Seedling Care and Troubleshooting

Four tomato seedlings in pots by a windowsill
Healthy tomato seedlings growing in pots

text-box-multiplePart 4 in our Seed Starting Series link

All the info you need to start your seeds indoors!

If you’ve been busy starting seeds indoors, you might already be at the point where your seeds have germinated and are growing into tiny seedlings (if not, be sure to check out our Seed Starting series to get started!) But the conditions that seedlings need to grow and thrive are different from what they need to germinate. Knowing how to properly care for your seedlings is important to get them off to the best start possible!

Lighting and heat for seedlings

Seeds typically need warm, moist conditions for germination and most veggie plant seeds do not need any light for germination. Once the seeds germinate though, it’s a whole different ball game! Seedlings need ample light as soon as they germinate. If you’re using heat mats you can turn them off once the majority of your seeds on the mats have germinated. Refer to All About Grow Lights and Heat Mats for Starting Seeds for more information on how to use grow lights and heat mats.

How to water seedlings

How much to water your seedlings and when depends on many factors such as: how well your seed starting mix holds water, the type of containers you use, and how dry your house is. Check your seedlings often- multiple times a day- and as soon as the soil starts to appear a bit dry water them right away. While you never want the growing medium to go bone dry, you may not have to be as meticulous with misting and watering as when germinating seeds. Also, if you were using a humidity dome for germinating your seeds, be sure to remove it once they germinate. Many gardeners prefer to bottom water their seedlings, but no matter how you water them, be sure your containers have drainage and don’t let your seedlings sit in water. Over time you will develop a ‘feel’ for the right amount of water to give your seedlings.

Young seedlings in a tray with water on the leaves

How to fertilize seedlings

Seeds and newly emerged seedlings do not need any fertilizer- they have all the nutrients they need inside their ‘seed leaves’ (‘cotyledons’) or inside the seed. Because of this, seed starting mix often does not have any nutrients added (although some do). It’s usually recommended to start fertilizing seedlings once their first set of ‘true leaves’ emerge. When fertilizing young seedlings, use diluted quantities of fertilizer to avoid burning them. I like to use a liquid fish emulsion fertilizer diluted to a half or quarter of the usual strength. Fertilize about once a week as the seedlings grow (you can increase the frequency if the leaves start to turn yellow).

Thinning and potting up seedlings

Thinning seedlings is the practice of removing extra seedlings so you end up with only one seedling per growing container. Gardeners often sow more than one seed per container in case not all the seeds germinate However you’ll need to deal with any extra seedlings so they don’t crowd each other out. Identify the seedling that looks the most stocky and robust, and snip off any other seedlings in the container. Cutting is recommended instead of pulling so as not to damage the roots of the seedling you want to keep. If you don’t have the heart to kill the extra seedlings and you have space for the extras, you can very delicately use a chopstick to gently tease out the seedlings for potting up.

Potting up refers to moving seedlings into larger containers with more growing medium. Potting up gives seedlings room to continue growing so they don’t become stunted in a too-small container. To know when to pot up your seedlings, regularly check the roots. As soon as you see roots starting to poke out of the bottom of the container it’s time to pot up. Select a larger container, fill it partway with potting mix, then gently extract the seedling from its existing container (soil and all) and put it in the larger container. Fill in any gaps with more potting mix then water well.

Young tomato seedlings in plastic containers

Common seedling problems and remedies

Mold and damping off: Mold on seedlings is often just an eyesore, but it can be a sign of a deeper problem. ‘Damping off’ is a type of mold that can actually kill your seedlings. To prevent mold, make sure you are not overwatering your seedlings. Using a fan on a gentle setting to circulate air can also help keep mold at bay.

Fungus gnats: Fungus gnats are small, black flying insects. On mature plants they are a nuisance, but the larvae can inflict catastrophic damage on your seed starts. Sterilizing your seed starting mix with boiling water before sowing seeds will kill fungus gnat eggs. To treat fungus gnats in your seed starts, water them with a hydrogen peroxide solution of 4 parts water mixed with 1 part hydrogen peroxide. Multiple treatments may be needed to quell an infestation.

Leggy seedlings: Leggy seedlings are weak seedlings that are stretched too thin. To prevent leggy seedlings, make sure they are placed close enough to a light source to receive ample light, and use a fan on a gentle setting to help strengthen the stems.

Leggy tomato seedlings in peat pots

Taking care of seedlings is not complicated, but it does require a bit of attentiveness. Keep a close eye on your seedlings to make sure they are not underwatered or overwatered, and that they have sufficient light and nutrients. Taking action as soon as you spot a problem can save your precious seedlings. You’ll be rewarded for your efforts when they turn into healthy plants you can transplant into your garden!

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