Seed starting is a fun winter and early spring activity that gives us an opportunity to exercise our green thumbs during an often dreary time of year, but missteps can lead to disappointment. To help you have a successful and delightful experience, this week I am sharing the top five seed starting mistakes and how to fix them or avoid them altogether.
Seed starting season is my favorite time of the year. Though we’re still a couple of weeks out from when we should really get going, I’ve already started a lot of seeds in my greenhouse and seed starting room. If you haven’t already gotten a jump on seed starting season like me, now is the time to get your equipment ready and think about your plan of action.
From the moment you put those seeds into your seed starting medium, you take ownership of those seedlings and watch over them like children, ensuring that they have everything they need to develop and thrive so they can one day move out of the house, or greenhouse, and into the garden. You’d hate for anything bad to happen to them during this delicate time, so take heed of these common seed starting mistakes and how to avoid them.
I want to address these mistakes so you won’t feel overwhelmed or lose hope when something doesn’t go the way you wanted it to, especially if you’re a beginner. I want to help you steer clear of those seed starting mistakes and learn what to do to get back on the right path if you had some missteps.
At this stage in my career and my experience in seed starting, which is decades now, I still make mistakes, which I embrace. Every mistake is a learning opportunity. I want to know as much as I possibly can though I acknowledge I’ll never know it all. That’s what’s so exciting about gardening: There’s always more to learn.
If you are interested in seed starting, you’ll want to know that my Online Gardening Academy™ Master Seed Starting course will open up for enrollment on January 23. It’s a comprehensive course with 75 instructional videos. It took two years to create this course, with trials and experiments that I documented to craft the many hours of course content.
You can sign up to be notified as soon as the enrollment window opens. You can also register right now for my free webinar Seed Basics & Beyond: 9 Things to Know Before You Start Plants From Seed. There will be four opportunities to attend between January 24 and 27.
Mistake No. 1: Not Planning Ahead
In our enthusiasm to get our seed starting going, we may neglect to get everything in place that we need for a successful seed starting season and may rush into things when we should have acted more mindfully.
I want you to think about where you’re going to start your seeds. Do you have a room that’s warm enough and not drafty and is safe from pets and young children? You need an undisturbed place where you have a controlled climate environment because seeds and the soil have to be within a certain temperature range to properly germinate.
A garage or an unheated basement may be your only available spaces, but if you have the opportunity to choose a location in your home with consistent warm temperatures, that should be your first choice.
Old advice called for starting your seeds on top of your refrigerator because back in the day the compressor and condenser were near the top of fridges and would throw off heat. That was like a built-in heat mat for seed trays. However, this is not the case with modern refrigerators. Those parts are now low on the fridge, and the top of the fridge won’t supply the heat seeds require for good germination.
Where will you store your seed trays? Do you have a bookshelf, table or rack that is sturdy and can withstand the weight? It helps to keep your seed trays in a place where they won’t be in the way. The less disturbance the better, so avoid putting seed trays in a spot where they will have to be moved all the time.
Also consider if there is enough room above the seed trays to hang grow lights. It’s very important to have supplemental lighting. You might think that your sunny south-facing window offers sufficient light, and you could probably get by with it, but you’re not going to have seedlings that you’re happy with. They’re going to be very leggy and stretching for the sunlight.
Even a south-facing window that gets all-day sun in the wintertime does not provide enough hours of good-intensity sun. It will be less than eight hours, and that’s not enough for young seedlings to stay stocky, build up thicker stems and leaves, and not be so spindly.
Seeds are only viable for so long. The older they get, the less viable they are — they won’t sprout as well or consistently — and eventually all the seeds in a pack will no longer be of use. It depends on how well the seeds were stored and what variety they are. If you find a pack of seeds that you just uncovered, check the date. If they are a couple of years old, they may be shot. When in doubt, buy fresh seed. It will be worth the investment of a few dollars and will save you in disappointment.
Once the seeds are sown, think about the water. Will you be around for the next four to eight weeks that those seeds will need to grow? If you are heading out of town, what plans do you have in place to make sure water is still provided?
Mistake No. 2: Assuming You Have Everything You Need
One of the reasons that people don’t get into seed starting is because they feel like they don’t know enough to get started or that they don’t have everything they need to begin the process. But let me set you at ease right now. You do not need a lot of fancy equipment at all. Seeds are designed to germinate without our help. It’s in their DNA — but we need to provide the right conditions. Outdoors, they’ve got internal mechanisms that have that all figured out, but inside we can disrupt that with temperatures that stay too cold for too long or too hot for too long, or the humidity’s different than the outdoors. So taking some time to do your homework before sowing seeds is worth your while.
