The aerial or above-ground portion of perennial phlox dies when hit by frost. The roots remain alive underground. Most phlox plants are hardy down to zone 4. Check the label on a specific cultivar to make sure it is hardy in your USDA planting zone.
Add a layer of mulch if the phlox was planted in the fall or only marginally hardy in your planting zone. Snow cover is nature's insulation. Do not shovel snow off your phlox in an attempt to protect it from the snow. Removing snow is not beneficial and may even be detrimental. Snow protects plants from drying winds.
Cutting Back Phlox For Winter
Cut back tall phlox in autumn after the stems and leaves have been killed by frost. Leave the top two or three inches of the stems above the soil line. Creeping phlox is evergreen, but its leaves may dry out during the winter months. There is no need to trim back creeping phlox unless it has outgrown its space in the garden. Inspect creeping phlox for insect infestations or diseases in autumn. Spray the plant with insecticidal soap if an insect infestation is detected. Remove diseased plants to prevent spores from spreading.
Phlox Winter Care in Pots
Move pots such as non-glazed clay pots that are meant to be left outside during freezing temperatures. Place them in a garage, storage shed, or on a covered porch where they are not exposed to rain or snow. The containers may crack during the winter when moisture inside them expands as it freezes. Trim tall phlox after it has died back for a neater appearance in your garden. The dead stems do not hold up well to snow and will break. Creeping phlox does not need trimming unless it needs to be reshaped or reduced in size.
Watering Phlox in Winter
Phlox goes through a dormant period during the winter months. Watering is not necessary unless you are overwintering creeping phlox as a houseplant. Water house plant phlox only when the top couple of inches of soil become dry.
Growing Phlox Indoors
Creeping phlox can be grown as a houseplant. Spray the plant with insecticidal soap and inspect it for disease before moving a container indoors for the winter to prevent a potential infestation. Water the soil when the top few inches dry out. Overwintered phlox does not need fertilizer. Tall phlox goes through a dormant period making it unsuitable for use as a houseplant.
Steps To Care For Phlox in Winter
Perennial phlox goes through a dormant period during the winter. It needs little to no attention during this time.
Step 1 - Water phlox well at the end of the growing season if the weather has been dry.
Step 2 - Cut back stems of tall phlox to within 2 or 3 inches of the soil line after they've been killed by frost. Creeping phlox does not need to be trimmed unless it has become too large or has dead sections.
Step 3 - Add an inch of mulch or shredded leaves over the stem stubs. This is optional but recommended if the phlox is marginally hardy in your planting zone.
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