349-Your Seed Starting Questions Answered

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To get the hang of seed starting, it takes practice. There is a lot to know and a lot to get right, including watering, lighting and timing. So in this week’s podcast, I am answering seed starting questions that listeners have submitted to help you have a successful seed starting experience and get over any hurdles you’ve confronted.

Last week, I discussed the five most common seed starting mistakes, a list that I came up with based on the 5,000 or so questions that I have fielded from the  Online Gardening Academy Master Seed Starting course over the past three years. I set aside this week to answer the questions you still had after listening to last week’s roundup of common mistakes.


Joe Lamp'l with seed starting gear.

Seed starting is one of my favorite gardening activities, and I am happy to share the best practices and solutions that I have learned over the years.


Master Seed Starting is now entering year four. The enrollment period runs through Monday, January 29, at which point the launch window closes until next year. 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 also register right now for my free webinar Seed Basics & Beyond: 9 Things to Know Before You Start Plants From Seed. Three opportunities remain to attend, one per day from January 25 to 27. This webinar is full of information to help you take your seed starting to the next level. In this webinar, I share what you need to know about seeds to understand how they work so that you can be more successful in your seed starting endeavors. I firmly believe that the more that you understand about working with seeds and what they need to be successful, the more success you will have. 

You can delve further into seed starting by checking out the many  “joegardener® Show” podcast episodes and joegardenerTV YouTube videos I have made on the topic. My video “How I Start Seeds Indoors Tips & Techniques” is a great place to start. In 15 minutes I take you through my entire process for sowing seeds indoors, from putting them into the cell trays and watering them in after sowing the seeds, to taking them down to my seed starting room and putting them under the lights. For even more, see the Links & Resources section at the bottom of this page. 

Check Seeds for Viability

One of the most important things to know when seed starting is whether your seeds are still good. If they were not packaged for the current year (this date should be on the seed packet) they may no longer be viable. The older they are, the greater the chance that the seeds will be duds, so fresh seed is the way to go if you don’t want to take any chances. 

Some seed varieties, such as lettuce, go bad quickly, while others, such as tomatoes, can last for years if stored well. If you have older seeds that you are unsure about, you can conduct a simple viability test. Take out 10 seeds and put them onto a damp paper towel, and fold the paper towel over a few times. Put the paper towel into a zip-lock bag and set the bag someplace warm — around 70°F. Three to five days later, check for seed germination. Did seven seeds sprout?  Your seeds have a germination rate of about 70%. Did one sprout? Your seeds have 10% viability.

If you discover that the seeds you saved have 50% viability, you should sow them at a density of two per cell so that you can expect one sprout per cell. 


germination test

A germination test is a simple way to estimate the viability of a group of seeds.


Extending the Viability of Seeds

Moisture and heat are the enemies of maintaining the viability of seeds. Dry and cool are the best conditions for seeds to ensure they last.

When saving seeds, ensure they are dry before putting them into storage, and add a silica pack or two to each seed envelope to absorb any lingering moisture. (You can find all of my seed saving and seed starting supplies and equipment at my Amazon shop.)

A good rule of seed storage is to never go over 100. Take the average temperature in the room and the average humidity and add them up. For instance, a 70° room with 30% humidity adds up to 100, and that will be safe for stored seeds. But if the temperature is 70° and the average humidity is 40°, now the total is 110, and that won’t be good for the long-term viability of your saved seeds.

Seed Starting Mix

Most seed starting mixes that consumers can find at big box stores do not include a wetting agent. Also known as a surfactant, a wetting agent is described as something that makes water wetter. It breaks the surface tension of water so the seed starting mix can readily absorb the water. 

Wetting agents are found in mixes available to the nursery trade, but probably not in the bag of seed starting mix you brought home. You may find that when you add water, the water just sits on the top of the mix. That’s because the mix is hydrophobic. Peat moss is notorious for this. Once peat moss is wet it holds water very well, but when dry it is difficult to get it to absorb any water.

I use a professional seed starting mix, Pro-Mix BX, which comes with a wetting agent. The water soaks in immediately, which avoids the tedium of patiently soaking straight peat moss by adding a little bit of water at a time and waiting. 

I use the shower nozzle of my kitchen sink on a light spray, and the water goes right into the seed starting mix with a wetting agent. If your mix is not absorbing water well, you can gradually add a little bit of water at a time, a couple minutes between applications, for about 20 minutes. Alternatively, you can premoisten the mix prior to adding it to your seed trays and sowing seeds. And once your seeds are sown, you can top the seeds off with a little more premoistened mix and sprinkle more water on top. Because the mix is presoaked, it will accept this additional water readily.

When watering newly sown seeds, take care not to blast the mix with a sharp stream of water. The seeds may float right out of the tray or into other cells. But when you practice, you’ll get a feel for this quickly.


Seed starting mix being watered in trays

Seed starting mix is also called soilless mix. It generally includes peat moss, coir or a paper-based product named PittMoss, but no soil.



Lori Jo Jamieson asks about using PittMoss for seed starting. I have used this product for several years now, and the more I use it the better I get at seed starting with it.

