9 Common Seed Starting Mistakes To Avoid

Last Updated on August 8, 2023

Truth me told, I don’t care for the word “mistakes”. Every time you try something new and work on improving your skills is a success in my book – regardless of the outcome! Particularly in the gardening world. Nearly everything we have learned over the years of gardening has been through trial and error. Lots of errors, in fact! Yet calling this article “The Top Seed Starting Struggles That Will Teach You A Valuable Lesson For Improvement” didn’t roll off the tongue quite as easy. So, for the sake of semantics, let’s talk about some of the most common seed starting mistakes that many new gardeners make – and how you can either prevent or fix them!

Every gardener has their own preferences for seed starting. For example, many like to start seeds in plastic trays and pots, while some may use soil blocks, biodegradable cups, or even eggshells. You can learn more details about our seed starting set-up and process here. 

However, no matter the method you use, there are a handful of best practices that apply to all seed starting routines – and are especially important when starting seeds indoors. When gardeners stray from these best practices, or “make mistakes”, there is an increased chance that things won’t work out the way we’d hope. Seeds may not sprout. Seedlings can be weak or unhealthy. Plants may even die. However, you can avoid those scenarios by using the tips below! 

At a glance, here are the most common seed starting mistakes:

1. Starting seeds at the wrong time
2. Not reading seed packages
3. Using the wrong soil
4. Not providing warmth
5. Insufficient light
6. Watering too much (or too little)
7. Fertilizing incorrectly
8. Not thinning or potting up
9. Skipping the hardening off process
   

Now let’s talk about each one of these things, so you can grow happy and healthy seedlings!

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THE TOP 9 SEED STARTING MISTAKES TO AVOID

1) Starting Seeds at the Wrong Time

Why this is an issue

Some crops like it hot, some like it not. It is important to understand what the preferred growing conditions are for the type of vegetable, herb, fruit, or other seeds you’re starting. Meaning, are they considered a warm season crop, or cool season? Will this variety be happy in your particular growing zone? Starting seeds that aren’t well-suited for your climate or the upcoming season can set you up for disappointment. 

Assuming that you have that first part dialed-in, and have selected seeds that should grow well in your garden, the next timing issue to sort out is: when to actually sow the seeds? If you start seeds indoors too early, you may be stuck tending to overly large and crowded seedlings while you impatiently wait for the weather to warm up enough to plant them outside. If seeds are started too late, you may be left feeling rushed and “behind”. The plants may not mature in time, and therefore provide less bounty during the growing season.

How to prevent or fix it 

The best time to start seeds will vary depending on your location, and the variety of plant. Familiarize yourself with a plants preferences by reading its description on the seed package, catalog or website listing. Most often, there will be instructions for timing, such as “sow seeds 6 weeks before your last spring frost date”. That means you also need to become familiar with your growing zone, the average first and last frost date, and then be able to work backwards or forwards from there. If you don’t know your hardiness zone, use this simple lookup tool to find out!

If that sounds complicated, don’t worry! To help simplify things, I created planting calendars for every USDA gardening zone. The calendar includes a list of common veggies and the recommended time to start seeds indoors, transplant seedlings outside, or directly sow seed outdoors. You’ll also learn to tweak your timing with experience. If you don’t live in the US, you should be able to look up your equivalent USDA hardiness zone and utilize the calendars too!

Finally, if time creeps up on you and it is suddenly too late to start from seed – don’t fret. There is no shame in buying started seedlings if needed! We purchase seedling plants from local nurseries from time to time as well. Check out this article with tips on how to choose the best and healthiest seedlings from the nursery. 

2) Not Reading Seed Packages

Why this is an issue

There is a lot of valuable information on a seed package! The ideal time to start or plant this particular variety, how deep to sow seeds, the recommended plant spacing, and more. For instance, did you know that lettuce actually prefers cooler soil to sprout (in the 50-60’s) as opposed to warm soil – like most other seedlings? So, you won’t want to put your lettuce seeds on a heat mat in most cases.

On the other hand, some types of seeds don’t like to be started indoors at all! For example, it is best to sow carrot and radish seeds directly outside. Root veggies don’t like to be transplanted. Read a full list of seed types that are best to direct sow outside versus those you can start inside here.

Last but not least, the germination rate provided on the seed packet also gives you an idea of how many seeds you should expect to sprout. If that number is below 75%, sow a few extra seeds as added insurance. If it is nice and high, just a few per hole or container will do.

