David C. Zlesak, University of Minnesota Extension Educator
The popularity of creatively combining different plant materials in window boxes has soared in recent years. Combining plants of various color, texture, and plant habit to design what in essence become miniature landscapes provides gardening enjoyment from both outdoor and indoor vantage points. Designing and having window boxes can become a very fun, creative, and rewarding endeavor. The diversity of plant materials to choose from and effects that can be generated seem almost limitless. Many people choose to try different combinations and overall themes each year. This makes it exciting for neighbors and passersby as they anticipate what will be next.
Photo 1: Window boxes can help to make outdoor spaces inviting. David Zlesak
A decade ago when I was a greenhouse grower at an independent nursery, designing and raising our own combination planters was one thing that helped set us apart from the competition. Combining different species or cultivars of plants that grow in harmony with each other throughout both the production period and then as they matured after purchase is challenging and requires looking beyond just what looks good together at the time of planting. For instance, dark foliaged sweet potato vine looked fantastic in hanging baskets and window boxes with trailing verbena and bacopa at first. However, as the season went along, the sweet potatoes being produced under the soil eventually crowded out the other plants if the containers were relatively small. Much of the fun is to experiment and discover combinations that work. Familiarity with the cultural needs and growth habits of different plant species and cultivars can go a long way to better choose combinations that will work well over the long run.
Photo 2: Experimenting with plant materials to see what combinations work well together is part of the fun. David Zlesak
Group plants with similar cultural requirements.
Combine plants that share similar light requirements. Plants that do well with half a day or more of full sun include ageratum, marigold, petunia, and zinnia. Plants that need to be shaded most of the day include common impatiens, tuberous begonias, and fuchsia. Higher and lower light-requiring plants can sometimes be successfully combined. For example, combine larger, higher light-requiring plants that cast shade over smaller, more shade-tolerant plants. Temperature preference is often closely allied with light requirement. Heat loving annuals like gazania and moss rose also prefer full sun and are also more tolerant of drying out between waterings.
Photo 3: Moss roses (Portulaca sp.) do well in full sun and relatively dry conditions. David Zlesak
Combine plants with similar moisture needs. Plants that can tolerate or prefer being kept on the dry side include gazania, moss rose, verbena, and zinnia. Those that prefer to have a consistent supply of moisture include coleus, tuberous begonias, and fuchsia.
Often not thought about, the pH of the growing medium influences nutrient availability and other aspects of plant growth. In addition to the starting pH, pH can change over time in response to water quality and type of fertilizer used. Often the pH of city water is elevated to a pH of over 7.5 with sodium hydroxide or other bases to protect metal pipes from slowly deteriorating. High water pH can elevate potting medium pH over time. The rate of pH increase is dependant on some properties of the water (dissolved minerals) and the particular planting medium and its pH buffering ability. Knowing your pH will aid in selecting better adapted plants. For instance, geraniums, celosia, and marigolds prefer a higher pH (alkaline) and petunia, pansy, and bacopa a lower pH (acidic). It often becomes clear in mixed plantings of for instance petunia and geranium if the pH is shifting more towards the acidic or alkaline side based on which is performing better.
Photo 4: Bacopa does best in acidic soil. David Zlesak
Combine plants considering their growth and flowering traits
Using plants with similar growth rates helps to prevent slower growing plants from being smothered by vigorous neighboring plants. A very vigorous spreading petunia cultivar, for instance, may choke out a neighboring compact-growing ageratum or geranium. One factor that influences growth rate is response to fertilization. Many of the more expensive, specialty annuals like spreading petunias, bacopa, and sun or solar coleus, for instance, are heavy feeders and can grow quickly in a nutrient-rich environment.
Photo 5: 'Tidal Wave Silver' petunia is a very vigorous spreading petunia and is overtaking its slower growing petunia neighbor and growing out into the walkway. David Zlesak
Some plants have looser, more open, airy growth habits and tend to intermingle and flower better with each other, while other plants have denser growth and more difficulty growing with other plants. Growth habits also vary between upright, mounding, or trailing. Loose-growing trailing verbena and bidens cultivars can intermingle and flower well together, while geraniums, tuberous begonias, spreading petunias and bacopa cultivars tend to make a thick, mounded plant or a relatively dense mat of growth capable of crowding out neighbors. Upright, taller growing plants are often placed towards the center or back of a container, while spreading or trailing plants are placed near the edge of the container where they can grow over the containers edge. Understanding growth patterns of plants can help in determining placement in the window box and how close to plant different species so each is showcased, not smothered. Window boxes typically have cascading types of plants planted near the edge that can spill over, while more mounded or upright plants are planted closer to the window.
Photo 6: Cascading lamium and moneywort are planted near the edge of this window box and more upright and mounded plants are planted towards the center and window. David Zlesak
Some plants flower continually, while others have more limited periods of bloom. For instance, osteospermums (cape or sunscape daisies), snapdragon, and pansies reduce or stop flowering during the heat of summer, but flower again once the temperatures cool down. Mixing plants so that something is blooming at all times during the season can help enhance season long beauty.
