Visitors and passers-by often compliment us on the petunias and other plants we grow in containers. Plant selection is obviously important, but here are two other important things you can do to ensure success with outdoor containers.
Fill any new container with a mixture of bagged topsoil and potting mix. Topsoil gives weight to the container; potting mix provides a consistently moist, airy place for roots to grow.
Many local nurseries sell peat or pine bark-based potting mixes (also called growing mix or planting mix). I prefer a mix without fertilizer because I want to control that myself. Don’t use soil from the garden that might have disease or insect issues.
Each April, I dump last year’s containers into a wheelbarrow. Any plant debris is removed and about a third more potting mix added. The containers are cleaned and refilled, placed where they will sit for the summer and watered well. Over the next couple of weeks the soil will settle a bit, so I top off each container before planting.
For very large containers that can’t be moved I dig out the top 6 inches or so of soil, break up the soil underneath that, then add potting mix to the soil I removed before refilling.
One petunia; over 800 blooms
Use self-irrigating planters (SIPs) that have water reservoirs that are filled using a tube external to the soil. As plants consume water the soil dries out causing water in the reservoir to evaporate. The biggest advantage of SIPs is that you don’t need to water anywhere near as often. Plants like it because they only get the moisture they need (excluding rainfalls, of course.) And because you are not watering from above, nitrogen in the soil doesn’t get washed out as quickly so fertilizer requirements are significantly reduced.
A combination of slow-release fertilizer and SIPs keeps plants happy all summer.
A variety of SIPs are available, and I have over 20 — large square planters on wheels, window boxes and large round rim planters.
For some of my existing pots, I bought conversion kits with quart or gallon reservoirs. For any SIP you use outdoors, however, make sure that the container has drainage holes in the bottom in case the reservoir overflows in a heavy rain. Otherwise your plants can get flooded and die.
(Jim Janke is an extension master gardener volunteer in Haywood County. For more information, visit https://haywood.ces.ncsu.edu, or call the Haywood County Extension Center at 828-456-3575. ©2020 NC State University.)