MSU Extension Gardening in Michigan

Many northern gardeners are using tropical, semitropical and desert plants in outdoor gardens and containers in the summer, but may not realize those same plants can be transferred indoors for the winter. Bringing tropical plants indoors adds an exotic flair to a northern home, drawing a visitor’s eye to lush foliage and unusual shapes. When making that transfer, there are four major things a gardener should assess: temperature, pests, lighting and fertilization/watering.

Temperature effects on plant survival

Tropicals and other cold sensitive perennials must be brought indoors during the winter for two main reasons.

• They lack the ability to adequately photosynthesis at low temperatures. With less photosynthesis, plants consume and deplete stored energy, which can cause plant death.
• Cold-sensitive plants cannot prevent ice formation between their tissues during a freezing event in the same way cold tolerant plants can. Even with adequate stored carbohydrates, a single frost may kill a healthy plant.

The ability to tolerate cold and freezing is referred to as hardiness. The USDA assigns hardiness values to regions and plants to designate what can grow in a particular area. These values range between 1 and 13 (1 is the coldest), with sub-increments of “a” and “b” (e.g., Zone 5a and 5b, where 5a is coldest). Plants with a lower hardiness number possess greater cold tolerance than those with higher numbers.

The USDA determines each value based on the lowest minimum temperatures that are expected to occur in each designated hardiness zone, e.g., Zone 5a equals -20 to -15 degrees Fahrenheit. Knowing the hardiness of your container plants and where they can survive year-round is important because a perennial in the south may not be a perennial in the north.

Tropical and temperate cold-sensitive species should be brought inside well in advance of the first frost. While many plants can survive sustained temperatures below 50 F, relocating them indoors before temperatures drop below 60 F during the day will help reduce any stress from sudden temperature changes.

Michigan’s varied climate across the state means there is no single best time in the fall to begin this process. Instead, indoor gardeners should stay updated with their area’s weather reports and plan accordingly. Once plants have been brought indoors, they acclimate to their new environment and should remain indoors until frost risk is gone in spring.

Preventing pest problems

Containers should be inspected thoroughly before being moved inside. Insects, fungi and bacterial infestations might not be readily apparent and can be hidden in various parts of the plant. For each container, check the undersides of leaves, within the tips of new growth, and just below the surface of potting mix.

In addition to looking for the insects themselves, look for signs of feeding such as chewing, stippling or mottling. Regularly check for the presence of sticky honey dew that sap-feeding insects leave behind, webs created by spider mites, or the exoskeleton remains of insects like aphids.

Insects can be removed by picking, wiping or scraping. Heavy infestations may also need to be treated with an insecticidal spray such as a horticultural oil or soap.

Fungus gnats can be very common hitchhikers of plants that move from the outdoors in. Adult fungus gnats are merely a nuisance, but their larvae in soil can damage roots. The most effective method to get rid of fungus gnats is to use an integrated approach, combining multiple forms of control: Avoid a constantly moist top layer soil of soil by watering from the bottom, use fresh soils, apply predatory nematodes and attach sticky tape to trap adults.

When possible, removing plants from their containers can reveal symptoms of an infected root system. Do so by gently inverting the container while supporting the base of the shoots, patting or massaging the bottom of the pot until the root ball comes free. A healthy root system will have lighter colored roots and fine root hairs. Unhealthy roots will be darker, may have larvae of harmful insects (e.g., fungus gnats) or may be covered with a thin layer of sticky film. This is also an ideal time to repot the plant into clean, fresh soil.

Although a mildly infested plant may survive, it is not recommended to bring a potential source of pests into the home unless appropriate control measures are taken. If pests are not apparent, keep new indoor plants separate from plants already indoors until you have time to observe them for infestation.

Powdery mildew should be a major consideration when relocating plants since this pathogen is rampant at the end of summer and early fall. If detected, keep the plant in question in the warmest/driest part of the house, limit handling to discourage further spread and apply a compound such as a sulfur-based fungicide to reduce the infection. Heavily infested plants are best discarded, particularly if they show signs of difficult to eliminate pests such as scale.

Read more about overwintering pest control tips in the Michigan State University Extension article, “Bring plants indoors now, but leave the pests behind”.

Assessing the light environment

In winter, the earth is tilted away from the sun on its axis, dramatically changing in-home light environments in northern latitudes. Areas within the home become darker and have less natural light, reducing what is available to plants. Otherwise healthy plants may become weaker and more susceptible to disease if they do not receive adequate sunlight.

Here are some important lighting considerations:

• Light coming through a south-facing window will be less intense but may still be too bright for full-shade species like Begonias and Fuchsias.
• Areas in the home with minimal natural light should only be used for a gardener’s most shade-tolerant plants.
• Plants that shed their foliage and go dormant can be placed in cooler, darker areas of the house.
• Avoid positioning containers in the direct path of vents and space heaters or in immediate contact with radiative sources of heat. Failure to do so may result in heat or water stress, exacerbating the loss of vigor from other issues (e.g., low light). Exceptions can be made when there is adequate light to sustain the growth and transpiration that a plant normally exhibits.

If there is insufficient natural light, supplementing with LED or fluorescent lighting can help. The primary considerations when selecting a light are (1) sufficient light intensity and (2) high spectral quality—typically either white light or a mix of red and blue. Gardeners growing under artificial light as a sole source of lighting are best served by selecting bulbs specifically manufactured for sustaining plant growth.

For the budget-conscious, using compact fluorescent bulbs or small LED fixtures is a good compromise over more expensive growing lights. Incandescent bulbs are not recommended due to their low efficiency and high heat output, which can burn plant leaves. For comparisons of different commercial horticulture lighting sources, see “Transplant production: Light considerations” from MSU Extension.

It is also important to consider the total time of available light in a 24-hour period. Many plants use perceived daylength as a cue for developing their flowers. These plants are referred to as photoperiodic. In Michigan, daylight falls under 12 hours starting in mid-September. Bringing cold-sensitive varieties of fall blooming species, such as kalanchoe, indoors at this time may help extend their lifespan but providing more than 12 hours of artificial light runs the risk of inhibiting the production of flowers. This can be avoided by not growing them under natural light, but instead setting light timers to turn on after the sun rises and off before the sun sets.

Fertilizing and watering during winter

Shade tolerant herbaceous perennials such as palms, philodendrons, colocasias and arums may continue to put on new growth and may still require some fertilization during winter months. In contrast, succulents, tender woody ornamentals and other slow growers may need less fertilizer and should especially not be fed prior to moving. As a rule of thumb, plants that are not actively unfolding leaves or elongating their stems (e.g., succulents) do not need fertilizer.

Additionally, avoid fertilizing plants that appear to have gone into dormancy. Loss of leaves may fool some gardeners into believing a tropical plant has died (e.g., bougainvillea) when it is simply entering dormancy. Spend some time researching tropicals that experience dormancy before pitching them. During dormancy, plants do not use nutrients at their typical rate, so adding fertilizer can stress the roots by increasing salinity to intolerable concentrations.

Ferns are particularly sensitive to over-fertilization and should be fed sparingly if fed at all. Plants under artificial lighting or in warm, sunny spots may benefit from some fertilizer albeit a reduced feeding schedule. For these plants, begin with one-fourth the amount of water-soluble fertilizer and slowly increase if plant growth rate accelerates.

Lastly, avoid over-watering indoor plants. As with fertilization, plants not growing as rapidly as they would outdoors do not require as much water. Allowing the top layer of soil to dry down in between watering will ensure that the potting mix stays aerated, giving roots ample oxygen to survive.