For those of us in the Northern Hemisphere, it’s seed-starting season. Starting seeds indoors is a great way to start gardening even though your garden outdoors may be covered in snow. It also provides you with better control over the health of the plants you will add to your garden – and is the perfect opportunity for experimentation.
I’ve started literally hundreds of seeds this season. I began a month ago and have planted more seeds each week. My earliest seedlings were started earlier than would be optimal – even for my Atlanta-area Zone 7b garden, but I wanted to try some new things and have really been enjoying observing all the different stages of development.
I hear from many gardeners who feel anxiety about the seed starting process. They are anxious to get it right and feel that the pressure is on. That couldn’t be further from the truth. Sure, being the caretaker of seedlings can be a lot of responsibility, and there are so many different ways to approach it. Yet, it’s important to remember – these are just seeds.
In the worst case scenario, everything goes badly and all your seedlings die or seeds fail to germinate. That’s okay. Take notes about what you did, where you may have gone wrong, and begin again. These are learning opportunities. Embrace them!
When should you start seeds indoors?
This is a question I’m asked often. As a general rule for everything you’ll be planting in your summer garden, you want to start seeds about eight weeks before the risk of frost has passed in your area. How do you know when that is? An easy internet search to find the “last frost date” for your area will provide the estimated date. Your local County Extension Office is another great resource for this information.
The last frost date is based on past year averages. As we all know, weather can be a fickle business, so don’t consider the estimated last frost date to be set in stone. Once that date arrives, keep an eye on the current forecast to make sure temperatures are remaining a few ticks at least above the frost level (32 degrees Fahrenheit) before you move your seedlings outdoors.
That said, eight weeks before that date is a good rule of thumb for most warm season edible plants.
What should you use for starting seeds?
The soil you use is important. Don’t dig up your garden soil and don’t use “potting soil” or “potting mix.” These options are too dense and heavy for delicate seedling roots. Those soils may also contain disease pathogens which can threaten the tender young plants.
Your best option is seed starting mix or what may be labeled as “soilless mix.” This material tends to cost around $5 per bag and contains materials like perlite, vermiculite, peat moss, and/or coir (ground coconut husk). Each of these ingredients are lightweight and, more importantly, they are considered sterile – a lower risk of carrying any disease pathogens.
You can certainly make your own soilless mix, and many gardeners do. However, I tend to feel that can become more expensive, and it can be more time-consuming.
I bought bags of soilless mix for years and had great success. However, in the past few years, I’ve been growing so many seedlings that I needed a lot of mix and turned to a different product. Lately, I’ve been buying soilless mix from a commercial supplier. You can find them online or one may even be located near you. They offer larger bags (which are still fairly inexpensive), and many offer a more sophisticated blend, which I’ve loved.
Some of the commercially-available blends include a wetting agent in the mix. That encourages the mix to absorb water more easily and to hold that water for longer. They sometimes include lime, which balances the mix closer to a neutral pH. Since peat moss is an acidic ingredient, some soilless mixes without lime tend to have a pH which is more acidic.
If you use a soilless mix without lime, don’t worry. This ingredient isn’t a game-changer, just a subtle difference which can give your seedlings a little bit of a leg up.
Commercial-grade soilless mix can come in a variety of different blends, depending on your needs. For example, you may opt for a blend with ground pine bark or another material which provides greater diversity in texture.
The blend that I’m using this year is Pro-mix BK25V. You may or may not be able to find that particular product. Other similar options I use with great success are MetroMix 360 or Fafard 3B.
There really is no right or wrong here – just finding the system and approach which works for you. The products and techniques I use may or may not work in your case. So, consider my approach as a guide if you’ve never tried this project before – or a way to gather new ideas to try.
How do you start seed?
If you use a soilless mix without a wetting agent, consider wetting the mix in a large bin or bucket before you add it to your seedling trays. That will distribute the moisture more evenly through the mix. If your soilless mix does include a wetting agent, you can wait to add water until after you’ve planted your seeds since the water is received so readily into the mix.
