Starting Seeds Indoors

Starting seeds indoors under fluorescent lightsStarting Seeds Indoors in Deep Root Trays

Starting Seeds Indoors: 
Timing Is Everything

I love starting seeds indoors in the early spring, when the gardening bug has hit me but it's still to cold out to get my hands in the dirt. But I have learned to restrain myself a little, and not start things so early that I end up with spindly little rootbound plants, that don't even get as big as the volunteers that come up on their own after the soil has warmed.

But take heart! I will show you how to get the timing right so that your plants are at just the right stage at exactly the right time. And if you live where it never freezes, just skip this section.

The first thing you have to know when starting seeds indoors is your average last frost date, the date beyond which the national weather service figures your area will be safe from further spring frosts. If you're in the US your State University Extension Service website will have that information.

Next, download and print the free Starting Seeds Indoors Planning Chart to figure out when to start different crops indoors. You will use this in combination with the information on your seed packets to come up with the exact date to start your seeds. And if you don't have your seeds yet, you can check out the some of the best seed catalogs here.

Here are some of the advantages (besides not going winter stir-crazy!) of starting seeds indoors:

  • it shortens your wait first that first juicy tomato
  • it is usually cheaper that buying transplants
  • it gives your crops a head start on weeds, and keeps you from inadvertently weeding out your freshly sprouted seeds
  • you won't have to go back and thin
  • If you have a really short growing season, you'll be able to grow things that wouldn't normally mature in your short season

After you have filled out your chart, you're ready to start! (That is, if it shows that your time is right.)

Starting Seeds Indoors:
Pots and Potting Mix

There are many different seed-starting setups on the market. As with tools (and for me, knives and stockpots!), I look for quality and durability.  I've learned over the years that paying for durability ends up being much cheaper in the long run - though I haven't always been able to afford quality up front.

When I was really short on cash I used to get free 2" and 4” square plastic pots in flats from the trash bin behind my local nursery (I always asked first). If you can source used ones, wash them well in hot soapy water or sanitize them in a mild bleach solution (1 part bleach, 10 parts water) before using to prevent plant diseases.

Potting Mix
(For use in trays,
NOT soil blocks)

A seed has everything it needs inside to grow until it gets its first true leaves. Pretty much any reasonably balanced storebought potting mix will work fine, or use:

1 part presoaked coir
2 parts fine compost
1 part vermiculite or perlite
1/3 part worm castings (if will be growing
awhile in the pot)

You'll also need the 10"x 20" flat trays that the pots go in. Another free option is to use discarded styrofoam coffee cups with holes poked in the bottom (but please don't go buy new ones for this - they're terrible for the environment.)

But my faves now, the best  trays I have ever used for starting seeds indoors, are the Deep Root Seed Starting Trays from Gardener's Supply. They are built to last forever (mine are 9 years old in 2020) and you can get them with an optional self-watering mat and grow dome. If you're apt to forget to water the mat is awesome, but if you're mindful you won't really need either the mat or the dome.

I use a variety of sizes, because some plants, like onions (from seed), do well in the 40-cells-per-tray size, while some plants need more room, like collards and chard, which I start in the 28's and later transplant up into the 15's as they get bigger. The trays do need some kind of drip tray underneath to catch drips. I use either boot trays from Home Depot, which hold 3 deep root trays side-by-side, or heavy-duty 10"x20" trays from the local pot-growing-supply store.

COVID-19 NOTE: Since the start of the pandemic, Gardener's Supply has only carried the 15-cell size tray. It's still unknown whether the smaller-celled trays will return...

Since the space on your shelves is usually at a premium, it pays to optimize the number of plants you can fit there by using the smallest cell size possible, at least until it's time to transplant up if they're getting rootbound.

do NOT recommend using any of the cheap, flimsy plastic systems for starting seeds indoors. Besides the added expense of replacing them when they break, (which they always do - planned obsolescence never goes out of style when you're selling something), they aren't recyclable. And we're gardeners of the earth after all.

I also don't recommend "compostable" or "plantable" peat or cardboard pots, but for a different reason. Even though they are more eco-friendly, they are very slow to break down in the soil, and in the meantime will impede the seedling's roots from freely spreading out in all directions, which decreases overall plant size and eventual yield.

In 2011 I did a side-by-side comparison and my yields of peat-pot grown peppers were 30-40% less than that of the free-root transplants grown in the deep root seed starting trays.

Starting seeds indoors using soil blocksSeeds Started Indoors Using Soil Blocks

And now... (drumroll please)... I've saved the best for last:

The Soil Blocker

The absolute best system for starting seeds indoors is the soil blocker. What the heck is a "soil blocker"? It probably should have been called a soil block maker, but it is a very cool metal form that has four, five, or even twenty cube-shaped cells.

You press the contraption down into a special potting mix. Once you've packed the cells with the sticky potting mix, you lift it over to an empty tray and squeeze the plunger, which ejects perfect little 1 1/2" or 2" cubes.

Soil Blocker Potting Mix:

1 part garden soil
2 parts
very fine coconut coir*
2 parts perlite
3 parts compost

Enough water to make it sticky, like brownie mix (but no licking the spoon!)

Each cube has a small depression in the top for planting a seed, which you then cover with a bit more mix. Because of the specific potting mix used, the cubes stand up without containers of any kind, and don't crumble apart when you water them!

The most obvious advantage to soil blocks is their sustainability. But they are also much better for seedling root growth. When the growing roots reach the edge of the block, they hit the air and just stop growing, instead of circling around.

