Tomatoes, peppers and cucumbers all start the same way: a seed, a little mix and a few weeks indoors. The hard part comes later, when the seedling has to leave its first container without losing half its roots on the way out.
Our Seed Starting Cups are built for that moment. This guide walks you through the whole run: setting up the cup, sowing, caring for seedlings, checking the roots, hardening off, and the push-out transplant into a fabric grow bag or the garden bed. Then you wash the cups and do it again next spring.
The short version: drop the plate into the cup, fill with pre-moistened seed-starting mix, sow at about twice the seed's diameter, keep the mix moist and the lights close, harden the seedlings off, then push the plate up from below and plant the root ball whole.
What the cup does
Each cup has a loose bottom plate. Push the plate up from below and the whole root ball lifts out in one piece, ready to plant. Here is what else is built in:
- Push-out bottom plate. The root ball comes out whole, so you never have to tip the cup over or tug on the stem.
- Three drainage paths and a raised base. Together they keep roots from sitting in water.
- 16 oz (475 ml), 3.75 inch rim. Room for a strong start.
- Dishwasher safe and reusable. Wash them at the end of the season and use them again.
They suit tomatoes, peppers, cucumbers, squash, herbs and flowers. Indoors, set them on a tray or saucer to catch the water that drains out.
Step 1: Set up the cup and fill it
- Drop in the plate. Lay the bottom plate inside the cup so the side marked "this side down" faces down, and let it settle flat at the bottom.
- Moisten the mix first. Use a soilless seed-starting mix, not garden soil, which packs down and can carry disease.[1] Wet the mix in a bowl before it goes in the cup.[1] You want it damp all the way through, not dripping.
- Fill and firm. Fill the cup with the moistened mix and gently firm the surface.[2] Leave a little space below the rim so water does not run over the edge.
Step 2: Sow at the right depth
Check the seed packet first. As a rule of thumb, most seeds go in at a depth of about twice their diameter, and very small seeds are pressed into the surface and barely covered.[2] Some seeds are "surface sown" and not covered at all, so the packet wins if it says so.[1]
How many seeds per cup? If your seed is a few years old, test it first: lay 10 seeds on a damp paper towel, and if 6 to 10 sprout, it is in good shape. Seed that sprouts well needs only a seed or two per cup. If fewer come up, sow 2 or 3 per cup and thin to one seedling later.[1] To thin, snip the extras off at the surface with small scissors.[2]
Step 3: Light, warmth and water
Warmth to sprout. Most seeds germinate well at 65 to 75 F.[2] A warm spot or a heat mat helps, though a heat mat dries the mix faster, so check it more often.[1] Once the seedlings are up, keep them on the cool side, since warmth makes spindly growth worse:[7] under 70 F by day and 60 F at night.[9]
Light, and plenty of it. Seedlings grown in the low light of most homes stretch and turn long and leggy.[7] A sunny south or west window can work.[1] Lights on a timer are more consistent: hang them 2 to 3 inches above the seedlings and run them about 16 hours a day,[2] raising them as the plants grow.[3]
Water. Keep the mix moist but never wet, because wet, cool conditions encourage damping off, a disease that kills young seedlings.[1][3] Seedlings can dry out quickly, so check them every day.[2] Water until a little drains into the tray, then pour off what is left so the cups are not standing in water.[9]
Step 4: Check the roots without pulling the plant
Here is where the plate earns its place. Once your seedling has a couple of sets of true leaves (the leaves that come after the first pair of seed leaves), lift the cup and press the bottom plate up from below just enough to see the sides of the root ball. Look, then let the plate and the root ball settle back down. The plant never leaves the cup.
What you are looking for: roots showing around the sides and a root ball that holds together. A good transplant comes out with a full root ball,[8] so if the mix still crumbles away, give it more time.
Step 5: Know when they are ready to move up
Three signs line up when a seedling is ready:
- Leaves. Seedlings are usually moved on once they have one to three sets of true leaves.[1][2]
- Roots. Roots show at the sides and the root ball holds its shape when you check it.[8]
- Weather, if the plant is going outdoors. Tomatoes and peppers go out after the last chance of frost, once the soil has warmed.[5][6]
Plan the sowing date backward from that. Typical times from sowing to transplanting:[3]
| Crop | Weeks from sowing to transplant |
|---|---|
| Cucumbers and squash | 3 to 5 |
| Tomatoes | 6 to 8 |
| Peppers | 8 to 10 |
Step 6: Harden off before they go outside
Seedlings raised indoors have never felt wind or full sun. Hardening off gets them used to both gradually, over one to two weeks before planting out.[2]
- Start with a few hours outside in the shade during the warm part of the afternoon, in a spot protected from wind.[1]
- Each day, leave them out a little longer and give them a little more direct sun.[1]
- Ease up slightly on watering, but do not let them wilt.[4]
- Bring them back inside for the night before the temperature drops. By the end of about two weeks, if no frost is forecast, they can stay out until you plant them.[1]
Keep the cups on their tray and you can carry the whole batch in and out at once.
Step 7: The push-out transplant
Moving into a fabric grow bag or straight into the garden bed works the same way. Pick a cloudy day or late afternoon, when the sun is past its peak.[1][9]
- Get the new home ready. For a grow bag, fill it with potting mix, leaving room for the root ball. A 1 gal bag is a transplant pot: the seedling grows on in it until the roots fill it, then moves to a bigger bag or the bed. Herbs and lettuce live happily in 2 gal, a compact pepper in 3 gal, and one pepper or a bush tomato in 5 gal, the size most people pick; vining tomatoes want 7 gal or more. In the garden, loosen the soil where the plant will go.