Once you have your mix and your seed trays, your best guide is the seed packet. It will tell you how deeply to plant the seeds. Many varieties of seed have different needs. Some – like tomatoes and pepper – only need a very light covering of soil. That thin layer of soil serves to keep the seeds moist during germination. Other seeds, like beans, need to be planted more deeply. Since they take longer to germinate, a deeper layer of soil keeps the seed protected and moist.
It’s not necessary to use seed trays for success. There are many ways you can save money by repurposing materials you already have in your home. Some gardeners prefer to use soil blocks. There are a number of benefits from soil blocks, but the molds to form them can be expensive. In the end, it’s all about discovering what fits your budget and which option brings you the most enjoyment in the process.
The same is true regarding the number of seeds you place in each container. The past two years, I’ve been using a dense-planting technique. I place 16-20 seeds in a 4” container. This allows me to grow a lot of seeds without using as much space or materials. However, as the densely planted seedlings grow, they need to be transplanted – more on that in a minute.
If you opt to plant one seed per container, you may be able to skip the transplant process, since each seedling will have ample room to develop a healthy root ball.
Once the seeds are covered, tamp the surface down very lightly. Your goal isn’t to compact the soil – and it shouldn’t become compacted. Your goal is a light touch just to ensure there is good seed-to-soil contact.
Next, add water. If you pre-moistened your mix, you won’t need much. In any case, you want to provide enough water that the entire container is about as moist as a damp sponge. In either case – once you think you’ve provided enough water to your seeded tray – pick it up and feel the weight. The tray should feel significantly heavier that before water was added. If it doesn’t, add more water.
Once your soil is moist, cover it with something transparent – either a tray dome or even plastic wrap. The purpose of the covering is to hold the moisture in the soil until those seeds germinate.
When do you remove the cover?
This is another question I’m asked frequently. I prefer to remove the covering just as soon as I see the first few seedlings pop up through the surface. Although there may be a majority of seeds which haven’t quite germinated, rest assured that they will show their little green heads within a day or so, and now it’s up to you to make sure the soil mix in the uncovered trays remains moist at the surface.
The reason I remove the cover at this early stage is to reduce the risk of a fungal disease known as “damping off.” Seedling tissue exposed to moist air is more likely to fall victim to this disease, which kills the seedling at the soil surface.
Once I remove the cover, I also place a small fan where it will direct air at a low setting across the surface of the seed trays. The air movement also reduces the risk of disease. Sure, it also helps strengthen seedling stems a bit, however it’s the key goal for good air movement is to control disease.
I keep the fans moving across the surface of seed trays throughout the eight weeks of their development. That’s not absolutely necessary. You could opt to turn fans off once the seedlings have developed their first true leaves. I’ve just found the best success with continued air movement throughout the process.
How much water do seedlings need?
How moist you keep the soil will, in part, determine how quickly your seedlings will grow. I’ve been keeping my seedlings in soil that is continually as moist as a damp sponge, which has encouraged them to really grow at a rapid pace. It’s been another interesting experiment.
I actually prefer to give seedlings a little tough love when it comes to water. I never let the soil dry out completely. However, I do typically wait to provide water until the surface of the soil has dried slightly; and the seedlings, themselves, begin to show the very first signs of wilting. At that point, I irrigate through bottom-watering.
Bottom-watering is placing water in a solid tray in which the seed trays are sitting. The soil quickly wicks up the water in the solid tray – from the bottom of the container. I really prefer this method, because it’s faster and it keep the foliage dry – another good way to avoid disease.
If you prefer to water from overhead with a watering can or a spray, that works just fine. Just find the method that works best for you.
When I bottom-water, I add fill the tray about a quarter of the way full, and I don’t need to do anything else until I notice those first signs that the seedlings need water. That little bit of tough love – allowing just a short period of dryness – can strengthen seedlings.
When do you put seedlings under light, and which light do you use?
Ah – the king of questions! Lighting just may be the single greatest source of anxiety for any seedling caretaker. There is a lot of information out there, so how do you know which option is the right option? The answer is trial and error and a little education.