People who are new to seed starting may assume that because they have a yard with soil in it, they have what they need to start seed indoors. But native soil, except in rare circumstances, will be too loose or too heavy for starting seeds indoors.
Heavy soil is mostly clay and is slow to drain. Sandy soill is too quick to drain. What you need is a growing medium that holds onto just enough moisture between waterings and doesn’t allow water to pool and drown roots. It should also be free of pathogens.
Seed starting mix, often called soilless mix, is a sterile medium that contains peat moss, coconut coir or paper products like PittMoss. It’s clean, it holds enough moisture, and yet it drains properly too. And that will help ensure success.
Next you’ll need cell trays, with drainage holes, to put the seed starting mix in. And because those trays let water run out of them, by design, you’ll also need solid trays for them to sit in that catch the water. Many seed starting kits come with bottom trays, but in case yours doesn’t, be conscious of that.
Once the seed starting mix is watered, keeping the moisture in the cell tray will also require humidity domes that go over the cell trays. But you don’t need to buy fitted domes. You can use plastic wrap that will keep the moisture in just as well as a dome. Then once the seeds germinate, the plastic wrap or dome should be removed.
The last thing you need is a grow light, and it doesn’t have to be expensive to be effective. Go to the box store and buy a 4-foot-wide or 2-foot-wide shop light or two. They run about $20 and are usually LED and very efficient.
Once seeds are sown, they take a couple of days to germinate, but you need to anticipate their needs by having lights ready to turn on before the sprouts emerge from the seed starting mix.
Mistake No. 3: Starting Too Early
When seed starting, timing is everything. The goal is to get seedlings to the size and maturity that they need to be for safe transplanting outside. If we’re talking about warm season seedlings, the classic summer crops, which is typically what we’re sowing coming into this time of year, we don’t want to plant them out too early — before the last risk of frost. If you get a frost after your seedlings are already planted outside, they’re probably going to die. This can feel crushing.
Seedings should only be planted out when all risk of frost is past. You can find your last frost date for your location by putting your zip code into Almanac.com. Take that last frost date and work backward. If a seed packet says to start seeds indoors six to eight weeks before all risk of frost has passed, count six to eight weeks back from your last frost date, and that’s your window for sowing the seeds indoors.
If you do what you’re supposed to do, those seedlings are going to take off. You’ve given them the right soil mix, you’ve watered them properly, you’ve given them good light and they’re happy and growing. Maybe you transplant them to a bigger pot halfway through the six- or eight-week period between sowing and transplanting. But if you start too early, they will have demands that you can’t fulfill indoors. They will continue to get larger, too large for their containers, their grow lights and indoor spaces. At this stage of growth, they want to be in good sunlight with great soil full of beneficial microbes.
Seedlings started too early will become rootbound, the lights won’t be strong enough for the overgrown plants, and the plants will start to look yellow, limp and unhappy. This is not a death sentence, however. When you do eventually get them outside, you can step back and watch Mother Nature take over. Things will improve dramatically in 10 days.
You can avoid this setback by getting your timing right in the first place. Plants that are at an ideal size when they are moved outdoors will be more vigorous from the get-go.
If you can start too early, can you also start too late? It’s not a bad thing to get a bit of a late start, however, you don’t want to start too late. For example, if your last first date is May 1 and you don’t get a tomato seedling outdoors until August, that leaves a very short window between the plant becoming accustomed to the outdoors and the first frost of spring killing the plant. It may not have the chance to bear fruit in that time.
Mistake No. 4: Overwatering
Overwatering kills more plants than underwatering, and this holds true when talking about seedlings started indoors. We love our seedlings, and one tangible thing we can do in an attempt to help them along is water them. But plants need to breathe, including their roots. They are living organisms that must have access to oxygen, which is found in porous soil. But when we water too much, we displace the air with water. If the seed starting medium doesn’t get a chance to dry out, the roots will drown.
Getting the watering right can be tricky. If your seedlings are growing on a hot mat and under a grow light, the soil surface may appear dry while the seed starting mix underneath is either dry or wet.