How to prevent or fix it

If I need to spell out how to “fix” this mistake, you may need more help than I than I can provide! 😉

3) Using the Wrong Soil or Potting Medium to Sow Seeds

Why this is an issue

Another common seed starting mistake that new gardeners make is using just any old soil to start their seeds in. Old soil from the garden or yard may contain and introduce diseases or pests to your new seedlings. On the other hand, some bagged potting soil is too dense, and has a less-than-ideal texture or drainage for seed starting. In the wrong soil, seedlings may grow less vigorously – or fail to sprout at all!

How to prevent or fix it

Always start with fresh, clean, bagged soil that is specifically made for starting seeds, like this one. It will have the right light and fluffy texture to promote germination, and allow the tiny hair-like new roots to grow with ease. Bagged seed starting mix will also be free of potential pests or disease. 

However, because it is so light and mild, seedlings won’t be happy for an extended period of time in seed starting soil alone. Sometimes we add just a small amount of richer potting soil and worm castings (no more than 30 % of the overall mix) to our seed starting soil. Or, the seedlings will eventually need to be fed or transplanted to get the nutrition they need. Be sure to keep reading to see the fertilizing and potting up tips below!

4) Not Providing Warmth to Seeds 

Why this is an issue

For most types of veggies and seeds, the optimum soil temperature range for seeds to germinate (aka sprout) is around 70 to 80°F. At the right temperature, seeds sprout readily – and move on to their next stages of growth more quickly too! Without adequate warmth, they can be sluggish or potentially not sprout at all. Note that a handful of plants do prefer cooler soil to germinate, such as onions, lettuce, and peas. They like it around 50-60 degrees. 

How to prevent or fix it

When you are starting seeds indoors, keep your seed trays in a warm location of your house. This could include the top of a refrigerator or tall shelf (heat rises!) or next to a sunny window, but not one that is cold and drafty. Sometimes those spots aren’t quite warm enough however, especially during the winter months.

A heat mat is a very handy gardening tool to have. In combination with a thermostat, a seedling heat mat can consistently keep the soil temperature in the perfect range. Ours has a probe that stays in the soil, and only turns on when the soil needs heat. That means that as things naturally heats up during the daytime, the heat mat turns off. As the temperature drops in the evening, it kicks back on. But do not heat seeds over 95°F! This can sterilize and kill them.  Also, remove or turn down you heat mat after the seedlings sprout, unless it’s extra cold (e.g. overnight in our greenhouse). Too much heat after germination can cause leggy seedlings.

Another way to help keep seeds warm (and damp!) during germination is to cover the seed containers. Humidity domes placed over the seedling trays trap in heat and moisture to help aid in rapid germination. But as soon as they sprout, it is best to uncover those babies – because they need light! 

5) Not Providing Enough Light for Seedlings

Why this is an issue

When it comes to seedlings, taller does not mean better. In fact, it is quite the opposite! Tall, lanky seedlings are usually the result of inadequate light. This is called getting “leggy” and happens when seedlings stretch in search of better light. Veggie seedlings need 12 to 18 hours of bright direct light, and at least 8 hours of dark. Providing insufficient light is arguably the most common seed starting mistake!

Leggy seedlings are usually weak, top heavy, and at risk of breaking. They’re also more prone to a condition called “damping off” – where they become skinny at the soil line, flop over, and die. It is also not ideal to have leggy seedlings come planting time.

How to prevent leggy seedlings

More often than not, a bright sunny window will not provide enough light for your seedlings – especially during short winter days. To prevent leggy seedlings, most gardeners find the need to provide supplemental light over them. Grow lights come in many shapes and sizes these days! Most often, I see lights hanging inside a shelving unit, suspended above the seedlings on the shelf directly below.

In the past, we used these cool white T-5 fluorescent lights with reflectors. However, that was when we were starting our seedlings in a greenhouse that also gets some natural light. We chose a slightly slimmer light design on purpose, so that the lights wouldn’t completely block out the sunlight. For an indoor set-up, it’s better to use wider, boxy lights with even more surface area. Now we have this awesome all-in-one LED grow light shelf. Check out this article for even more information on how to choose and use grow lights, including several top-rated and popular grow light options!

Ideally, all of your seedlings should be able to fit under the grow lights for maximum light exposure. Otherwise, plan to rotate your seedling trays every day or two so that everyone gets a chance under the spotlight! Hang the lights in a manner that can be easily adjusted. Keep them low (just a couple inches above the seedlings, unless your specific light directions say otherwise) and raise them as the plants grow. If you’re forgetful or not home at the right time, a programmable light timer can be helpful to turn them on and off for you!