First things first: I prefer not to force my seedlings to seek out the light for even a moment. The goal for anyone starting seeds indoors is growing sturdy, stocky seedlings – not seedlings which are gangly and stretched.
One of the causes for seedlings to become stretched or leggy is when they must reach for the light. I track how many days each seed will require to germinate, and I turn lights on the day before I expect seeds to break through the surface. For example, my tomato seeds take three days to germinate, so I turn the lights on during day two.
That way, the plant tissue is receiving the light it wants immediately – without have to stretch for it. I’ve had great success timing the lighting this way.
If you aren’t certain which day you’ll see germination or if you are growing different varieties within the same space, you could opt to turn the lights on a day or two early (based on the estimated germination date on the seed packets), or you can just keep a close watch and provide light once you see germination start.
Again, a day or two without supplemental light after plants have been up and growing for at least a week or so won’t be detrimental to seedling success. Plants are resilient. Even if they stretch for a day or two, they will be just fine once you begin providing the light they need.
So, what supplemental light do they need? The truth is all seedlings will need more than just a sunny window. You can get by with that, but you will have leggy seedlings. If you have the ability, I strongly recommend you provide some form of supplemental light.
I used inexpensive 40-watt fluorescent shop lights for years. I grew great seedlings that way and typically kept the lights on for 16 hours each day. Last year, I switched to LED lights. LED lights are more efficient in energy use. What I love most about LED lights, however, is that I can raise them a higher distance from the seedling trays.
If you use 40-watt fluorescent lights, the lights need to be about an inch or two above the tops of the seedlings in order to provide the light the plants need. I like to get in there and see what’s happening. The LED lights I purchased are 300-watt. The higher wattage means there are more light waves being provided to the plants, so the bulbs can (and must) be further away from the leaves.
With all lighting, there is a learning curve. Read the instructions provided with your lighting unit. When I switched to LEDs, I made the mistake of placing them very close to the seedlings – just like I had been doing with the fluorescent lights. I quickly noticed that the seedlings turned yellow and looked bleached. They were getting burned by too much light. It’s not the heat that burns foliage. It’s an overabundance of light energy.
I continued to raise the lights incrementally until I found that the foliage was responding well when the light was 54” away. That sounds like a lot right? Well, it is, but because I had damaged the foliage with too much light, it needed that distance to recover.
This year with the same lights, I’ve found that a height of 24” above the tops of my healthy seedlings is the sweet spot. With foliage that hasn’t been damaged by too much light early on, they can withstand – and they love – the quantity and quality of light provided by the 300-watt bulbs at a distance of 24”.
This will be a case of trial-and-error for you too. Your best guide is observation. Check your seedlings daily to watch for telltale signs. Are they stretching? Move the lights closer. Are they becoming pale? Raise your lights further away. It may take a few attempts, but you will find the sweet spot too.
This year, I’ve also opted to leave the lights on for 24 hours initially. That has really encouraged my seedlings to grow rapidly. With more light at a proper level, the foliage is coming on faster and sturdier than ever before.
Once seedlings set their true leaves, I began to turn lights off for 8 hours each day. What are the true leaves? The first set of leaves on every seedling is known as the cotyledon leaves. The true leaves are the second set – the leaves which develop above the cotyledon.
Now, there are all kinds of lighting options out there, and you may even be overwhelmed by the diversity. LED and fluorescent, various wattages, full-spectrum or red or blue and more. How do you make sense of it all? With a little bit of education.
There is more to the relationship between plants and light than you may be aware of. Recently, I shared a podcast on the science of light. It’s a fascinating look to help you understand what is important about light duration and quality. For example, plants utilize red and blue light waves more than the other spectrums. It’s why you’ll see grow lights commonly sold with just the red and blue lights. The light cast by this combination appears pink to purple – what we see by the blend of the red and blue spectrums.
I use full-spectrum lights. Although I know that it’s not quite as efficient, I just prefer to look at the white light provided by full-spectrum bulbs. I’m not a fan of the look of the colored bulbs.