PittMoss is a non-peat, non-coir alternative for seed starting. It’s made of recycled paper and cardboard that’s been engineered to mimic soil. It’s very finely ground but still a little bit chunky. It looks and feels very different than peat moss or coir, but it is an alternative that has promise. 

I am currently experimenting with 2-inch-deep cell trays filled with 100% PittMoss that I am comparing to trays that use a mix of PittMoss and peat moss or PittMoss and coir. 

When I spoke about PittMoss with Dr. Charles Bethke, the manufacturer’s horticultural soil and nutrition consultant, he said that PittMoss is really designed for 4-inch-deep trays, because shallow trays tend to get waterlogged since they don’t have the depth of gravity to move excess water out. 

Figuring out how to use 2-inch trays with PittMoss took me some time. It is a different experience than sowing into peat moss or coir, and there is a learning curve there.



PittMoss is a peat-free option for seed starting mix.


The Best Humidity Domes

Heather C. asked about using humidity domes over seed starting trays. These range from 2 inches high to 7 inches or even taller. I am a firm believer in using 2-inch domes. If you are challenged for space overhead, you don’t want tall domes. A tall dome will make it difficult to adjust the distance of your lights from your plants. The 2-inch domes also cost less, which is another plus. 

A shallow dome over the seedbed will more effectively hold moisture in the seed starting mix. A taller dome gives the humidity more room to spread out, which can lead to the seed starting mix drying out on its surface.

I use the taller domes for propagating cuttings. That’s when I find I need more than 2 inches of head space.


Humidity domes.

Two inches is the perfect height of humidity dome for seedlings.


Heat Mats: Nice to Have

I put heat mats, also known as propagation mats or seed starting mats, in the category of “nice to have” but not “need to have.”

Seeds have a preferred temperature range in which they will germinate. The range is different from one seed variety to the next, and if the soil is either too cold or too hot beyond the range, the seeds are not going to germinate. If your seed starting room is cooler than that range, a heat mat will help get the soil temperature where it needs to be. You can even buy a mat with a thermostat to dial the mat into a specific temperature.  

I use heat mats all the time for my warm-season crops, especially tomatoes and peppers. 

Once the seeds germinate, the heat mat should be turned off. Leaving the heat mat on too long can create an adverse reaction. Cool soil is better for root development, and the air temperature is more important than the soil temperature at this point.


Germination Mats

If your seed starting room is cooler than 70°, a heat mat will help get the soil temperature where it needs to be. You can even buy a mat with a thermostat to dial the mat into a specific temperature. Your cat will appreciate the mat as much as your plants do.


How to Know Your Moisture Is Correct

A good way to know if your seed starting mix is moist enough is to compare the dry weight of the mix to its weight when it has reached field capacity, which is as much water as it can hold. Aim for a moisture level, and weight, about halfway between totally dry and field capacity. 

If sewing in soil blocks, you won’t know the dry weight because you are starting with moist mix pressed into blocks. Soil blocks inherently dry out faster than mix in plastic cells because soil blocks are exposed on all sides. You may not know how light the blocks are when completely dry, but you can still get a sense of how dry the blocks are based on their weight. You can also get a better sense visually of whether soil blocks are dry, compared to looking at seed starting mix in cells, where you can only see if the top is dry. 

You can also reach in and tap a block. If it feels hard and crusty without a lot of give, your blocks need water. 

I don’t bottom water soil blocks because it is hard to know if all of the blocks are getting water. The blocks on the outside of the trays could suck up all the water, leaving the blocks in the middle dry. 

I water soil blocks from the top using a narrow spouted watering can to precisely direct water. If you use a floret that has more of a shower spray, you risk the water beating down the tender seedlings. With a narrow spout, you can get water where it needs to be without pummeling the seedlings.


Watering a seedling tray

Seed starting mix should be kept at a moisture rate that is halfway between dried out and field capacity.



Grow lights come in full-spectrum types and dual-band lights.  Full spectrum lights are white light, and dual-band lights emit a purple hue — I call it burple — which is a combination of the blue light frequency and the red light frequency. The red and the blue light frequencies do the most to promote the growth of the plants. 

The problem with dual-band lights is that purple-ish look is not a pleasing look. If you have to deal with it every day in your office or living room, it’s annoying. Though dual-band lights are the most effective grow lights, it doesn’t make enough of a difference to justify putting up with the unattractive and unpleasant burple light. So I only use full-spectrum lights; it’s just easier on the eyes.

If you will keep your grow lights someplace where they are always out of sight, dual-band lights could be the way to go for you. 


Seed starting lights.

Full-spectrum lights, seen here, are more pleasing to the eye than dual-band lights, which are purple.



Do you need to fertilize seedlings when starting seeds in soilless mix that is free of fertilizer? There is a great debate about this, but I can tell you that I personally rarely fertilize my seedlings between the time they are sown and when they are transplanted outdoors.

Some seed starting mixes contain a trace amount of fertilizer, known as a “charger,” to give seedlings a tiny kickstart. I use these mixes sometimes, but the fertilizer does not last.

