Pruning Perennials in the Fall: A Zone-by-Zone Guide for Tennessee Gardens
Most perennials should be cut down when the foliage is first browned or mushy by frost‚ which in USDA Plant Hardiness Zones 6b through 7b in Tennessee is normally late October or early through mid-November․ However‚ in many cases‚ the stems are best kept up through the winter‚ either to help discourage pests‚ provide food for animals‚ or keep the seed pods that you want․ The order of business is therefore more important than the timing of the pruning․
Unlike the Northeastern and Midwestern United States‚ where the first frost occurs at about the same time every year‚ the first frost in East Tennessee can occur as early as mid-October in the mountains‚ while in west Tennessee it may not occur before mid-November in the Memphis area․ You have to watch the actual foliage‚ not just the calendar․ You can't simply circle a date and prune everything at once․
Fall Pruning in this Area's Situation
The real enemy in Tennessee gardens is humidity․ Our wet falls and mild winters allow fungal spores and slug eggs to overwinter in dead leaves and explode into problems the following spring․ But if your plants are diseased and holding wet foliage over the winter in the humid Tennessee air‚ this could be a problem come April․ And likewise‚ some plants don't like a hard cutback and might be better off waiting until spring․
Bee Balm (Monarda)
Bee balm is highly susceptible to powdery mildew‚ which overwinters in old stems of the plant which are left untouched during winter․ As with most herbaceous perennials‚ they should be cut back to the ground after the first frost has blackened the foliage‚ usually late October in Middle Tennessee․ If you had the white powdery coating on your cuttings this year‚ the fungus was present‚ and you should bag and discard the trimmings to avoid reinfection next spring․
Blazing Star (Liatris)
Liatris is tougher than it looks: cut the spikes back to a height of 4-6 inches as the spikes fade and leaves turn brown․ Most garden books suggest cutting back the stems back in the fall‚ but if you wish to collect goldfinch seed heads‚ wait until January or the new leaves emerge‚ and cut back in March․
Daylilies (Hemerocallis)
You can cut daylily foliage back to within 2 to 3 inches of the earth after the first frost․ Due to mild winter conditions in Tennessee‚ daylily foliage may be semi-green throughout the month of November․ There is no real concern for when to cut your daylilies back based on the autumn solstice․ When leaves yellow and flowering ceases‚ they should be divided by digging up the plant and slicing through it with a spade․ Replant at least 18 to 24 inches (46 to 61 cm) apart․
Hostas
Most hostas need a hard frost to blacken the foliage before the plant is cut back to within an inch (2․5 cm) of the ground․ Doing this too early while the foliage is still green robs the hosta of energy that would otherwise be stored in the roots․ Everything that's been blackened‚ you have to cut back to the crown․ There's no choice about it in Tennessee․ Slugs will overwinter in the old hosta junk․ In the Pacific Northwest‚ winter is mild and wet‚ so the slugs never stop moving even in the month of January․ Should be divided every 3 or 4 years in early fall‚ well before frost‚ to allow the divided clumps to establish․
Irises
The bearded iris is not uncommon in Tennessee․ Iris borers are a potential pest which overwinter as eggs on old foliage‚ which‚ when brown‚ should be cut back to a height of 6 inches (15 cm)․ Cut back shorter than normal and‚ if borers were a problem this year (look for streaked and tunneled leaves)‚ discard clippings‚ don't compost them․ Irises should be divided every 3-4 years; do so in late summer or early fall․ This gives them time to settle in before the ground cools․
Lilies (Lilium)
True lilies (as opposed to daylilies) are bulbs and need the leaves to produce food for as long as frost-free weather persists‚ so it is best to leave them alone until they are fully brown and can be cut down․ Another approach is to cover the crop with 2 to 3 in (5․1 to 7․6 cm) of mulch․ This protects the Tennessee bulbs from freeze-thaw cycles‚ which are more damaging than a uniform freeze․ Straw and shredded leaves make good mulch․
Garden Phlox
When frost kills the blossoms‚ cut the stems down to 4-6 inches (10-15 cm)․ If powdery mildew is present (the most common condition affecting garden phlox in the hot humid summer of Tennessee)‚ removal of dead leaves from around the plant is as essential as cutting back the stems․ Good airflow next season starts with a clean bed this fall․
What Not to Cut Back
Some perennials are most valuable left up through the winter; many fall pruning lists fail to note that coneflowers and rudbeckia produce seed heads that attract birds through the winter․ Ornamental grasses add structure and winter interest but in fall they can also risk killing the crown in their winter dormancy by cutting them down to ground level․ Sedum and hellebores should not be cleaned out in the fall‚ as the dead foliage provides winter interest․ Unless the plant is recognizable‚ it is usually best to leave clumps standing through the end of February․
Preparing Your Beds for the Next Spring
Once you're done cutting back‚ planting perennials into those holes or gaps is another option․ It is preferable to do so in the fall‚ as this will allow roots to establish prior to the intense spring growth․ Since winter temperatures in Tennessee are milder and the ground does not usually freeze to the depth that root growth stops‚ TN Nursery grows field grown perennials like daylilies‚ hostas‚ and irises that are suitable for the Southeast and it is a good time of year to plant and clean them up․
After pruning‚ mulch the roots with 2 to 3 inches of shredded hardwood mulch or straw․ Winter weather can do as much or more damage as the cold temperatures‚ so mulch the soil surface a few inches away from the plants to protect the roots and make the garden ready for the spring months․
Things like fall sanitation‚ adding disease resistance‚ insect control and an overall way of giving your perennials a real jump start when the soil is warm again in the spring․ The bottom line is that if you're pruning too early in the fall‚ you're probably creating next year's problems․
