Knowing when to transplant seedlings can mean the difference between healthy, vigorous plants and those that are stunted and root bound. Seeds for vegetables, herbs, and flowers are sown in cell packs, plug trays, or peat pellets with most outgrowing their containers after 4 to 5 weeks. Transplanting the small plants into larger containers allows seedlings to develop robust root systems. Knowing when to transplant seedlings is a skill that is easy for gardeners to learn, even those just starting out. Below you’ll learn how to tell when the time is right to repot seedlings.
Why is it important to know when to transplant seedlings?
Transplanting seedlings, also called ‘potting up’, at the right time ensures consistent, uninterrupted growth. This gives your vegetable and flower seedlings the chance to grow bigger and more vigorous. Transplanting offers increased space for the root system to develop. This, in turn, reduces the risk of transplant shock when the seedlings are eventually moved to the garden.
When to transplant seedlings: 4 easy options
There are four options for when to transplant seedlings:
- The first option is based on the stage of growth. The majority of vegetable, flower, and herb seedlings can be potted up once one or more sets of true leaves have developed.
- The second option for timing transplanting is based on plant density. Many gardeners like to sow seeds thickly, but when they start to crowd out their neighbors its time to prick them out and move them to larger pots.
- The third indication that it’s time to transplant seedlings is when the young plants outgrow their original containers. More on this below.
- Finally, let’s look at legginess. When certain seedlings, like tomatoes, have grown leggy repotting can help encourage sturdier stems.
Option 1: The number of sets of true leaves
Many gardeners use the number of sets of true leaves as a signal of when to transplant seedlings. To use this technique you need to understand the difference between cotyledons, also called seed leaves, and true leaves. When a seed, like a tomato or zinnia seed germinates the cotyledons are the first leaves that open.
After the cotyledons open, the true leaves are next to emerge. These leaves look the same as those of the mature plant. So the first true leaves of a tomato plant look like mature tomato leaves. It’s when the true leaves develop that photosynthesis really begins. I typically repot my seedlings when they’ve developed one to two sets of true leaves.
Option 2: Transplanting seedlings based on plant density
There are several ways to start seeds indoors. Some gardeners plant just one or two seeds per cell pack or pot, while others prefer to sow their seeds thickly in seeding trays. Either technique works, but if you’re planting densely, you’ll need to prick out seedlings and move them to larger pots when they start to crowd out their neighbors. You don’t want seedlings competing for light, water, and nutrients.
Overcrowding seedlings can also impede air flow which may prompt issues like damping off. Damping off is a fungus or mold that causes seedlings to fall over and die. Repotting densely planted seedlings can reduce the risk of damping off.
Prick out seedlings using a small dibbler, wooden skewer, or pencil. Carefully separate seedlings and repot them into larger containers filled with a high quality potting mix. Never hold seedlings by the stems, as this can damage their delicate tissues. Instead gently handle the young plants by the leaves.
Option 3: Transplanting seedlings based on plant size
The third option for when to transplant seedlings is based on seedling size and whether they’ve outgrown their containers. Seedlings grown in cell packs, plug trays, or other small containers quickly become root bound. One sign that it’s time to repot seedlings is when roots begin to grow out of the drainage holes on the bottoms of the containers. You can also check root systems by carefully slipping seedlings from their containers. If the roots are circling around the root ball, it’s time to repot the seedlings.
Seedlings started indoors too early also become root bound. It’s good practice to follow the instructions listed on the seed packet or in a vegetable gardening book to encourage stocky seedlings. Start tomato seeds, for example, indoors 6 to 7 weeks before the last frost date. Knowing the best time to start seeds indoors is an important step for promoting healthy plant growth.
Option 4: Transplanting when seedlings have become leggy
Legginess is a frequent complaint from seed starters and typically caused when the young plants stretch towards a light source. This issue is most common when seeds are started on a windowsill where light is less dependable. Leggy growth can also happen beneath grow lights if the fixtures are too high above the plants or the bulbs are old. Temperature also plays a role in stretched seedlings. Leggy growth occurs if the seed-starting room is very warm or a seedling heat mat is kept on for too long.
