What Can You Plant in Outdoor Containers for Winter in Pittsburgh?

Outdoor containers in Pittsburgh can be filled for winter in two ways: with living cold-hardy plants, or with cut evergreen boughs arranged in the potting mix. For living plantings, the Pennsylvania Horticultural Society recommends conifers such as junipers, pines, spruces and firs; winterberry holly for its red fruit; and red-stemmed dogwoods for color that holds from December to March. 

The easiest version starts with the pot you already have. Penn State Extension advises against emptying summer containers in fall. Cut the annuals off at the soil line and leave the roots in place, because the old root mass gives better support to evergreen stems that will have to bear the weight of snow. The same article puts the timing at mid to late November, with the arrangement staying in place until the potting mix thaws in spring. This post covers which containers survive a freeze, which plants and greens to use, when to put a winter planter together, and how to care for it.

What is a winter container?

A winter container is an outdoor pot or window box planted or arranged to look good from late fall until spring. Gardeners use the term for two different things.

A living winter container holds rooted plants that tolerate cold: small evergreens, shrubs with colorful stems or berries, and sometimes bulbs.

A cut-greens container, often called a porch pot, holds branches with no roots at all. University of Minnesota Extension describes how spruce tips are pushed into the soil of a container and watered in, so that the water freezes the soil and keeps the cut tips from drying out. It notes that porch pots made this way can last all winter until spring arrives.

Which containers survive a Pittsburgh winter? 

Freezing is the test. NOAA's 1991 to 2020 normals for Pittsburgh International Airport put the median first 32-degree night at October 23, and Oakmont sits in USDA Hardiness Zone 6b on the 2023 map, where the average coldest night of the year falls between 5 below zero and zero. 

The Pennsylvania Horticultural Society sorts containers this way: 

• Plastic and fiberglass withstand cold and freezing temperatures.
• Stonecast and concrete containers are durable as well.
• Terracotta should be avoided. It is porous; the pores absorb water, and when that water freezes and expands, the pot cracks over time.

Penn State Extension gives the same advice from the other direction: store pottery and terracotta for the winter so they do not break, and make sure plastic and metal pots have plenty of drainage holes.

Drainage still matters when little is growing. The Pennsylvania Horticultural Society notes that a drainage hole is important even though winter watering is minimal. 

Which living plants work in a winter container?

The Pennsylvania Horticultural Society groups its recommendations by what each plant contributes. 

Conifers form the backbone: junipers, pines, spruces, firs and cedars.

Colorful stems carry the display once leaves are gone. Red-stemmed dogwoods such as Cornus sericea 'Cardinal' and Cornus sanguinea 'Midwinter Fire' are named for their salmon, orange and red stems, along with golden willows and white-barked birch. 

Berries add the red. Winterberry holly, Ilex verticillata, is the standout, with selections such as 'Winter Red' and 'Winter Gold'. For small pots and window boxes, American wintergreen offers evergreen leaves and large red fruit on a low plant. 

Foliage adds contrast. Yucca 'Color Guard', with yellow-centered leaves, and golden or variegated Japanese hollies are among the suggestions. 

Bulbs can be tucked in for the end of the season. Daffodils, snowdrops and snowflakes planted in the pot emerge in late winter.

One caution applies to the local climate. The society recommends Fatsia japonica only for USDA zone 7 or higher, so it is a poor bet in Oakmont's zone 6b. Downtown Pittsburgh is rated 7a. 

For plants that are meant to live in a pot through winter, University of Illinois Extension says the key is to prevent the soil mass from freezing and to maintain moisture in the soil. A large soil mass, an insulated container and a protected location all help.

How do you build a cut-greens arrangement? 

Penn State Extension describes the method step by step.

Start with the existing container. Cut annuals at the soil line and compost only the tops. Tall perennials can be trimmed to about 3 inches. Leave the root mass in the pot as an anchor. For a brand new container, add a dense potting mix, water it thoroughly, and let it settle before adding stems.

Then design with the same thriller, filler and spiller pattern used in summer. 

• Thriller: the attention-getter, such as branches with bright berries or a seasonal accent
• Spiller: evergreens that trail over the edge of the pot
• Filler: everything in between, including more evergreens, colored twigs, cones and seed pods

Penn State's list of reliable evergreens includes boxwood, false cypress, holly, red cedar, spruce, pine, yew and arborvitae. It advises avoiding short-needled evergreens, especially eastern hemlock, because they drop their needles quickly.

Two cautions come with collecting materials. Do not overharvest from one tree or shrub, since a big branch will not grow back. And leave invasive plants alone. Penn State specifically warns against using oriental bittersweet, because birds spread its berries.

Once the stems are in, water the pot well so the mix freezes around them. 

When should you put a winter planter together?

Mid to late November is the window Penn State Extension describes for swapping fall plants for evergreen boughs. The goal is to get stems into the potting mix while it can still be worked.

The NOAA normals help with the timing. The median first night at 28 degrees at Pittsburgh International Airport is November 4, so by late November freezing nights are common and a watered pot will firm up around the stems. 

Living plantings can go in earlier in fall, while there is still time for the Pennsylvania Horticultural Society's planting steps: fill the container about two-thirds with potting soil, set the plants, backfill to about one inch below the rim, and water thoroughly.

How do you care for a winter container?

Watering is the only regular job, and it depends on the weather.

For living plantings, the Pennsylvania Horticultural Society says containers may only need water on warm winter days. University of Illinois Extension adds that watering containers of needled and broadleaf evergreens, such as hollies and boxwoods, is essential in winter. 

For cut-green pots, University of Minnesota Extension advises watering during prolonged stretches above freezing, the same way summer flowers would be watered. If a warm spell is coming, it suggests moving the pot out of the sun and into shade to keep the soil from thawing.

Snow can sit on the arrangement. The anchored stems are there to carry it.

FAQ

Can terracotta pots stay outside in winter? 

Both the Pennsylvania Horticultural Society and Penn State Extension advise against it. Terracotta absorbs water and cracks when that water freezes.

What hardiness zone is Pittsburgh?

Oakmont, Verona and Fox Chapel are zone 6b on the 2023 USDA Plant Hardiness Zone Map, and downtown Pittsburgh is zone 7a. The USDA bases the map on the average annual extreme minimum winter temperature. 

Do winter planters need to be watered? 

Sometimes. Water living evergreens on warm days, and water cut-green pots during long thaws.

How long do cut evergreens last in an outdoor pot?

University of Minnesota Extension says porch pots of cut spruce set in frozen soil can last all winter until spring.

Which evergreens should you avoid in a winter arrangement?

Avoid short-needled evergreens, especially eastern hemlock. Penn State Extension notes that they drop their needles quickly.

Can you add bulbs to a winter container? 

Yes. The Pennsylvania Horticultural Society suggests daffodils, snowdrops and snowflakes, which emerge in late winter.

Custom Winter Containers From Roots to Petals in Oakmont

Roots to Petals is an independent, woman-owned florist and plant studio at 527 Allegheny Avenue in Oakmont. The shop designs custom container gardens and hanging baskets for front porches and entry areas at homes and businesses, offers spring, summer, fall and winter plantings, and provides care instructions for the plants in each design. A free design consultation can be requested by email. Container gardening is also one of the topics in the shop's workshops, and the Plant Studio carries a wide variety of pots. 

For more seasonal reading, see the shop's posts on flowers to plant in the fall in Oakmont, on the best house plants for Pittsburgh homes and apartments, and on where to find flowers, plants and workshops in Pittsburgh.

Visit Roots to Petals or call 412-822-6023 today. Contact us for more information.