Transitioning Container-Grown

By Mandy Bayer, Ph.D.

The arrival of the spring planting season is a great time for a refresher on best practices for planting and establishing containergrown plants in the landscape. It is important to keep in mind how production impacts plants when installing them in the landscape. Irrigation frequency during production, soilless substrates, and circling roots should all be considered when transitioning container-grown plants to the landscape.

Container plant production is a resource-intensive process, with water and fertilizer generally applied in large amounts. The soilless substrates used in containers are meant to be lightweight, making them easier to handle and transport. The materials used in these substrates, such as bark, peat, and perlite, are meant to aid drainage, but their waterand air-holding capacities can be highly variable. Daily (or up to several times a day) irrigation is common. The combination of soilless substrates and frequent irrigation during production can result in increased water stress during landscape establishment.

Although producing plants in containers benefits the grower and consumer, the container itself can create plantgrowth problems because containers have limited rooting volume. In the ground, roots can grow horizontally, anchoring the plant and providing water and nutrient uptake. In a container, roots reach the side of the container and begin to circle around the pot. Failing to address circling roots before installation in the landscape can result in stem-girdling roots that disrupt water and nutrient flow in the xylem and phloem. Over time, this causes crown die- back, early fall color, canopy thinning, and eventual death. Newer containers such as fabric containers and containers with air holes help reduce circling roots. Root issues such as circling or girdling should be addressed during planting. Circling roots should be cut, shaved, or teased apart. Stemgirdling roots should be removed.

Root establishment is important for plants to succeed in the landscape. For a container-grown plant to maintain its growth, it must receive similar irrigation in the landscape (i.e., daily irrigation). This is generally not realistic, but should be kept in mind, as irrigation frequency will determine how vigorously the plant grows. Irrigation should be monitored closely during the first growing season and adjusted according to rainfall and again as plants become established. Keep in mind that more frequent irrigation (around every 4 days) will result in more vigorous plant growth and that applying large volumes of water will not make up for infrequent irrigation. Less frequent irrigation can be adequate depending on rainfall, but will impact how rapidly the plant grows. As roots grow out into the native soil, irrigation demand lessens, and frequency should be reduced. It can take months for a plant to become established in the landscape, and larger plants take longer to establish. Directed irrigation applications, via drip irrigation or hand watering, should be used to ensure water is applied to the root area. Overhead irrigation often falls on areas with no plants roots or is intercepted by the plant canopy, not making it to the root area where it is needed.

The trunk flare on trees should be located to ensure trees are not planted too deeply. Buried trunk flares are unfortunately common and are a major reason for tree decline or death. The planting hole should be 2–3 times