How to “fix” leggy seedlings

Once seedlings become leggy, you can’t make them short again. However, all is not lost! As soon as you notice your seedlings are becoming leggy, provide them stronger and longer light as quickly as possible to prevent further stretching. Then, you’ll want to work on gently increasing their strength. Read more details in the “hardening off “section below (number 9). Once their stems are more strong and tough, you should be able to bury a portion of the leggy seedlings stem – either when they’re potted up, or transplanted outside.

Most types of veggies don’t mind if their stem is partially buried. For example, it is a common practice to bury the seedling stems of lanky tomatoes, peppers, and members of the brassica family (cabbage, kale, and broccoli) up to their first set of true leaves. Many will even grow new roots from the buried portion! Only do this once the stems have become firm and slightly tough. Super young and tender seedling stems will likely rot if buried. 

6) Watering Too Much, or Too Little… or Both!

Why this is an issue

Too much water can cause seedlings to rot and die. Yeah, you drowned them. Plants “breathe” (respire) both through their leaves and their roots. So while they need water to survive, they will not be happy campers if left sitting in constantly soggy soil. Furthermore, overly wet conditions can lead to fungal disease, “damping off”, and sudden death, as described in the light section above.

I’m sure we can all imagine why too-little water leads to unhealthy plants. If allowed to dry out, seedlings will wilt and can eventually die as well. However, over-watering is a more common seed starting mistake, and also more likely to cause issues that they can’t bounce back from! On the other hand, I have seen very wilted plants fully recover – even after their soil was left dry for a couple days.

And then there is the yo-yo issue. When plants are watered inconsistently, they’re going to be stressed. Stressed seedlings are not robust and healthy seedlings like we want, and will be more susceptible to all other types of issues. 

How to prevent or fix it

Maintaining the “perfect” moisture level can be tricky, but here are a few tips: Pre-moisten your seed starting soil before adding to containers. That way, you don’t have to dump a bunch of water on it after planting the seeds. The goal is to get it evenly damp, and the consistency of a wrung-out wet sponge.

During germination (before the seeds have sprouted), keep the seedling trays covered with a humidity dome or other plastic to prevent the soil from drying out. As needed, mist the top of the soil with a spray bottle to keep it damp. Seeds need consistent moisture to sprout.

After your seeds sprout into seedlings, one of the best ways to water is by “watering from below”. This means adding water to the tray that the seedling containers are sitting in, rather than pouring or spraying water directly into the containers themselves. The soil will draw up water as needed. It also encourages roots to grow downward. Spraying or watering from above is less consistent, and can lead to damage, disease, or damping off. 

As long as the seedling tray is sitting level, watering from below is the easiest way to ensure every cell or container is getting the same amount of water. Provide enough water for all the soil/containers to draw up water and become damp, but not so much that they’re sitting in standing water. The water you add from below should be “sucked up” within a few hours of watering. 

Before watering, assess the soil moisture level. It is best to err on the light side, so allow soil to change from wet to damp (or even ever-so-slightly dry) before watering again. Develop a consistent watering schedule.

7) Fertilizing Seedlings Incorrectly

Why this is an issue

When seeds first sprout, they don’t need any fertilizer at all! That’s because seeds are amazing. The seed itself contains all the food and nutrients that its seedling needs for a few weeks after sprouting. If seedlings are fertilized too soon, it could harm or even kill them.

On the other hand, if seedlings are left without any fertilizer for a month or two after sprouting, they will be starving for nutrients and grow far less robustly than they could be. This is especially true if they’re not potted up and are left in their original seed starting soil. Finally, using too strong or the wrong type of fertilizer can burn or shock seedlings, even if it is applied at the “right” time. 

How to prevent or fix it:

Leave your seedlings alone for the first few weeks. I mean, give them water and light and love of course… but don’t fertilize them! The best time to start feeding seedlings is after the first couple sets of “true leaves” appear. True leaves are the ones that come after the very first two sprouts (often heart-shaped), called cotyledon.

We like to feed our seedlings using a dilute seaweed extract. It is gentle, sustainable, and effective! As long as you mix it according to the instructions, it is virtually impossible to burn or shock seedlings with seaweed or kelp fertilizer. To do so, we simply mix it with water and pour it into the trays below the seedling containers (watering from below) in place of their routine water. Repeat every other week. See this article all about feeding seedlings with seaweed extract for more details. 

