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Deadhead mums in late spring to mid summer. Find blooms that are wilted or dead. Pull off dead blooms with your fingers. Clip off bunches of dead flowers with shears. Shear at the bottom of the stem for major deadheading. Don’t prune outdoor mums after they flower.
The best time to deadhead or prune mums you are growing outside is during the late spring up to mid summer. This is right before blooming season, so the flowers have time to branch off from the cut stems. Deadheading during the warm season also makes it less likely that the vulnerable cut areas will be exposed to cold temperatures.  If you are growing your mums in a greenhouse or indoors, you can deadhead them as soon as you see dead growths since the mums won’t be exposed to cold weather. When it’s time to deadhead your mums, inspect the plant to find the flowers or branches that are spent. Most mum blooms are orange or yellow in color when healthy, and brown when dead or dying. They also are likely drier than the other healthy flowers of the plant, often with a papery and brittle texture to their petals. If your mum plant only has a few dead blooms, you can simply pull off the browned flowers with your fingers. To do this, grip the stem of the mum right under the dead flower, then simply pinch and snap off the dead bloom. If you are growing your mum indoors or in a greenhouse, this method is probably best for you since you can continuously deadhead throughout the year and don’t have to worry about exposing your mum to cold temperatures. If many of the blooms of your mum plant are dead, but the stalks appear to be alive and are growing buds, use a pair of garden shears to clip multiple flowers off at a time. Angle the shears right under the dead blooms and clip, taking off the blooms from a whole section of the plant. Repeat until you have taken off all the dead blooms. If most or all of the blooms and stalks on your mum plant are dead, you’ll need to use a pair of gardening shears to deadhead your plant. As soon as you see new green growth from your mum plant emerging from the ground, take a sharp pair of gardening shears and clip all of the dead sections of plant as close to the ground as you can. This will eliminate not only the dead flowers, but also the dead stems of the mum plant so that it can start anew.  You may feel nervous about cutting off so much of the plant. However, if your mum is a perennial plant that went through a winter season, it’s necessary not only to cut off the flower but also the dead stems. Make sure that if you are doing a major deadheading that you do it during spring or early summer. This will give the new growths of the plant time to grow fully before blooming season in the fall. Once your mums flower in the fall, you may be tempted to deadhead any dead flowers you see among the healthy blossoms. Resist this temptation if you are growing outdoor plants in a region that has a cold winter season, as clipping back flowers right before cold weather will leave your plant vulnerable.