We've all shimmied a seedling out of a plastic pot and seen it: roots spiraling in stifling circles. Stunted growth. A dense knot of disappointment.
Plants didn’t evolve to grow in plastic containers. So, when we force them into a home they don’t recognize, things get wonky. Roots grow down until they bonk against the container walls. With nowhere to go, roots end up circling over and over, becoming a tangled mess. The result? A plant that can barely breathe and barely grow.
Plastic pots (left) lead to roots coiling up in a stifling knot, stunting growth; Kelp Pots (right) are porous and allow for air exchange and root penetration. This encourages air pruning naturally, resulting in thriving network of roots.
What is air pruning?
Air pruning is a growing technique that exposes roots to air, convincing a plant to think and develop differently than in a plastic container. When roots meet air, they die back due to lack of moisture and humidity. This signals to the plant, “Oh boy, I should stop putting all this energy into these dying roots and grow new ones.” This process happens over and over again, until there’s a thriving network of young, vigorous roots.
Think of plastic containers and air-pruning as ways of building a team of roots for the growing season. Who would lead their crew to victory, a couple of old veteran players or an enormous squad of talented young recruits? What happens if a veteran blows out their back and can’t compete anymore?
Why plants <3 air pruning
📏 Growth spurts
Air pruning triggers plants to grow loads of tiny root hairs, dramatically increasing surface area for nutrient and water uptake.
🥨 No more knots
Roots don’t coil around each other or choke the plant.
🧳 Better transplant success
With a rich root network, plants establish more quickly when transferred to their new home.
Ways to air prune
🛍️ Fabric bags
These bags allow air to move through fabric, resulting in air-pruning on all sides. Lightweight, portable, and some can be washed and reused.
💩 Plastic air pruning containers
Look similar to traditional plastic pots, but include holes or slats to expose some roots to the air. We don’t endorse these, however, because, well...plastic blows.
🌱 Biodegradable seed-starting pots
Some of these pots, like Kelp Pots, are designed for roots to penetrate walls as they grow, naturally air-pruning as the plant lives its life.
How Kelp Pots breathe life into your roots
Kelp Pots are made from a porous blend of kelp fibers and organic matter. That structure does two things at once:
1) Allows roots to easily grow through pot walls, so root tips self-prune instead of circling.
2) Holds onto just enough moisture, thanks to kelp’s natural gel-like properties, to prevent seedlings from drying out completely (a common flaw with peat or paper pots).
Early on, roots branch and strengthen inside the pot. Later, as the pot softens in soil, roots naturally grow through the pot wall into the garden bed. It’s the best of both worlds: air pruning first, then root penetration for rapid establishment.
Air pruning tips
💦 Water vigilantly
Growers new to air-pruning often under-water. Because of constant exposure to air, there’s a risk of soil drying out quicker. Also, consider bottom-watering in a tray to avoid damaging vulnerable seedlings.
🏁 Don’t get ahead of yourself
Some gardeners start seeds indoors too early, leading to rootbound and stressed seedlings. Most seed packets will provide info on the best time to start seeds indoors. Typically, you should start seeds indoors 6-8 weeks before the last frost date. Check out the USDA Planting Zone Map for understanding which crops are hardy in your area.
🙏 Keep the faith
One week is a good, standard amount of time to air prune. It’s tough to see root tips curl up and shrivel when they’re exposed to air. It feels like you’re doing some wrong, but it’s an important part of the process. Resist the urge to helicopter parent and let your plant babies struggle and figure out. They’ll be stronger for it!
Plants that benefit most from air pruning
Annual vegetables with massive root systems like tomatoes and peppers, and perennial herbs and plants like hostas greatly benefit from air pruning. Allowing them to spend extra time in Kelp Pots before transplanting will enable them to grow without stress or become root-bound while you prepare their forever home.
- Tomatoes
- Peppers
- Broccoli
- Eggplant
- Celery
- Lettuce
- Kale
- Swiss chard
- Squash