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How to Care for Plant Seedlings

The Spruce / Margot Cavin

When you start seeds indoors, the tender seedlings are dependent on you for all their needs.   This includes getting fed. Some gardeners think their seedlings will grow faster if they give them fertilizer right away. However, while those tiny plants may look helpless, they don’t need anything other than water, warmth, and light for their first few weeks. They are capable of feeding themselves up to a point. After that, it’s time to start feeding them, following a few standard guidelines.

When to Start Fertilizing Seedlings

When seedlings first poke out of the ground, they are still feeding off the food stored in the seed.   The first couple of leaves that form are not leaves at all. They are called cotyledons or seed leaves, which are part of the seed or embryo of the plant. Cotyledons contain the remainder of the stored food reserves of the seed, and they keep the seedling fed until the first true leaves sprout and the plant can begin photosynthesis.

Usually, the cotyledons disappear shortly after the first true leaves form and begin photosynthesizing. It is at this point that the seedling can use a little boost of fertilizer.

Before you reach for the plant food, make sure you haven’t used a potting mix that already contains fertilizer. Some do, and some don’t. If the mix has fertilizer, you shouldn’t need to add more. For the future, because seedlings can initially feed themselves, you don’t need to use a potting mix with fertilizer for starting seed. Using a mix without fertilizer is cheaper, and more importantly, you can control how much and what type of food your seedlings get.

Selecting Fertilizer

Seedlings tend to need a fertilizer that’s high in phosphorous. Phosphorus stimulates root development and is a component of photosynthesis. Look for a 1-2-1 N-P-K (nitrogen-phosphorus-potassium) ratio on the fertilizer label. A liquid or water-soluble fertilizer is typically the easiest and quickest way for the seedlings to access nutrients. You’ll also have a choice between organic and synthetic fertilizer, which often comes down to personal preference.

  • Synthetic fertilizer: If you are using synthetic fertilizer, feed your seedlings weekly. However, it’s often wise to dilute the label’s recommendation by at least half. Tender seedlings can be easily burned by too much fertilizer. Young seedlings commonly can get away with a quarter of what the label recommends for full-grown plants.
  • Organic fertilizer: There are several liquid organic fertilizers available, though they sometimes can be hard to locate. A mix of fish emulsion and kelp can also give your seedlings the nutrients they need to get started and reduces the risk of burning your seedlings. As with synthetic fertilizer, give your seedlings a dose of organic food weekly. Unless the product is labeled specifically for seedlings, dilute it by at least half the recommended dose. It’s better to give your seedlings a little food regularly than to risk burning those tender roots with too much fertilizer at once.
  • Another option: Mix a granular organic fertilizer into the potting soil. Many gardeners do this when their seedlings are ready to be moved from their starter containers to larger pots. However, granular fertilizer can take a while to release nutrients and impact the plants, so adding it when you are starting your seeds is often a better option. Try to add it to the lower layer of potting mix, and don’t let it come in direct contact with the seeds. Even organic fertilizers can burn if you use too much.

Knowing When Seedlings Have Had Enough Food

How much to feed seedlings will take some experimentation. Keep an eye on how well your seedlings are filling out. Too much fertilizer can cause a flush of tender, lanky growth, which is not what you want. Ease back on the fertilizer if this is the case.   At this point in a seedling’s development, you should be more interested in growing a healthy root system than sending up a lot of green leaves.

Moreover, each plant—even those of the same species—will react a little differently to fertilizer. But in time you should get a feel for how much food it takes to keep your seedlings robust while they build up the strength to be moved outdoors into the garden.

New plant seedlings can feed themselves up until their first true leaves appear. Here are tips on how, when, and what to feed your seedlings.

Common Mistakes Made While Growing Seeds Indoors

Guidance on Watering, Lighting, and Other Growing Factors

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The Spruce / K. Dave

It is quite economical to start seeds indoors, especially when the seedlings grow into robust plants. However, growing seeds indoors can be challenging. To significantly increase your chances of success, avoid these common seed-starting mistakes.

Watch Now: Mistakes to Avoid When Growing Seeds Indoors

Not Supplying Enough Light

Seedlings need a lot of light to grow into sturdy, healthy plants. No matter what anyone tells you, chances are that you do not have enough natural light in your home to grow robust seedlings. Even a south-facing window usually will not do. You can, however, use artificial light to achieve the right amount of light required by seedlings. To do so, obtain grow lights explicitly designed for plants. Or, for a more economical solution, purchase large fluorescent shop lights outfitted with one warm bulb and one cool bulb.