I hope you’ll check out that podcast on the science of light to learn more about light spectrums and the other aspects of light science which will help you make an informed decision that’s best for you.
One tool that’s helpful is a timer for your lights. I’ve used one for years so that I don’t need to remember to turn the lights off and on when I switch to a 16-hour light day for my seedlings. Whatever schedule you find that works for you, an automatic timer will really be helpful.
One last note about lights: The quality of the product you buy will make a difference. It’s not necessary to buy the top-shelf products, but do pay attention to quality. I paid $70 for the 300-watt units I purchased last year. There were more expensive options, but I did a little research and settled on these. Considering these units will last me for years, I felt the cost was pretty reasonable.
Unfortunately, these units are no longer available. There is a 600-watt version which may work well for you, but keep in mind that the higher wattage will require a greater space between the bulbs and your seedlings.
Should you use heat mats?
Heat or germination mats are just a thin layer of heating coils contained together in a waterproof cover. They come in various sizes to fit under a single tray or multiple trays. Their purpose is to warm the soil.
There are three elements which contribute to seedling growth. Light, moisture and heat. It’s not the foliage of seedlings which needs heat. It’s the soil. Warm soil encourages root development.
Heat mats raise soil temperature about 10 degrees, on average. The preferred soil temperature for most warm season edible plants is 70-80 degrees Fahrenheit. So if your seedlings reside in a room of your home that’s 65 degrees, for example, a heat mat will increase the soil temperature to the ideal range.
If you grow seedlings in an unheated space, like a basement, you may opt for a propagation mat with a thermostat. The thermostat gives you the control of dialing in a specific temperature. Regardless of the temperature of the surrounding air, a propagation mat with a thermostat set to 70 degrees Fahrenheit will maintain the soil at that temperature.
Once your seedlings have germinated, you could opt to remove them from the heat mat or turn the mat off. I prefer to leave them on. By continuing to provide warmer soil – one of the three elements necessary for growth – I’m encouraging more vigorous root development and quicker growth.
In other words – if you have the extra time and patience, you can get by without a heat mat or without continued use of a heat mat. Seeds will germinate and roots will develop in soil that is cooler than the ideal 70-80 degree range. It will just take them longer.
When should you transplant seedlings?
Transplanting time will depend on a number of variables. What size container did you start with? How many seeds did you plant in each container? How quickly are roots developing?
As a general rule, there is no set timeline. Check the bottom of the container. If roots are creeping out of the bottom drainage holes, they are looking for more room. That is the key signal that seedlings are ready to be transplanted to a larger space.
Try not to allow your seedlings to become root-bound. If they do, the plants will recover but avoid this if you can.
Because I have been using the dense-planting technique, I don’t wait until I see roots under the containers. I know those seedlings are crowding together, so I need to take action sooner.
In this situation, I wait until the seedlings have their true leaves and typically wait until four weeks from the time I planted the seeds. I gently remove the cluster of seedlings from the container and tease them apart. Because I’ve used a good soilless mix, it’s actually very easy to separate them.
I place each seedling in its own 4” container and that provides them with enough room for root growth until the last risk of frost in my area has passed, and I can move them into the ground.
That said, I’m doing so many experiments this year, that I might find some of my seedlings develop much more quickly than I’m accustomed to. If I notice the individual seedlings in those 4” pots are sending roots out of the bottom before I can plant them outdoors (or if my area experiences an unusually cold spring), I may need to transplant them into larger containers. Observation. Observation. Observation.
Can you transplant seedlings too early? Yes, and I recently made that mistake. I got anxious and eager to bump them up, and I jumped the gun a little bit. As a result, my transplanted seedlings flopped over and looked terrible. This was due to transplant shock.
If the stems or roots haven’t quite developed enough, seedlings can’t recover immediately. You know what? It’s all okay. Even though they looked as though I had killed them, my transplanted seedlings recovered within 48 hours. I lost about 8 of 128. But all the rest bounced back and are doing great.