For certain types of seedlings, like tomatoes or tomatillos, legginess is remedied by transplanting into new containers. When transplanting, I generally plant most seedlings slightly deeper in their new pots. This, as well as providing at least 16 hours of direct light each day, can help reduce legginess.
The best containers to use when transplanting seedlings
There are a bunch of container options when transplanting vegetable, flower, and herb seedlings. These include plastic pots, large-sized cell packs, fibre pots, and up-cycled containers like yogurt or plastic milk containers. Whatever items you choose, ensure there are drainage holes on the bottom of the pot.
My go-to containers are 4 inch diameter plastic pots which I save from season to season. I rinse them clean and re-use for transplanting seedlings. I’m not a fan of fibre pots because I find they dry out very quickly making it necessary to pay extra attention to soil moisture. Plus, they can get moldy on the outside which can impact seedling growth.
The best soil to use when transplanting seedlings
I typically start my seeds in a high quality seed starting mix, but when transplanting I just use an all-purpose potting mix. These lightweight, soilless growing mediums offer excellent drainage and some nutrients. Most are peat based, but you can also buy peat-free potting mixes. It’s best to pre-moisten the growing medium before filling your containers. I use a large Rubbermaid tote to mix potting soil with water. Once it’s lightly moist, I fill the new pots.
How to transplant seedlings
When you’ve determined it’s time to transplant, or pot up, seedlings, start by prepping your supplies. Moisten the potting mix and gather pots, labels, and a waterproof marker. Carefully slip the young plants from their seed tray or cell pack, leaving each root ball intact if possible. Tease seedlings that are growing thickly apart and plant them individually. As you transfer the seedlings, hold them by a leaf, not the stem which is fragile. Replant each seedling into the larger pot, setting it slightly deeper. Water with lukewarm water to eliminate any air pockets in the growing medium and place the pots back under your grow lights or in a window with direct sunlight.
When to transplant seedlings grown in soil cubes
I love starting seeds like tomatoes and basil in soil cubes created by block molds. They offer a plastic-free way to start seeds and promote healthy root systems because the roots are air pruned when they reach the outside surface of the soil cube. I have a set of block molds that makes 3 different sized soil cubes. This allows me to transplant the seedlings into larger cubes when they outgrow their initial small cubes of soil. It’s time to size up to a larger block of soil when you see roots growing along the outside surface of the cube.
Learn more about when to transplant seedlings in this helpful video:
When to transplant tomato seedlings
Tomatoes are the most popular vegetable grown in home gardens with gardeners starting the seeds for their favorite varieties indoors in early to mid-spring. I use cell packs and sow 2 tomato seeds per cell, eventually pricking them out and transplanting them into their own containers. Other gardeners prefer to start tomatoes by sowing the seeds thickly in seed trays and transplanting when the plants reach the first true leaf stage. The stems of tomato plants develop adventitious roots. Because of this they can tolerate deep planting in the larger containers. I typically bury about half of the stem beneath the soil.
Should all types of seedlings be transplanted?
Nope! Not all seedlings benefit from transplanting. Cucumbers and squash, for example, don’t transplant well. I therefore move the seedlings directly into the garden when they outgrow their cell packs or pots. I also recommend direct sowing the seeds for root vegetables like carrots and radishes. Transplanting root crops can result in stunted or misshapen roots. I also don’t start quick growing crops like zucchini, peas, and snap or pole bean seeds indoors as they grow so quickly when direct seeded.
Tips for transplanting seedlings
- Fertilizing – When I water newly transplanted seedlings I add a diluted dose (diluted to about half strength) of an organic fertilizer to the watering can. This provides a steady source of nutrients to the young plants.
- Culling – Don’t be shy about culling weak seedlings when transplanting. I discard stunted or discolored seedlings, or those not growing as well as the rest of the plants.
- Hardening off – About a week before you intend to transplant the seedlings outdoors into a garden bed or container start the hardening off process. This transition acclimatizes indoor grown plants to outdoor growing conditions like sun and wind. I check the weather forecast and try to pick a cloudy day or overcast day to transplant. This reduces the risk of transplant shock.
For more information on growing from seed, be sure to check out these awesome articles:
- How to grow tomatoes from seed
- Growing broccoli from seeds
- Learn how to start basil seeds indoors
- How to harden off tomato plants like a pro