You want soil that is in between dry and saturated. It’s the Goldilocks principle: not too dry, not too wet, but just right. You can test with a moisture meter, which is a good tool to have, though personally, I don’t use a moisture meter. I go by weight.
I start by filling the cell tray with seed starting mix and seeds then weighing it before adding any water. Then I saturate it and let the excess water run out the bottom of the drainage slits. What remains in the mix after the excess has had a chance to disperse is called “field capacity.” That’s the most water the mix can hold, just like when a sponge is wet all the way through but no longer dripping.
Now put the tray on the scale again and record its weight at field capacity. Now you know the bookends of the weight, from completely dry to maximum moisture. And when you are unsure if you need to add more water, you can weigh the tray. And you’ll soon learn how to tell if it needs water just by getting a feel for how heavy it is. If it feels light, it needs water.
I aim for the sweet spot between totally dry and totally wet. I want to know that there is enough porous space in the seed starting mix for oxygen to get in. The roots need oxygen to form, so if the tray is kept at 100% saturation all the time, the seedlings will be doomed.
Mistake No. 5: Insufficient Light
All plants need some amount of light to grow. That’s what they use to make their energy via photosynthesis. Unless you’re a professional grower who’s spending a lot of money on lighting, you won’t be able to come close to replicating the amount of sunlight outside. Consumer-grade grow lights can’t get you there — but they do provide enough light for the few weeks your plants will be reared indoors before they are planted out.
Even a really good grow light provides just a 10th of the amount of photons that a plant would get if it spent all day outdoors from sunrise to sunset. But that 10th is enough for our seedlings because their demands are not as much during this stage in their growth. You don’t need a huge light or to spend a ton of money. A $20 shop light will do the trick.
There isn’t one grow light that I always recommend. Many lights work, both LED and fluorescent, and it’s not which light you buy but how you work with it. You need to understand the amount of output from that bulb or those diodes.
Note that fluorescent bulbs and tubes lose their brightness as they age. A bulb that worked great the first year may not work as well in later years of seed starting, and you may need to move the light closer to the plants to make up that difference.
The best way to know if your light is enough is to just gauge how your seedlings look. If they appear spindly and leggy, the light is inadequate. Your first step is to either lower the lights closer to the top of the plants or to increase the number of hours a day that you leave the lights on — or a combination of both.
It takes some time to get a feel for this. It will probably take all season to get it down.
Read the instructions that come with the grow light. If it’s a reputable company, the instructions or the company website will tell you what distance from the plants the lights should be and how often to leave them on. Though if you brought a shop light that was not intended to be used as a grow light, the company won’t offer that information, so you’ll have to experiment.
A T12 fluorescent tube is the diameter of a quarter and can be placed very close to the top of the seedlings without burning them. A more modern style fluorescent tube is a T5, and it’s about the diameter of a dime. There is one in between called a T8, which is about the diameter of a nickel. The smaller the diameter, the more efficient the light is. So a T5 should be a greater distance from the top of the seedlings compared to a T12.
You’ll have to observe and adjust accordingly. That’s the only way to do it unless you wish to shell out $500 for a light meter.
LED lights have all but replaced fluorescent lights when it comes to grow lights. LEDs are deceptively powerful — they will bathe that plant in a good amount of light, and you’ll want a greater distance between the light and the plants than you would have if using fluorescent lights.
When the light is too close to the plants, you risk burning the plants. The plants will look bleached, pale or tan as a direct result of being hit by too many photons for too long. The solution will be to raise the light or reduce the number of hours that the light is on. I leave my grow lights on for 16 hours a day on an automatic timer.
Don’t worry if you don’t have your lighting dialed in perfectly in the first year. You’ll figure it out with some time and observation. Monitor the plants, take notes and pictures, and rely on your documentation.
If you haven’t listened to the top five most common seed starting mistakes, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.
What seed starting mistakes have you overcome? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course. The course is designed to be a comprehensive guide to starting, growing, nurturing and harvesting your favorite vegetables, no matter what you love to eat, no matter where you live, no matter your level of gardening experience.
joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out. We will open up for enrollment on January 23. Sign up to be notified.
joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier.
joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast was based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Corona Tools, Milorganite, Soil3, Greenhouse Megastore, Territorial Seed Company, Earth’s Ally, Proven Winners ColorChoice and Dramm. These companies are either Brand Partners of joegardener.com and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.