose an organic fertilizer that’s formulated for roses.
• Fertilize about once a month or so from spring through mid-summer. Stop fertilizing mid-summer. By the end of summer, the plant is getting ready for winter, and you don’t want to coax it into creating a lot of new growth.
• Fertilize when the soil is moist, not dry, or you might burn the roots.
• Don’t overfertilize, or your plant will be heavy on the leaf growth but short on those spectacular blossoms.
Knockout roses bloom on new growth. It’s best to prune your bush in the early spring, right after the frost-free date. Cut it back — yes, every year! — to about 12 to 18 inches above the ground. It’ll triple (or more) its growth by the end of the summer.
If you cut your plant in late summer or early fall, you might encourage it to grow too late in the season and not have time to harden off.
BTW, Knockout Roses don’t need to be deadheaded (have the old flowers lopped off), but they look prettier, I think, if you do snip the spent flowers.
Once your rose is established, it’ll only need watering about once a week. Give it a good drink, and then let the soil dry out a bit before watering again. And, of course, if Mother Nature takes a turn watering, you can skip!
Water your rose at the base of the plant, not over the leaves. (A drip hose works well for watering the base of the plants rather than the leaves. So does careful watering with a watering can that has a long spout. No sprinklers!) Water in the morning so the soil can dry off before evening when the temps drop.
For harsh winters, mulch about two or three inches deep around the plant stem. (Remove the mulch around the stem in the springtime.) Some people like to show their roses TLC by wrapping them in burlap for the winter, too.
Yellow or brown leaves — You may have overwatered or overfertilized the plant. Let it dry a bit between waterings. A plant may also turn yellow if the weather is too hot. Not much you can do about that, but the plant should recover (like we do) when the weather improves!
No blooms — Your plant probably needs more direct sun! Remember, six to eight hours of direct sunlight per day is what your plant craves.
Japanese beetles — There are sprays and a product called Milky Spores that help eliminate Japanese beetles, but I just flick them off into a container of soapy water.
Holey leaves — Your plant might have rose slugs. You can spot the tiny green slugs on the undersides of the leaves. They won’t hurt the plant long-term, other than perforating the leaves. Some people like to use neem oil to keep critters off their roses. It’s natural, and it works. I have a strong aversion to the smell. Instead, I pick them off by hand. (Tip: They’re easiest to find in the early morning!)
Important note: If you do use neem oil, don’t spray the flowers, because it’s toxic to bees. (Yes, I think it would be hard to spray a Knockout Rose without getting the oil on the flowers!)
Powdery Mildew — This white coating is a fungal disease (remember, the plants are disease-resistant, not disease-proof!) that strikes in high humidity and mild temps. There are horticultural oils you can apply, but mildew often goes away once summer is in full swing, so consider waiting it out. In the meantime, trim away the badly affected areas. Also be sure to remove the affected leaves from the plant and the ground around it.
Black spot — This ugly blight happens in high humidity, too, though Knockout Roses are less prone to it than other roses. It won’t hurt your plant, but it isn’t pretty. To prevent black spot, only water your bush at the base, not over the leaves.
Bright red shoots and distorted flowers — Uh oh. Your plant has dreaded rose rosette, caused by a virus that’s spread by tiny mites. Cut the red branch off (down where the wood is still green and healthy). When winter comes, cut the plant back by two-thirds. This will get rid of any mites and eggs hanging on over winter. Then cross your fingers.
Last year I planted a rose that’s similar to the Knockout Rose but more compact. It’s called Drift, and, so far so good! Do you grow Knockout Roses or other roses? What are your favorite varieties?