How to Plant Seeds in Egg Trays
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Knowing how to plant seeds in egg trays can cut the cost of starting a garden indoors. Cut the carton into cups, poke a drainage hole in each one, fill the cups with moist seed-starting mix, and sow each seed at the depth printed on its packet. Iowa State University Extension says cardboard egg cartons are “easy to use for germinating and growing seeds”. The steps below follow what several university and state extension guides say about containers, moisture, light, and moving seedlings outdoors.
Quick answer
- Choose cardboard cartons if you plan to plant the cups with the seedlings. Styrofoam cartons must come out before transplanting.
- Poke drainage holes in each cup and set the carton on a tray or in a shallow pan.
- Sow each seed at the depth on the packet, keep the mix moist but not wet, and give seedlings 12 to 16 hours of light a day.
- Harden seedlings off for about two weeks before they go outside.
Pick the right carton
The carton comes first, and the material decides what happens at planting time. Mississippi State University Extension says “You can use egg cartons as a seed-starting tray”. Its blog adds that, depending on the type of carton you have, “you can even cut apart the individual sections and plant them”. The carton breaks down in the soil, which is why the cut sections can go into the ground with the seedling.
Iowa State University Extension makes the same point for cardboard and draws a firm line for foam. Its guide says “Styrofoam egg cartons can also be used; however, remove the plant from the styrofoam carton before transplanting because it does not break down in the soil.” A foam carton therefore adds a removal step when you move the seedling, while a cardboard cup can stay with it.
Iowa State’s advice on cardboard tubes is written for tubes, not cartons, but the same material raises the same question. The guide says “Make sure all portions of the cardboard are planted below the soil surface.” It also warns that “Any exposed edges will act like a wick and dry out the transplant, even if planted in moist ground.” Our reading is that the cut edges of a cardboard cup can wick moisture the same way, so the whole cup should sit below the soil line at planting.
Make the cups drain and label them
Drainage comes first. Mississippi State says to “Be sure to poke small holes for drainage” and to set the cartons on a tray or in a shallow pan, which catches “any residual water.” Ohio State University Extension, in its Wayne County guide, gives the same instruction in plainer words: “make sure you poke a small hole in the bottom of the carton so the extra water can run out.”
Labels matter once several cups sit side by side. The Ohio guide says “Label your trays or cartons before you plant so you do not get different varieties mixed up.” The University of Connecticut’s seed-starting guide goes further and says containers should be labeled “with masking tape or wooden plant labels using an indelible marker with the name and planting date.” For label options, see our page on plant labels for seed trays.
Fill the cups and sow at the right depth
Start with the mix. The Connecticut guide says “Mix and moisten enough soilless seedling starter mix for all your containers.” Oklahoma State University Extension describes the feel to aim for: “The mix should be moist like a sponge, but not so wet that water will wring out.” It also advises moistening the mix before it goes into the containers, and it says warm water works best for that job.
The packet sets the depth. Connecticut says to “Follow the recommended planting depth on the seed packet when sowing seeds”, and it gives a rule of thumb: “This is generally 1-1/2 to 2 times the width of the seed.” Ohio State makes the same point: “You should plant the seed to the correct depth by checking on the back of the seed package.” Connecticut also notes that some seeds should be surface sown and left uncovered.
Connecticut’s guide says “If the germination rate is lower, 2-3 seeds can be planted in each cell”, and then “Thin to just one seedling if necessary.” That advice is written for cells, but a cup in an egg carton plays the same role, so the rule applies by analogy.
| Step | What the sources say | Where it comes from |
|---|---|---|
| Container | Cardboard cups can go into the ground with the seedling | Mississippi State, Iowa State |
| Drainage | One small hole in the bottom of each cup | Ohio State, Mississippi State |
| Mix | Moist like a sponge, not wet enough to wring out | Oklahoma State |
| Depth | About 1-1/2 to 2 times the seed width, or the packet rule | Connecticut, Ohio State |
| Light | 12 to 16 hours a day under fluorescent lights | Connecticut, Ohio State |
| Hardening off | About two weeks, or 7 to 14 days | Ohio State, Oklahoma State, Connecticut |
Light, warmth and moisture until germination
Seeds need light and warmth to start. Ohio State puts it simply: “Most seeds need light to germinate so it is recommended to put them by a warm window or use fluorescent lights.” Connecticut recommends a timer, and says to “Use a timer to keep fluorescent lights on for 12 to 16 hours a day.” Ohio State also warns against leaving lights on all day, because plants only need “12-16 hours of light a day.”