Another way to feed seedlings is to pot them up into a larger containers with richer soil, explained next.

8) Not Thinning Seedlings or “Potting Up”

Why this is an issue

Crowded, cramped seedlings are not happy seedlings. When they’re left un-thinned, those several sprouts will compete for nutrients, root space, and water. Furthermore, crowded seedlings get less air circulation and may lead to disease or damping off. They may also shade each other out and not get the best light, leading to leggy seedlings. 

Even once they are thinned out, seedlings will not be healthy if they’re left in small containers for too long. Their roots will run out of space to grow and wind around themselves, becoming “root bound”. Plants that are allowed to become root bound are often stunted long-term. Meaning, even once they’re potted up or transplanted into a garden bed, they will not grow as large and healthy as they could have. They will not reach their full potential!

How to prevent or fix it

Different gardeners have varying methods and preferences for thinning seedlings. Some like to pluck and pull out the “unwanted” ones. Personally, we prefer to trim out the excess sprouts with little pruning snips, reducing the risk of disrupting the keeper. We start this process around the same time we start feeding seedlings, just a few weeks after sprouting. By then, we can determine who the strongest, healthiest looking sprout is to keep. The sooner, the better!

Other gardeners like to keep every sprout. You can thin and pot up seedlings at the same time by removing a cluster of sprouts from a small cell, gently pulling the seedlings and their roots apart, and then planting each one into a larger container.

In regards to potting up, we reduce the urgency and need to pot up as soon by starting many of our seedlings in 4” nursery pots. They are large enough to comfortably house a tomato seedling, pepper plant, or other veggies that tend to get big quickly for at least a month, if not longer. If you start seeds in smaller cell trays, they will need to be potted up into larger containers (or transplanted outside) sooner. Check the bottom of the containers. Are roots poking out through the drainage holes? If so, it may be time to pot up.

To read more about potting up, check out this article all about it. Or, learn more details and methods on thinning seedlings here.

9) Skipping the Hardening Off Process

Why this is an issue

Imagine this: you’ve dedicated your love, effort, and time to raising healthy little seedlings. You’ve even avoided many or all of common seed starting mistakes you’ve learned here today. It is finally time for your baby seedlings to move out of the house into the big world waiting for them outside! You plant them in your garden. The sun shines down on them, they sway in the wind, they’re dewy with mist… Everything is looking good. And then they die. 

Uh oh. Did you forget to harden your seedlings off? Without a proper “hardening off” process, seedlings that were raised indoors (or in another sheltered environment, such as a greenhouse) are not ready for the conditions of the great outdoors. They are susceptible to sunburn, snapping in the wind or rain, and general shock.

How to prevent or fix it:

“Hardening off” is the process of slowly introducing seedlings to conditions that mimic the great outdoors, and also gradually bringing them outside to become more hardy. This prepares them to be planted outside in a smooth, healthy, stress-free transition.

Start early! Just a couple of weeks after sprouting, we introduce movement and airflow to mimic wind by using an oscillating fan nearby. If you are using a heat mat, you should start to slowly reduce the temperature and time it is used as it gets closer to planting time.

Finally, a week or two before actually planting them out in the garden, seedlings should be brought outside little by little to get exposure to direct sun, wind, and other outdoor conditions. Start with just a couple hours per day in a shady location. Then, gradually increase the time and sun exposure over the course of a week. Here is a step-by-step guide to hardening off seedlings.

And that concludes your crash-course on the most common seed starting mistakes to avoid.

What do you think? Have you perhaps committed some of these seed starting mistakes? Are there any other best practices that you utilize that I forgot to mention? I hope that you learned something new, and feel empowered to start and raise healthy seedlings of your own! Now that you know the most common seed starting mistakes, it should be easy to avoid them.

If you enjoyed this article, you may also like:

• Beginner’s Guide to Using a Hobby Greenhouse
• How to Design & Build a Raised Garden Bed
• Seed Starting 101: How to Sow Seeds Indoors (more detailed step-by-step instructions than provided here today)
• Using Grow Lights for Seedlings and Indoor Plants
• How to Prevent Green Algae or White Mold on Seedling Soil

Please feel free to ask questions, and spread the gardening love by sharing or pinning this article! Tag us in your seed starting adventures by using #homesteadandchill or @deannacat3 on Instagram. Happy sprouting!