Remember, these tips and tricks are current for zone 5 and need to be adjusted one week earlier for each 100 miles south of the southern tip of Lake Michigan that you live, and one week later for each 100 miles north.
August
Many rosarians (anyone who grows roses is a rosarian) like to set back and enjoy the fruits of their labor in August. I'm tempted to do that too but now is the time to start planning for fall. Yes, fall. You can give your roses one more feeding but not after August 15th. Foliar feeds can be continued into September. The point is to avoid new succulent growth that will not have time to harden off before it freezes. If you have kept up your fungicide spraying schedule and have kept the bushes pruned and cleaned up properly you should have little or no blackspot. It's important to continue to spray regularly all the way until the first hard frost. Keep the spent blooms deadheaded and the beds well watered and you should start to see taller growth and wider rosebushes as they get ready for a spectacular September bloom. My end of August, first of September bloom is my best of the season. As the temperatures settle down and the nights get cooler, the buds have time to develop into bigger and more beautiful flowers. This along with all the food made available to the rosebushes due to the lush green growth can only result in a wondrous and colorful display. Now is the time to start any projects you might want for the winter. I'm rebuilding my polyhouse again, hauling in lots of horse manure and keeping my eyes open for clearance sales on items I might need next spring.
You should not apply fertilizer after August 15th in Zone 5. Otherwise the new canes will not have a chance to harden off before the first hard frost does them in. After September first don't prune anymore. When you prune you signal the bush to keep sending out new flowers and growth. Leaving the dead flowers tells the bush to start setting seeds and start hardening off for the winter.
Don't forget to keep up your weekly spray program until frost. Powdery mildew can be vicious with the cool nights and warm days and you need those bushes strong to survive the upcoming winter. Dead and weak spring rose bushes are started now if you don't prepare.
Don't let up now, the best is yet to come.
September
Stop deadheading your roses now. When the blooms wither you can pluck the petals to keep the bush looking neat. Leaving the dead buds on the bush signals it to make seeds (rose hips) and to start hardening off for winter. If you wish to feed them just use a very dilute water soluble fertilizer (half strength or less) and spray it on the leaves for a foliar feed, but don't continue this much longer than past the middle of September.
After plucking the petals, this would be a great time to make potpourri. You can dry them by spreading them out on newspapers, putting them in little net bags and adding a drop or two of fragrant rose oil to them. They make great personal gifts or put one on each of your wrapped Christmas gifts for a personal touch.
Start planning what you need to do to amend your soil for next spring. Call your county extension agent and find out how to take a soil sample and where to send it. Doing it now will give you time to put down slow acting minerals such as lime if you need it. Lime takes 2-3 months to start raising your ph IF it's needed. You should not apply these minerals until a soil test indicates you need them.
You might also start looking for a source of wood shavings, compost, mulched oak leaves, or soil to cover the bud unions of each bush when the time comes. It seems kind of early to think about this now but a good gardener is always planning ahead.
If you have done everything right, your roses should be in the height of their glory this time of the year. As the hot summer days change to more temperate weather and the nights get cooler, the rose buds have longer to develop. They are therefore larger which makes for larger and longer lasting blooms. My best bloom comes after September 1st because the rosebushes are fuller, filling all the spaces between them. I like to think of my yard as a cornucopia of color this time of the year. I saw an Emergency Ambulance stop in front of my house today and when I went out to see what they wanted, I saw they were just admiring the roses. That's what makes all the work worth while.
Keep up your weekly spray program. If you haven't you probably have a lot of blackspotted leaves laying on the ground under your rosebushes. You will probably have new growth on the canes where the diseased leaves were. All this takes vigor from next spring's growth and makes the rosebush more susceptible to damage or dying during the winter. Spraying also helps the leaves stay free of powdery mildew when the nights turn cooler. See, you can't win. Hot, humid days and warm nights make for blackspot and warm days and cool nights bring on powdery mildew. We've got to love those roses to put up with this, don't we?
Now is the time to fertilize your yard and put down a broadleaf weed killer. It's also wise to watch for those end of season sales on fertilizer, spray chemicals, and gardening tools and supplies. I try to buy all of next year's garden supplies in the fall. If you plan right it saves big bucks and gives you extra money to buy more rosebushes next spring.
This is also the time of year to plan for your next year's new rose bush purchases. Finding the right places for them are important as well. A wise Rosarian once said that you should never order more than 10 more roses than you have room for. I always order 20 more than I have room for but always find a place to put them.
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