The University of Illinois Extension guide, written by Knox County Master Gardeners in 2010, gives temperature targets for each stage. Germination needs about 70 to 75 degrees Fahrenheit. Once the seeds sprout, the seedlings do better at 50 to 60 degrees, though they tolerate warmer air. The same guide suggests keeping the light fixture about 4 inches above the seedlings. Oklahoma State says most windows do not give seedlings enough light, so a sunny sill may not be enough on its own.
Cover the cups until seeds sprout. Connecticut says “Plastic domes, cling film or even clear plastic bags can cover the trays and hold in moisture until germination.” Then the cover comes off. Keep the top of the mix damp but not soggy. The Connecticut guide says “Keep the soil moist, but never wet”, and Oklahoma State explains the cost of letting it dry: “If the potting mix dries out, it forms a crust that prevents seeds from popping through.”
Connecticut adds a warning about drying: “Once a seed dries out, it will not germinate.” If you use a heat mat to keep the cups warm, check them more often, because the guide says a heat mat “will dry the soil more quickly.” Oklahoma State describes bottom watering as a gentle option, with pots set in water so it wicks up, and says “Plants should not sit in standing water for more than 30 minutes.” If the windowsill is not bright enough, see our page on grow light options for seedlings.
Thin, feed and move the seedlings
Thinning means keeping the strongest seedling in each cup. The Illinois guide describes it as choosing the strongest and healthiest seedlings and removing the weaker or smaller ones, which gives the rest room to grow. Connecticut’s version is shorter: “Thin to just one seedling if necessary.”
Fertilizer timing is one place where the guides differ, and the disagreement is worth showing. Ohio State says “Fertilizing your new plants does not need to happen until they have multiple sets of true leaves.” Connecticut says to wait until the true leaves appear, then use a quarter-strength solution of a high-phosphorus fertilizer. Oklahoma State is the earliest of the three and says to give a quarter dose “Within 5 days of germination”. Ohio State waits the longest, and Oklahoma State gives the first dose soonest.
When seedlings outgrow the cups, the transplant step depends on the material. Ohio State says “If you used containers made from organic matter, you can plant that straight into the ground.” It suggests gently breaking open the bottom of the container so the roots can spread. Connecticut says seedlings “can be transplanted to larger containers if they outgrow their first cell packs or begin to get leggy.” Oklahoma State adds that seedlings without enough light grow tall and fall over.
Harden off before the seedlings go outside
Ohio State defines the process plainly: “Hardening off is when you get the plants accustomed to the outside.” It times the process as well: “This should take place around two weeks before you want to plant them in the ground.” Its schedule starts small: “Start off with a couple hours a day in full sun then move them to a shady spot and increase that time outside each day.”
Oklahoma State gives a shorter range, saying “It takes 7-14 days to harden off seedlings.” It also warns about wind: “Beware of windy days that will dry out and whip and kill seedlings.” It recommends a protected spot for the first days outside.
Connecticut says transplanting should happen on “a cloudy day or late in the afternoon when the sun is past its peak.” It also reassures that “Hardened off plants may droop a bit but should recover.” The guide says drooping seedlings should recover.
Mistakes to avoid
- Leaving foam cups in the ground. Iowa State says styrofoam “does not break down in the soil”, so the seedling must come out first.
- Letting cardboard edges stick out. Iowa State warns that exposed edges “will act like a wick and dry out the transplant”.
- Letting the top of the mix dry. Oklahoma State says a dry surface forms a crust that stops seeds from breaking through.
