How to keep native plants alive in containers

Native plants are sold on the promise that they need little or no watering once established. That is true in the ground. It is not true in a pot. The Theodore Payne Foundation puts it directly: no plant is drought tolerant in a container except succulents, and container plants should be watered regularly.

A plant in the ground can send roots outward and downward to find moisture. A plant in a pot cannot. Expect to water a container through the growing season even if the same species would be left alone in a bed.

The National Wildlife Federation notes that many native plants have deeper root systems than the nursery plants most potting advice is written for, so large and deep containers are the better bet.

The Colorado Native Plant Society gives a floor: do not bother with containers less than 15 inches deep, unless you are making a small tabletop arrangement with naturally small plants. For capacity, the Illinois Department of Natural Resources estimates a 10 to 12 inch container will hold three medium plants started from four to six inch nursery pots.

When you move a plant up, the Theodore Payne Foundation advises using a container no more than two to four times the size of its current one, and repotting to a larger size every year or two once the roots have completely filled the pot.

The Theodore Payne Foundation is permissive about what a pot can be: nearly any durable, solid-walled object with a drainage hole will work. The hole is the requirement, not the material.

If you use saucers, empty the excess water out of them rather than letting the pot stand in it.

Per the Theodore Payne Foundation, glazed ceramic and plastic pots hold soil moisture longer and need watering less often than porous wood, unglazed terra cotta, or low-fire terra cotta.

In a full sun spot, avoid dark-colored or metal pots. The same source warns they heat up enough to damage roots. On a balcony with a south or west exposure and a reflective wall behind it, this is not a small effect.

The Theodore Payne Foundation recommends a mix that crumbles easily and drains quickly, such as a cactus mix containing perlite, a light mineral that aids drainage. Garden soil compacts in a container and holds water where you do not want it.

The Theodore Payne Foundation describes the target as soil that stays slightly moist at all times, with only the top inch or two allowed to dry between waterings. Water thoroughly, until it drains from the bottom.

Never let the soil dry out completely, unless you are growing bulbs that want to be dry through summer dormancy. If a mix gets so dry it stops absorbing water, the same source says to break up the surface with a fork or cultivator, then soak the whole container in a tub or bucket until the soil is thoroughly moistened.

A balcony is not a yard with less space. Building walls, the railing, and the overhang of the balcony above you all cut into the light, and the number changes through the day and across the season.

Before you buy anything, watch the space across one clear day and count the hours of direct sun it truly gets. That number, more than any other input, decides what will grow there. It is also the first question worth answering, because it is free.

Most natives are fine without fertilizer. The Theodore Payne Foundation makes a narrow exception for containers: feed sparingly, with fertilizer diluted to a quarter strength, once or twice a year at the start of the growing season. Do not fertilize during dormancy or immediately after transplanting.

In hot spells, the Theodore Payne Foundation suggests clustering containers together to retain soil moisture and keep roots cool, or double-potting by slipping smaller containers inside larger ones.

This is where most potted perennials are lost, and the reason is structural. A container gives roots much less insulation than open ground, so the root zone runs colder than the surrounding soil would.

Two organizations give the same guidance on how to handle it. The National Wildlife Federation advises choosing plants able to withstand at least two hardiness zones colder than your own, with the worked example that in Indiana in Zone 6 you would look for plants hardy to Zone 4. Plant NOVA Natives gives the same two-zone rule.

Beyond plant choice, the National Wildlife Federation suggests moving containers near building walls, using mulch or fallen leaves on the soil surface for insulation, and watering sparingly through the winter rather than not at all.

Plant NOVA Natives adds a point that catches people out: the bigger winter problem is often moisture rather than cold, because many plants cannot tolerate wet roots. Drainage matters as much in February as in July.

A practical one, not a legal one. Watering until it runs out of the bottom is the right technique, and on a balcony that water has somewhere to go. Use a saucer, and empty it rather than leaving the pot standing in water. Check your own building rules before hanging anything from a railing.

Many native perennials die back completely and spend winter as roots and bare stems. A pot that looks like a container of sticks in February is usually a plant doing exactly what it should.

Resist the urge to pull it and replace it. Give it until late spring in your area before you decide anything is gone, and keep the soil from drying out completely in the meantime.