Occasionally, as I pot up seedlings to larger containers, I add a light dose of fish emulsion fertilizer or something else with an NPK ratio of around 2-3-1.

You shouldn’t feel like you need to add fertilizer. The seed embryo — the material inside the seed coat — has the food that the seed needs to germinate and grow to the point that it’s on its own. First the seed leaves (cotyledons) develop to start the photosynthesis process, and then the true leaves come on. At this point, the plants create their own food using the light emitted by your grow lights. 

I generally don’t add fertilizer when seedlings near the time they are going to be moved outside because I’m not entering them into a beauty pageant or state fair where they have to look tremendous or perfect. I know once they are planted outside in true soil, natural sunlight and good rainwater, they will be fine — and they’ll take off. They will make up for any lost time when they didn’t get any supplemental nutrients. 

If you still feel the need to fertilize, use an organic fertilizer solution that has a low NPK ratio and dilute it to a quarter or half of what the bottle recommends. (If it says 1 part solution to 10 parts water, add at least 20 parts water). The worst thing you can do is overdo it on fertilizer when plants are still indoors. Stimulating growth in that artificial environment will lead to plants that don’t have sufficient space for root growth and top growth, and your lights probably won’t meet the demands of the larger plant. 


Seedlings under lughts

Small seedlings have everything they require inside their seed coat. You can wait to fertilize until they are transplanted outside and growth takes off.


Why Cool Season Crops Are Started in the Summertime

Cynthia Scott writes from Florida, where for much of the year it’s hotter than most U.S. gardeners are accustomed to. She asks about starting cool season crops and why the seeds are started in the middle of the summer. 

The reason cool season crops are started when temperatures are still high is because there needs to be sufficient time for the crops to mature before the days get too short and everything stops growing as much. And for those living in cooler climates, the first frost date of fall is another concern. When it is too cold, the growth of cool season crops will be hindered, and they will eventually die in a freeze.

The only way to ensure plants mature in time to beat shorter days and the first freeze is to start them early. Cool season crops have no issue, for the most part, getting started when temperatures are warm or even hot. What’s important is that they mature in cool weather, so they don’t bolt and become bitter. 

Cool season crops can germinate in soil that is up to around 85°F, depending on the variety. Even in an un-air-conditioned home, soilless mix that is kept consistently moist should be under this top-end of the range.

When it comes time to plant the crops outside but it is still blazing hot, you need to provide some relief. You can do this with shade cloth. Suspend the shade cloth up high to leave lots of airflow between the plants and the cloth, so no heat is trapped. This could knock the temperature down by up to 10°, depending on what percentage of shade cloth you use. It will take some trial and error to figure out what is sufficient. 

Keep the soil mulched so it stays cooler, and keep drip irrigation or soaker hoses under the mulch to maintain soil moisture.

For those in Florida and warm climates, I recommend starting cool season crops indoors rather than direct sowing. Your house is hopefully cooler in summer than the temperature outside. 


Joe Lamp'l with seedlings under a shade cloth

Keeping cool season crops under a shade cloth can make the temperature more tolerable for them.


If you haven’t listened to my answers to listeners’ seed starting questions, 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 questions do you still have? Let us know in the comments below.

Links & Resources

Some product links in this guide are affiliate links. See full disclosure below. 

Episode 037: Starting Seeds Indoors: The Non-Negotiables for Success, Pt 1

Episode 038: How to Start Seeds Indoors: The Non-Negotiables for Success, Pt. 2

Episode 039: How to Start Seeds Indoors: Digging Deeper, Pt. 3

Episode 083: Gardening Indoors: The Science of Light, with Leslie Halleck

Episode 125: Saving Seeds: The Basics, the Benefits and Beyond

Episode 238: Peat Moss: Examining the Challenges of Its Ongoing Use in the Face of Climate Change

Episode 256: The Challenge (and Solution) for Using Coconut Coir as a Seed Starting and Growing Medium

Episode 259: Getting to Know PittMoss, a Peat-free Growing Medium Alternative, with Dr. Charles Bethke  

Episode 297: Seed Starting Essentials

Episode 348: The Top Five Seed Starting Mistakes and How to Fix Them

joegardener blog: The Best Soil Temperature for Seed Germination

joegardener Seed Inventory Chart & Seed Longevity Chart

joegardenerTV YouTube: How to Know if Seeds Are Still Good 

joegardenerTV YouTube: How I Start Seeds Indoors Tips & Techniques

joegardenerTV YouTube: How to Save Tomato Seeds

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. Open for enrollment through Monday, January 29. 

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.

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Growing a Greener World®  


joegardener Amazon shop

First and last frost date finder

Silica packs

Pro-Mix BX Mycorrhizae Growing Mix 3.8 sq. ft.

Fish emulsion

Plant Label Tags

Garden Markers

Seed Dispenser

Propagation & Humidity Vented Domes

Large Waterproof Seeding Heat Mat

PittMoss Plentiful

PittMoss Performance

Greenhouse Megastore – Our podcast episode sponsor and Brand Partner of – Enter code JG10 for 10% off your order

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Territorial Seed Company – Our podcast episode sponsor and Brand Partner of 

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 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.

About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

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