- Dig the hole. Make it about the size of the cup: 3.75 inches across at the top and a little deeper than the root ball for tomatoes (see the tips below).
- Push the plate. Hold the cup upright in one hand. With the fingers of your other hand, push the bottom plate straight up from below. The root ball rises out of the cup in one piece.
- Lift it by the root ball. Cradle the soil in your palm. If you need to steady the plant, hold it by a leaf, not the stem.[3]
- Set it in. Most plants go in at the same depth they grew in the cup, not deeper or shallower.[2] Tomatoes are the exception.
- Firm and water. Firm the soil around the root ball so it makes good contact, then water thoroughly but gently.[2][6]
- Keep the plate. It goes back in the cup for next time.
If you are moving into grow bags next, our guide on how to use fabric grow bags covers filling, watering and choosing a size. The Seed to Pot Set pairs five of these cups with the five 1 gal grow bags they move into next.
Tips by crop
Tomatoes: plant them deeper
Tomatoes can root along any part of the stem that ends up under the soil.[6] Take off the lower leaves and set the plant deep, so only the top two or three sets of true leaves show above the soil.[4][6] A leggy tomato can even go in on its side, with roots forming along the buried stem.[4] In a grow bag, put less mix in the bottom so the root ball sits lower.
Peppers: same depth, warm soil
Peppers do not root along a buried stem as readily as tomatoes, so plant them at the depth they grew in the cup.[7][2] Wait until after the last chance of frost and until the soil has warmed.[5] Peppers handle early-season stress less well than tomatoes, cucumbers and squash,[8] so do not rush them out on the first warm day.
Cucumbers and squash: start late, move them gently
Cucumbers and squash do not transplant well,[1] and damage to their roots makes it harder for them to settle in.[8] Give them a short indoor stay of 3 to 5 weeks[3] and move them while they are young. This is where the push-out plate helps most: the root ball comes out whole, with no tipping or tugging.
Herbs and flowers
Follow the packet for depth and for whether the seed is surface sown,[1] then move them on in the same way. Herbs do well in a 2 gal grow bag.
Wash and reuse
At the end of the season, push out any old mix, take the plates out and rinse off the soil. Wash the cups and plates with soap and water, or run them through the dishwasher; they are dishwasher safe. Extension guides also suggest sanitizing reused seed containers after washing, with a rinse of one part chlorine bleach to nine parts water, to keep disease from carrying over to next year's seedlings.[2][1] Rinse well, let them dry, and store each plate in its cup so the pairs stay together.
Frequently asked questions
Does the plate go in before or after I fill the cup?
Before. Drop the plate in flat, with the side marked "this side down" facing down, then fill the cup with moistened seed-starting mix.
How many seeds should I sow in each cup?
One or two with fresh seed. With older seed, sow two or three and snip all but the strongest seedling at the surface once they are up.[1][2]
Do I need a tray under the cups?
Indoors, yes. Set the cups on a tray or saucer to catch the water that drains out, and pour off what collects.
How do I know when to transplant?
Push the plate up a little and look at the sides of the root ball. When roots show around the sides, the root ball holds together and the seedling has one to three sets of true leaves, it is ready to move up.[1][8]
Can I start cucumbers and squash in these cups?
Yes. Sow them later than tomatoes and peppers, about 3 to 5 weeks before they go out,[3] and transplant them while they are young.
What size grow bag comes after the cup?
A 1 gal bag works as a transplant pot for any seedling, before it moves on to its final home. For that final home: 2 gal for herbs, lettuce or strawberries, 3 gal for a compact pepper, 5 gal for one pepper or a bush tomato, and 7 gal or more for vining tomatoes.
Can the cups go in the dishwasher?
Yes. They are dishwasher safe and reusable, season after season.
I have another question.
Email hello@onlinegardenco.com. Fastest customer service: get responses in minutes, not hours or days.
References
- UConn Home and Garden Education Center. "Seed Starting." University of Connecticut. homegarden.cahnr.uconn.edu/seeds/seed-starting
- Dorn, S. and Pennisi, B. V. "Starting Plants From Seed for the Home Gardener." UGA Extension Bulletin 1432, reviewed January 2025. fieldreport.caes.uga.edu/publications/B1432
- Clemson Cooperative Extension, Home and Garden Information Center. "Starting Seeds Indoors" (HGIC 1259). hgic.clemson.edu/factsheet/starting-seeds-indoors
- Clemson Cooperative Extension, Home and Garden Information Center. "Tomato." hgic.clemson.edu/factsheet/tomato
- Clemson Cooperative Extension, Home and Garden Information Center. "Pepper." hgic.clemson.edu/factsheet/pepper
- Marquesen, S. "Tomatoes: From Seedlings to Fruit." Penn State Extension, updated June 2026. extension.psu.edu/tomatoes-from-seedlings-to-fruit
- Lerner, R. "Leggy Tomatoes." Purdue University Consumer Horticulture, May 2003. purdue.edu/hla/sites/yardandgarden
- Johnson, G. "Transplant Shock." Weekly Crop Update, University of Delaware Cooperative Extension, May 2010. sites.udel.edu/weeklycropupdate
- Geist, L. "Plant now for seeds of success." University of Missouri Extension, February 2025. extension.missouri.edu/news/plant-now-for-seeds-of-success