Again – if something goes wrong, don’t panic. Plants are resilient and can recover from our mistakes or missteps.
Should you fertilize seedlings and, if so, when?
Did you know that seeds hold the amount of nutrients they need to sustain through the development of true leaves? They don’t need or want fertilizer until that stage. In fact, it can hurt – rather than help – your plants to provide fertilizer too soon.
Many soilless mixes include a very small amount of fertilizer as part of the blend too, so adding more fertilizer just isn’t necessary or advisable.
Once the true leaves come on, that’s the time to consider providing fertilizer. It’s not necessary for success, but it’s one way to push your plants for more growth. The truth is, I rarely use fertilizer and have had great success without it.
This year, I’m experimenting with fertilizer. I’ve set aside three trays of seedlings at the same growth stage. I’ve begun misting the foliage of one tray of seedlings with a liquid kelp fertilizer product. Since my fans provide continuous air flow, I don’t need to be too concerned that moistening the foliage will encourage fungal disease.
To a second of these three trays, I’m adding an OMRI-approved, liquid, organic fertilizer to the water as I bottom-water. The nutrients in this product come from distilled seed oils.
For both fertilizer products, I’m carefully following the instructions on the packaging. If you use any fertilizer product – whether indoors or out – following the instructions is key.
To the third of these three trays, I am not providing any supplemental nutrients. They will continue to grow and develop with only light, water, and warmth from the heat mats.
I’m anxious to begin to observe the differences in seedling health through the next few weeks. I’ll be sharing short video and photo updates on progress on my Instagram channel. I hope you’ll join me there and watch the progress as I document this seedling journey.
How do you harden seedlings off?
Hardening off is the process of acclimating seedlings to the great outdoors. After all, the seedlings have been growing in the comfort of a warm and controlled environment for weeks. If you plant them outside without any adjustment period, it can really take a toll.
It can surprise many gardeners to discover how quickly the sun’s rays will scorch young seedlings. Regardless of how much supplemental lighting you’ve provided indoors, the sun – even on a cloudy day – is even more powerful.
The hardening-off process should begin 7-10 days before you plan to put the plants in the ground. Each day, you move seedlings outdoors for a brief period – then, you move them back inside. The time spent outside becomes gradually longer throughout the hardening off phase. For example, on Day One you may choose to put seedlings outside for 30 minutes – Day Two, an hour – Day Three, 90 minutes.
If you have 7 days to harden off, increase the time by an hour each day so that seedlings are prepared for 8 hours of sun on Day Eight. Ideally, you can take advantage of ten days to ease plants a little more slowly. An hour more of direct sun for new seedlings can sometimes be too much.
This moving in and moving out can be tedious and time-consuming, of course. There are a few ways you can work around this if you aren’t available to follow a stringent schedule. Be careful, but if you find you’ve scorched your plants, don’t panic.
Remind yourself that your little seedling champs are resilient and will bounce back when you ease off on the daylight hours.
Pay attention to cause and effect. These are all learning opportunities, so I encourage you to have fun with it. Don’t put so much pressure on yourself that you lose the joy of the process. Take notes, take pictures, but most of all observe and learn. This can truly be the best part of the experience.
What types of seeds are you starting this year? I would love to hear about your experiences in Comments below.
If you haven’t already listened to the recording, scroll to the top of the page and press the Play icon in the green bar under the page title. You’ll hear me share a few more observations I’ve had this year, and it may spur you to try something else new. Happy indoor gardening!
Links & Resources
Register for My Free Live Webinar – 5 Fundamental Principles for a Healthy Garden (with real-time Q&A) – Four sessions only
Beginning Gardener Fundamentals – Online course details
Beginning Gardener Fundamentals – Registration wait list: Sign up to be notified about the next class!
Here are the products I use and mentioned in this episode:
Griffin Greenhouse Supply (The company I source for soil needs, trays and containers.)
600W LED Full-Spectrum Grow Lights (note: The 300W lights I have are currently not offered by Amazon or anywhere else I can find.)