- Setting seeds too deep. Connecticut ties depth to the packet and to the width of the seed.
- Moving seedlings out too fast. Ohio State and Connecticut both describe a gradual process of about two weeks.
When to start the cups
Timing comes before the carton. Connecticut’s guide says “Check packets for days to harvest to determine when seeds will need to be started indoors.” It notes that “Seedlings that have a 4-6 week headstart indoors can extend that period” of the growing season. Oklahoma State advises that growers “calculate back from your last frost date” and allow a week for hardening off.
Ohio State makes a related point for warm-season crops, which are started indoors because they should not go out early: “you should wait to plant those until after the first frost.” Counting back from the planting date is the step that decides when the cups should be sown.
How we chose
We chose six extension and university pages that cover seed starting in containers, including egg cartons. Each was opened on 2026-10-22 and read in text form. We kept only the claims that a page states in plain words, and we copied each quoted sentence from the fetched text to check it. Where two pages gave different figures, such as fertilizer timing or the length of hardening off, we show both and say they differ. We left out product pages and retail listings, so no product is recommended here.
What we could not verify
The pages we opened cover seed starting in general, so the egg-carton specifics are thinner than the container and light advice. The Mississippi State page is a short blog post, and the Ohio State page is a county news item. Where a claim rests on one of those short pages, the post says so in the text.
What the sources do not say
None of the pages we opened gives a time for a cardboard carton to break down in garden soil, so the post does not give one. They also do not give the size of a cup or the number of cups in a standard carton, which means the planting plan in your own tray must be counted by hand. No page in this set compares germination in egg cartons with germination in plastic trays, so the post makes no such comparison.
Frequently asked questions
Can I plant a whole cardboard egg carton in the ground?
Not always as a whole piece. Ohio State says “If you use paper egg cartons, you can plant the section right into the ground because the carton will biodegrade.” That refers to the sections, which can be cut apart. Iowa State’s advice on cardboard tubes says all of the cardboard should sit below the soil surface, so bury the cup fully. Foam cartons are different, and Iowa State says they must come out before transplanting.
How deep should I plant seeds in an egg carton cup?
Use the depth on the seed packet. Connecticut gives a general rule of “1-1/2 to 2 times the width of the seed”, and says some seeds are surface sown and should stay uncovered. Ohio State gives the same instruction in its own words, so the packet is the first and most useful guide.
How many hours a day should grow lights run?
Connecticut recommends “12 to 16 hours a day” with a timer on fluorescent lights. Ohio State says plants “only need 12-16 hours of light a day” and warns against leaving the lights on around the clock. Both guides give the same range and neither picks a figure inside it.
When should I fertilize egg carton seedlings?
The guides disagree. Ohio State waits until “multiple sets of true leaves” appear. Connecticut waits for the true leaves and then uses a quarter-strength high-phosphorus feed. Oklahoma State gives a quarter dose “Within 5 days of germination”. If you want to follow the most conservative guide, use Ohio State’s timing, and if you want to follow the earliest, use Oklahoma State’s.
How long should seedlings harden off before planting?
Most of the sources say about two weeks. Ohio State says the process “should take place around two weeks before you want to plant them in the ground.” Oklahoma State gives a range of “7-14 days”, and Connecticut describes exposure that lengthens each day over about two weeks. Start in shade on a calm day, and lengthen the time outside each day.
Sources
- https://extension.msstate.edu/blog/start-seeds-indoors-using-egg-cartons (opened 2026-10-22)
- https://yardandgarden.extension.iastate.edu/how-to/containers-starting-seeds (opened 2026-10-22)
- https://wayne.osu.edu/news/warming-spring-planting (opened 2026-10-22)
- https://extension.illinois.edu/sites/default/files/starting_from_seed_indoors.pdf (opened 2026-10-22)
- https://extension.okstate.edu/e-pest-alerts/2025/tips-for-starting-seeds-at-home.html (opened 2026-10-22)
- https://homegarden.cahnr.uconn.edu/seed-starting (opened 2026-10-22)
