Coleus is a tender perennial, meaning it can live for more than two years under ideal conditions. Outside of tropical USDA Hardiness Zones 10 and 11, it is cultivated as an annual because it cannot withstand freezing temperatures. Temperatures consistently below 50°F cause the plant to struggle, and frost exposure results in irreversible damage. Overwintering involves moving the plant indoors to protect it from the cold, preserving favorite varieties for the following season.
Preparing Coleus for Indoor Transition
The transition from the outdoor garden requires specific preparation, ideally before nighttime temperatures consistently dip below 50°F. The primary concern is inspecting and eliminating pests that may be hitchhiking on the foliage or in the soil. Spider mites, mealybugs, and aphids are common outdoor pests that multiply rapidly in the warm conditions of a house, quickly infesting other houseplants.
Carefully examine the undersides of leaves and stem joints for insects or sticky residue, treating any infestations with insecticidal soap or neem oil before the move. If the coleus is planted directly in the ground, repot it in a clean container with fresh potting mix to minimize the risk of bringing in soil-dwelling organisms. Pruning back long, leggy stems by about one-third helps the plant adapt to lower indoor light levels, reducing the foliage the roots must support through winter.
Overwintering by Maintaining the Whole Plant
Overwintering the entire mature plant requires sufficient indoor space and careful management of environmental factors. Coleus needs bright light indoors to maintain vibrant foliage colors and prevent legginess. A south-facing window provides adequate light, but bright, indirect light from an east or west exposure is often better to prevent scorching.
If natural light is scarce, supplemental lighting from a full-spectrum LED grow light is beneficial to sustain growth over the short winter days. Temperature management is also important, with 60°F to 75°F being optimal for continuous growth. Keep the plant away from cold drafts near windows and heat sources like radiators, which cause sudden temperature fluctuations and dry the air.
The plant’s growth rate slows significantly during winter, demanding a reduction in watering frequency. Allow the top inch or two of the soil to dry out before rewatering, as consistently wet soil is a primary cause of root rot. Increasing humidity is helpful, especially during heating season; this can be achieved by misting the foliage or placing the pot on a tray filled with pebbles and water. Avoid fertilizing during this dormant period, as excess nutrients can damage the roots.
Overwintering by Propagating Cuttings
Propagating cuttings is a space-saving alternative that produces small, vigorous starter plants for the next spring. Select healthy, non-flowering stem tips that are four to six inches long. Use clean scissors or shears to cut just below a leaf node, where new roots will emerge.
Remove the lower leaves from the bottom two to three nodes, leaving only two sets of leaves at the top to reduce moisture loss. Cuttings can be rooted in fresh water or directly into a moist, sterile potting medium like perlite or a seed-starting mix. If rooting in water, change the water every few days to prevent stagnation; roots should form within one to two weeks.
Once roots are visible and about a quarter-inch long, or after two to three weeks in soil, transplant the new plants into small, individual pots. Place these cuttings in a warm area with bright, indirect light to encourage them to grow into bushy starter plants over the winter. Pinching out the growing tip once established encourages side branching, leading to a fuller, more compact plant for spring planting.
Reintroduction to the Outdoors in Spring
The final step in successful overwintering is “hardening off” the plants before permanently moving them outdoors. Whether you kept the whole plant or grew new ones from cuttings, they spent the winter in a sheltered, stable indoor environment and are highly susceptible to sun scald and wind damage. Do not move the plants outside until all danger of frost has passed and nighttime temperatures are consistently above 60°F.
The hardening-off process involves gradually introducing the plants to the elements over seven to ten days. Start by placing the plants in a shaded, protected location outdoors for only two hours on the first day, bringing them back inside afterward. Each subsequent day, increase the duration of outside exposure by an hour or two, and slowly move them into an area that receives slightly more sun and wind. This incremental exposure allows the plant’s cellular structure to adapt, preventing shock and preparing the foliage for harsher outdoor conditions.