Suspend the lights from chains so that you can raise the lights higher as the seedlings grow. Keep the lights as close to the seedlings as possible without touching them (2 to 3 inches). When seedlings first appear, keep the lights turned on for 12 to 16 hours per day. To reduce your hands-on time, use a timer to turn the lights on and off automatically.

Applying Too Much or Too Little Water

The amount of water you supply can make or break seedling growth. Watering is one of the most challenging aspects of seed starting. Because seedlings are so delicate, there is very little room for error when it comes to watering. You must keep the sterile seed-starting medium damp but not wet.

To increase your chances of getting it right, here are a few things you can do:

  • Create a mini-greenhouse to keep soil moist: cover the container with plastic until the seeds germinate.
  • Water from the bottom to enable the seedlings to soak up water through the container drainage holes. There is less chance of over-watering when you use this approach. Add water slowly for 10 to 30 minutes, and use your finger to touch the top of the soil to ensure that moisture has reached the top of the container.
  • Check soil moisture at least once a day.
  • Buy a self-watering, seed-starting system.

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The Spruce / K. Dave

Starting Seeds Too Soon

Many plants do not tolerate cold temperatures, and exposing them to chilly air or cold soil will stress them out. Chas Gill, who runs the Kennebec Flower Farm, agrees that one of the biggest mistakes people make when starting seeds is starting the seeds too early. Stressed-out plants are more susceptible to pests and disease. Most plants are ready to go outside four to six weeks after you start the seeds.

Planting Seeds Too Deeply

Seeds are finicky when it comes to how deep they are planted. Some seeds need complete darkness to germinate and others require light to germinate.   Proper planting depth is usually provided on the seed packet. If there is no information on the packet, the rule of thumb is to plant seeds two to three times as deep as they are wide. Determining depth can be a challenge, but if you are not sure, err on the shallow side.

For seeds that need light to germinate, make sure the seeds are in contact with the seed starting medium but are not covered. To do this, gently press the soil medium to create a firm surface. Then, place the seed on top of the medium and gently press down, making sure the seed is still exposed.

Moving Seedlings Outdoors Too Soon

There is no benefit to a tough-love approach with seedlings when they are young. They will either instantly die or become weak and then fail to thrive. Even the most stalwart plants, when young, need a considerable amount of coddling and attention.

When your seedlings are large enough to plant outdoors, you need to prepare them for the transition by hardening off.   Hardening off gradually prepares them for outdoor conditions like wind, rain, and sun. The hardening-off process is simple, though it can be time-consuming; it involves exposing your plants to the elements gradually. The first day of hardening off, place your seedlings outdoors for one hour, and then bring them back indoors. Gradually increase the amount of outdoor time every day for 6 to 10 days. You will need to make some judgment calls based on the outdoor temperature and the fragility of your seedlings. If it is a particularly cool day or very rainy, you will want to decrease the time of that hardening-off session.

Sowing Too Many Seeds

When sowing seeds, begin modestly if you are a beginner. If you sow more seeds than you can reasonably maintain, it will become challenging to nurture the seedlings into adulthood. Depending on the type of plant you want to grow, you might be able to direct-sow seeds in outdoor containers or in the ground when outdoor temperatures warm up.

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The Spruce / K. Dave

Keeping Seeds Too Cool

For seeds to germinate, most must be kept warm: about 65 to 75 degrees Fahrenheit. A favorite place to keep seeds warm in order to germinate is on top of the refrigerator. Or, you can purchase seed-warming mats to place under the seed trays. Once a seedling emerges, they can tolerate fluctuating temperatures (within reason). Whatever type of light you use, natural or artificial, make sure it produces enough heat to keep the plants in the 65- to 75-degree range.

Failing to Label Seeds

To be able to identify seedlings as they grow and to know when they will be ready for transplanting, you should label the seed containers as you are sowing. For every type of seed sown, use popsicle sticks or plastic plant markers and permanent ink pens to record the plant name and date sown. Insert the plant labels into the soil near the edge of the container or tray.

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The Spruce / K. Dave

Giving Up Too Soon

Starting seeds can be a difficult process. However, one of the most satisfying benefits of this labor of love is eating a tomato or marveling at the flowers that you nurtured from day one. Growing plants from seed takes dedication, attention, and time. Recognize that you might make mistakes along the way, but you should not give up. The results outweigh the challenges along the way.

Growing seeds indoors isn't hard, yet keeping them alive can be challenging. You can save a lot of money by starting plants from seeds.