This is known as "air pruning". If roots hit a hard physical limit like the side of a pot, they turn sideways and circle, and never quite recover. The plant knows it's reached a hard limit, and it gets that stuck in its little head, and in its growth pattern, forever.

With air pruning, as soon as you transplant it out the roots just start growing again, but out and down like they would have if the seed had just been planted in the ground in the first place.

Use the soil blocker potting mix recipe to make soil blocker mix. Peat moss was used in the original "Eliot Coleman" recipe (from his book The New Organic Grower), but peat is a non-renewable, unsustainable resource - at least in human timeframes - so for sustainability reasons I now use coconut coir (pron. "COY-er") instead. It's ground up coconut husks and is a byproduct of coconut processing.

If you live in a state where pot is legal, the pot-growing supply stores are an affordable source for coir, perlite, soil minerals and other "potting" supplies. I don't imbibe myself, but I do buy growing supplies there.

Now there is one disadvantage to soil blocks: it takes time to make them. It takes longer than stuffing store-bought potting mix into deep-root trays. (Note to self: remember to make soil blocks when you're bored next winter)

Starting Seeds Indoors:
Shelves and Lights

Light Requirements

Seedlings need very bright light to keep from growing spindly and weak. Contrary to popular belief, a windowsill is not a good place for starting seeds indoors because the seedlings will grow sideways and spindly trying to reach the light, even if you turn the tray around every day. Also, the cold air coming off a window at night will slow germination and growth. 

Start seedlings under bright fluorescent lights. Seedlings do not need full-spectrum light, only the red and blue frequencies which are provided by cool white fluorescents. Cheap shop lights work great! (Now if you wanted to get plants to flower, you'd need a different bulb with a broader spectrum of light.)


As a DIYer, I've used a variety of different plant shelves over the years - mostly home-built ones made out 2x4's for uprights and 1x2's for cross-slat shelves. But a few years ago I discovered that the ideal, adaptable, moveable replacement is the ubiquitous wire shelf unit sold by Costco and many sellers on Amazon. These are relatively low-cost and I now have a bank of them in my basement, where I grow lettuce and microgreens all winter, and all my own vegetable and flower seedlings in late winter and early spring. (And one shelf does double-duty as a drying rack for my wool socks.)

I attach S-hooks under each shelf and suspend the shop lights from little chains, which then shine on the seed flat on the shelf below. I can raise and lower the height of the lights by hooking the chain higher or lower on the S-hook. (There are no lights above the very top shelf, but there are no plants on it either -  it's too high to reach for watering. I store my extra heat mats and deep root trays up there.

If you want to purchase a top-notch, living-room-worthy, ready-made seed starting shelf unit with lights, I highly recommend the SunLite Garden by Gardener's Supply. It's beautiful, the lights are very easy to adjust, and they come in a bunch of different sizes and designs to fit every need and taste.

Give your seedlings 12-16 hours of light per day, but no more than this – they need a dark cycle, too. The lights should be adjusted so that they stay within 1-3” of the top of the seedlings. Raise the lights as the plants grow, and put the lights on an outlet timer so you don’t forget to turn them on or off at the right time.

Starting Seeds Indoors:

One of the biggest challenges I used to have starting seeds indoors was finding a place that was warm enough, because most seeds need temperatures between 65˚and 80˚ F to germinate.

If you have a furnace room or other warm place, set up your seed-starting shelves nearby. 

Since I don't have that, I use seedling heat mats, which keep my seeds and seedlings warm and greatly speed germination and growth, encouraging strong healthy plants. This is especially helpful for really slow-growing seedlings like peppers and parsley. Don't use a heating pad - even on low they get too hot for seedlings and are not built for wet environments. If you don't have a warm place or a heat mat, just start your seedlings a few weeks earlier.

Starting Seeds Indoors:
Planting the Seeds

The back of your seed packet will tell you how deep to plant each variety of seed that you have. A rule of thumb is to plant them about 3 times the diameter of the seed, but I have found it is generally better to plant seeds a bit shallower than recommended when starting them indoors..

Plant at least two seeds in each hole as insurance against poor germination. If both seeds sprout and grow, snip off the smaller one to let the other grow strong without competition. Snip instead of pulling it out, so you don't damage the root of the one you're keeping.

Beginning gardeners sometimes have a hard time killing these beautiful little seedlings, but your garden will be much stronger if you do this. I always say a silent "thank you" to the one I’m culling, and either eat it or add its little stem to the compost, out of respect. Nature is brilliant in this: it's why it oversupplies itself with masses of seeds per flower in the wild. To us it may appear cruel, but it is the cycle of abundant life and keeps the species adapted to conditions.

Starting Seeds Indoors:
Hardening Off and Transplanting

Our pampered little indoor seedlings need time to adjust to the great outdoors. They have to adapt to strong sunlight and wind, both of which stress plants and thereby make them strong.

About a week before you transplant your seedlings out, set the seed flats outside during the warm part of the day. in the shade at first, for a an hour or so. Every day for the next week increase their time outside and their exposure to sun. They will gradually adjust their “sunlight antennas” and strengthen their stems until they can take a full day of sun and wind without burning up or blowing over. This is known as "hardening off".

It is very easy to want to skip this step and think "but it's so nice outside!" I sometimes get impatient at this point, but one of the challenges of starting seeds indoors is being patient and giving them the time they need to adapt.

starting seeds indoors: hardening offHardening Off Seedlings That
Were Started Indoors

When they are hardened off, transplant them out at the recommended spacing on the seed packet. Sometimes this is listed as "thin to x inches apart". I always grow a few extra in case I need to replace any that don’t make it. Sometimes they get eaten by birds or slugs, or otherwise mysteriously disappear overnight.

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