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Egg Carton Petunias
An Unusual Old Way of Starting Flowers
February 22, 2022

My mother used to start seedlings in egg cartons on the kitchen windowsill. In a bit of a nostalgic mood, I tried starting petunias in egg cartons about ten years ago. The experiment turned out well, although the petunias quickly outgrew their egg carton cells. But the baby petunias add some nice color to a kitchen windowsill in the winter.

Our Egg Carton Petunias Getting Big

The first petunias I start each January are somewhat trailing varieties for hanging baskets. Other than one terrible mistake, I plant pelletized (pelleted) Supercascade and Double Cascade seed from Twilley Seeds for the first planting. Petunia seed is tiny, so the pelletized version makes planting much easier. Twilley packages most, if not all of its seed now in foil packets. The pelleted petunia seed ships in a reusable plastic vial...in a foil packet. That may seem like overkill in packaging to some, but we appreciate it, as we freeze leftover seed for use in later seasons.

Preparing the Egg Cartons

While I'm not a fan of Styrofoam coffee cups, Styrofoam egg cartons are perfect for this kind of planting. Obviously, cardboard egg cartons wouldn't hold up.

I first cut off and discard the narrow flap on the egg carton before splitting the egg cell section from the top. One of the advantages of Styrofoam cartons over cardboard is that the top can go under the egg cell section as a watertight drip pan. Before putting the halves together, I punch a drainage hole in the bottom of each egg cell with a sharp pencil.

Cutting egg carton Punching holes Halves assembled
Cutting egg carton Punching drainage holes in cells Halves reassembled

Planting

The cells of the egg cartons get filled with sterilized starting mix. We make our own from potting soil and peat moss, heating it in the oven for an hour or so at 400° F to kill off any damping off fungus that might be (and often is) present in potting soil. I've also tried using peat pellets in egg cartons but didn't have much luck with the pellets in egg cartons or otherwise.

Watering Pellitized petunia seeds Seeding and melting pellet Under lights
Watering before seeding Petunia seeds (under arrows) Melting seed pellet Under lights, on heat pad

Before seeding, I water the starting mix thoroughly with warm water. Our petunia seed comes as pelletized seed, so planting is just a matter of getting one seed in the center of each cell, something harder to do than it sounds. A few extra seeds do provide insurance in case not all of the centered seeds germinate.

Petunia seed needs light to germinate, so I don't cover the seed. To help the pellet dissolve and release the seed on the soil surface, I go back and drip several drops of warm water on each seed with an eyedropper or an old syringe.

Egg cartons in tray over heat mat and under lights

Gro-mat with wire rackHydrofarm Digital Thermostat for heat matsI part with Mom's practice of just setting the egg carton on a windowsill to germinate, as it's a bit cold on our available windowsill these days. Petunias also benefit from a bit of bottom heat during germination, so ours go into a planting tray with a clear cover on our heat mat and under our plant lights. For the time being, the egg carton tops that will later serve as drip pans get set aside, allowing me to bottom water the petunias if needed. Our soil mat thermostat has a probe that goes right into the soil, so we can be pretty exact with our seed starting temperatures. For petunias, I set our thermostat between 75-80° F. A warm, sunny windowsill might work as well.

Once germinated, I keep the petunias on the heat mat for a day or two more before removing the clear cover and shutting off the heat. They acclimate a bit under our plant lights in the basement before being moved to our kitchen windowsill. The petunias have to be watered every other day once they get started, due to the small size of the egg cells. The egg carton tops used as drip pans make bottom watering pretty easy.

Back Under Lights

Moving petunias from egg cartons to fourpacksEgg carton petunias now in hanging basketsAfter four to six weeks, the petunias outgrow their egg cells. I use a teaspoon to remove the petunia plant and rootball from each egg cell, moving them into roomier fourpacks filled with sterile potting mix. The tray of petunias in fourpacks goes under our plant lights.

At this point, I seed two more egg cartons of petunias. For the second planting, I use varieties that do well in our garden plots. That's often the Celebrity variety, along with whatever I might have picked up off a plant rack.

The petunias for hanging baskets eventually outgrow their fourpack cells. At that point, they get moved to their final pot. That's often a large (10-12") hanging basket with three petunia plants per pot. Occasionally, I put single petunia plants in eight inch hanging basket pots. There's usually not space for such large pots under our plant lights, so the hanging basket pots in trays go onto bookshelves in our sunroom or on our dining room table by our bay windows.

Hardening Off

Petunias
Supercascade petunias in hanging baskets

Hanging basket plantsSometime in March, I begin getting our hanging baskets of petunias outside to begin hardening off. Hardening off is the essential step of gradually exposing plants to bright, hot sunlight, cool nights, UV radiation, and strong winds to toughen them up. I generally start this step by lining the plants along the side of our house on the back porch. There, they receive some sunlight and are exposed to the wind a bit.

After about a week of hardening off, I begin hanging the plants from the porch for a few hours each day. I have to be careful with this step, as the strong winds we frequently experience can permanently damage the plants. Sadly, we almost always have a hard freeze late in March, so bringing the petunias back inside on cold nights has become a bit of a ritual at our house. In 2016, we had a 28° F low on April 2-3 that required bringing in not only our hanging basket plants, but everything that was under our cold frame!

Starting our petunias as early as we do gives us gorgeous plants by April. A drawback is that the plants begin to require a good bit of pruning and fertilization by July or August. It sometimes seems that the plants just wear themselves out. But with a little care, some of the plants make it, and we have blooms well into fall.

A bonus with our hanging basket plants is looking out our kitchen window in the summer and seeing lots of hummingbirds flitting around the plants and visiting our feeders.

Botanical Interests Burpee Gardening Required FTC Disclosure Statement: Botanical Interests, Burpee, Renee's Garden, and True Leaf Market are some of our Senior Gardening affiliate advertisers. Clicking through one of our ads or text links and making a purchase will produce a small commission for us from the sale. Renee's Garden True Leaf Market
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