Vast plant selection options
Annual bedding plants are traditionally used for window boxes since plants are typically discarded after each season and replanted in the spring. For many traditional annuals there are newer, compact versions of old favorites that are more amenable to confined spaces. For instance, the profusion series of zinnias are more compact and more amenable to confined places than the old fashioned taller zinnias. The Profusion and Pinwheel series of zinnias are crosses between the small, disease resistant Mexican zinnia (Zinnia angustifolia) and the traditional large flowered garden zinnia (Z. violacea, Aka Z. elgans). More colors and doubled flowered versions have been added to these series.
Photo 7: Ageratum is a common sun loving annual bedding plant used in window boxes. David Zlesak
Many plants used as annuals in Minnesota are actually perennials in their native climate. Examples include geraniums and fuchsias. Also consider using hardy Minnesota garden perennials in containers. At the end of the season they can be nestled into a perennial bed to extend their value, or discarded as an annual. Due to window boxes being exposed to the air from all sides during winter, typically Minnesota hardy plants may not overwinter if left in a window box. Root are typically less hardy than stem tissue and the strong insulation ability of soil and snow prevents roots from experiencing winter air temperatures. Frequently-used, Minnesota-hardy perennials for containers include lamium (Lamium maculatum, L. galeobdolon - plants are low and spreading and variegated foliage looks great even when not in flower) and golden-leaved creeping Jenny or moneywort (Lysamachia nummularia ‘Aurea’). Compact versions of standard perennials that have a long flowering season make great choices as well and include astilbe ‘Sprite’, heliopsis ‘Tuscan Sun’, and scabiosa ‘Butterfly Blue’.
Compact versions or carefully pruned specimens of an especially ornamental woody plant can make a great accent or focal point in a container. Some woody plants have especially interesting flowers, foliage, or texture. One potential option includes the new compact, variegated weigela, ‘My Monet’.
Photo 8: Dichondra 'Silver Falls' has a uniquely shaped, silver leaf and vigorous training habit. David Zlesak
Foliage color, shape, texture, and size all can add unique elements to a design, even if the plant never blooms. In addition to traditional foliage plants such as dusty miller, coleus, alternanthera, and dichondra, consider using herbs and vegetables with interesting foliage color and texture such as fennel, parsley, and rosemary. Gold, purple-black, and variegated cultivars of ornamental sweet potatoes have become very popular and are quite versatile in containers. One unique consideration with sweet potato is that in relatively small window boxes the tubers it produces can easily crowd out other plants as the season progresses.
Creating one or more window boxes dedicated entirely to herbs is also a great option, especially window boxes conveniently located outside a kitchen window or bedroom or other window where the fragrances can particularly be enjoyed. Many herbs have a compact and/or spreading growth habit and would work well in a window box and include lavender, creeping rosemary, basil, some of the mints (like chocolate mint), and thyme.
Photo 9: Commercial potting medium often contains peat moss, perlite (white pieces), and vermiculite (tan pieces). David Zlesak
Basic container garden growing recommendations
Make sure window boxes have holes for drainage and are also a suitable size to accommodate your chosen plants as they grow throughout the season. Often too small a container is chosen or too many plants are put in a container at planting time which leads to overcrowding problems by midsummer. Look beyond how full, colorful, and proportional the pot looks at planting time and envision the container and size of the plants throughout the rest of the growing season. The larger the container typically the better (heavier containers will need stronger anchoring). Larger containers is beneficial for added root development and also allows for greater reserves of moisture and nutrients and allows for extended time between thorough waterings.
Potting medium needs to have a balance of both moisture retention as well as porosity so oxygen can get to the roots. If the medium does not have good aeration and frequently becomes waterlogged, then roots can be deprived of oxygen, slow in growth, and more likely rot. On the other hand, a medium that is too porous can be challenging to water frequently enough to keep moist. Most standard, commercially sold mixes for containers are high in organic matter (often containing peat moss, compost, and/or bark) and may contain perlite or vermiculite for added porosity. Instead of filling up a window box with planting medium, another option is to place a series of potted plants into an empty window box. This allows great flexibility to replace plants and to have a spot to set some plants that typically serve as houseplants during the summer months.
Photo 10: Window boxes can add a lot to enhance ones home and can be as simple as a row of geraniums. David Zlesak
Containers can dry out quickly, especially on hot windy days and after plants grow in size and their demand for moisture increases. To help with watering needs, leave at least half an inch from the surface of the media to the rim of the container when planting. This will provide space for water to puddle and then slowly soak in without quickly running off when you water.
Water crystals or beads (hydrogels) are commonly found for sale as a means to help hold moisture in potting media. Water beads are dry crystals that can absorb multiple times their weight in water. They can be hydrated and mixed into the medium before planting. Although water beads or crystals can hold water, research conducted by Dr. Jeff Gillman at the University of Minnesota and others suggest that they may be holding onto water too tightly and not benefiting plants because the water they are holding may not be readily available to plant roots. Work continues to compare different types of hydrogels.
The options for developing beautiful window boxes seem almost limitless, just like our garden beds, and they provide a wonderful, creative, and versatile accent to our home. If you haven’t tried window boxes yet, hopefully this article will inspire you to give them a try this season.