Starting Seeds Indoors in 6 Easy Steps (+ Tips for Success)

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Regardless of which zone you are in, starting seeds indoors can allow for the best maximization of time to grow a healthy flower or vegetable garden. Read on to learn when and how to start seeds for tomato, cucumber, pepper, and squash plants, and read this post on the advantages of planting seedlings indoors.

Dozens of seedling pots with plant starts growing.

Direct Sow vs. Starting Seeds Indoors

Direct sowing seeds simply means planting the seed directly in your garden as opposed to starting the seeds indoors and transplanting those seedlings out into the garden at a later date.

Many people choose to start seeds indoors for warmer weather crops such as tomatoes, peppers, cucumbers, and melons. These crops have a longer growing season and many areas don't have long enough growing seasons to get these plants to full growth.

Starting these seeds indoors instead of directly sowing them in the garden can help extend your growing season by weeks and sometimes even months.

What Seeds Should Not Be Started Indoors

There are definitely some plants that don't lend themselves to being started indoors. For instance, all root vegetables such as potatoes, sweet potatoes, carrots, beets, rutabagas, parsnips, turnips, and garlic should be direct sown.

Vegetables such as leaf lettuce, spinach, arugula, kale, kohlrabi, Swiss chard, green beans, and radishes are fast-growing and have no need to start from seed indoors as well.

Additionally, vegetables such as cucumber, squash (summer and winter), melons, corn, peas, and okra do not like their roots disturbed, and do best when direct sown.

If your growing season (last frost date of spring to the first frost date of fall) is less than 100 days, you could try to start these from seed where you otherwise wouldn't be able to grow them. But again, there's a reason these vegetables are recommended to be direct sown, so proceed with caution (and at your own risk).

For some, the risk is better than not being able to grow these crops at all.

Tiny seed starting pots with seedlings growing.

What Seeds Should Be Started Indoors

Tender vegetables such as tomatoes, peppers, broccoli, cauliflower, Brussel sprouts, cabbage, celery, eggplant, and head lettuce all do best started indoors regardless of how long the growing season is.

Starting these crops indoors allows for more control over these delicate crops to ensure that they flourish.

When to Start Seeds Indoors

Avoid being tempted to start them too early!

Begin by referring to your seed packet. There is usually a chart on the back of the packet that indicates germination time. Determine the number of weeks for germination and seedling growth.

Next, do an online search for your area to get an estimated date for last frost. Count backwards from that date to figure the date you need to start your seeds indoors.

I always want to be sure there's no chance of losing my baby seedlings to a late frost. So, I will usually wait to plant until the average last frost date is well past. When I’m getting close to our last frost date, I will keep a close eye on the 10-day forecast and plan my planting date from there.

To learn more about your growing season and how to find your growing zone, check out my post on garden planning basics.

Tiny seed starting pots with seedlings growing.

Materials and Supplies Needed

The materials needed for starting seeds indoors are minimal. However, you will find you can spend little or break the bank. I purchased some of my seed trays and other supplies from Bootstrap Farmer and really like the products they have.

  • Seeds – If you are new to vegetable gardening, my seed shopping method can help take the overwhelm out of getting started. This post on where to buy heirloom seeds shares my favorite resources to find seeds. I have also put together my MUST grow garden list to give inspiration for your own garden. For those who are more experienced gardeners, check out this complete guide to seed saving. Also, the best methods for successful pollination when saving seeds to become more self-sufficient when it comes time to source more seeds.
  • Seed Starting Containers – Think outside the box for this one if you want to save money. You can go to your local big-box store and find lots of growing products on the shelf. There are great options out there, but I’ve found with a little imagination, I can find other options that are cheaper. Pro Tip: Do not forget to add drainage holes to repurposed items.
    • Colored Plastic Cups: I like the 18-ounce size Solo cups. These are big enough to hold the plant until planting time without the need to pot up. The smaller sizes might require transplanting as the plants grow. I do not recommend using foam cups. They may initially be cheaper, but they will not last from year to year whereas plastic cups can be reused. Pro Tip: If you are at a gathering where plastic cups are served, ask the host if you can ask the guests to set their used cups aside as opposed to throwing them away. The cups can be washed, and now you have seed starting containers for free!
    • Clear Plastic Cups: Clear cups allow light in exposing the root systems. Although I've never had trouble with this, it could be problematic for your plants.
    • Paper Cups: These can work if you have them, but note that they might break down with moisture before you have the chance to transplant them. If you have to transplant to new cups, this will be harder on the seedlings, and cost-prohibitive.
    • Egg Cartons and Toilet Paper Rolls: These work well for initial germination, but just like the smaller cups and paper cups, will need to be potted up before they are translated into the ground.
  • Potting Soil – I like organic soil, but the main thing to look for is loose, potting mix. Stay away from buying bagged garden soil as it tends to be too dense.
  • Heat Source – Most things you plant do not need light to germinate, but they do need heat. Plants such as lettuce and perennial flowers need light, but most vegetables only need heat. Warmth to germinate and light to grow! 70°F to 80°F is the optimal temp for germination. If you don’t have a heated greenhouse you can start the plants inside your home near a south-facing window. Once the plant has pushed through the soil and has its first set of leaves, you can move them, but make sure the temp does not drop below 45°F in the evening. In some growing zones, you might want to invest in fluorescent lights (or grow lights) and heating mats.
Tiny seed starting pots with seedlings growing.

How to Start Seeds Indoors

  1. Begin by labeling the container with what you are planting. If you are using plastic cups, you can write directly on the cup if you wish.
  2. Heap the container with soil. Loosely press the soil, and be careful not to pack it too tightly. The goal here is to eliminate the air so that it’s not too loose, but firm enough to hold the seed in place so that watering will not dislodge it.
  3. Water the soil lightly before planting the seed, I actually like to dampen a large amount of soil in a separate container so it's a faster process.
  4. Press a small indentation in the soil with your fingertip to plant the seed. The depth you need to plant the seed is 2 to 3 times as deep as the seed is wide. For tiny seeds, only sprinkle them across the surface of the dirt and mess up the soil a little to cover them. For larger seeds, place them in the hole you made with your finger and brush soil across the top to cover. 
  5. Water daily to keep the soil moist. A good way to visualize the moisture level you want the soil to be at is to think of a sponge. When a sponge is taken out of a bucket of water and squeezed, it’s damp, but not drippy. That’s the moisture level you need for your soil. Too dry, and the plant dies. Too wet, and the seed rots or the plant drowns. Pro Tip: A good way to keep the soil damp but not soggy is to use a spray bottle.
  6. When they are mature and the danger of frost has passed, transplant seedlings into your garden space.
Hands separating seedlings to pot up.

FAQ's

Should I Thin My Seedlings After Germination?

For most plants, I will have up to 10 planted seeds in one larger container for the germination step. Transplanting these plants into their own container once they have produced their first set of sturdy leaves optimizes their full growth potential.

If you don’t want to transplant all of them, pinch them off at the soil line and discard. However, if you wish to transplant, dig down around the root system with your fingers. Gently work the plant loose, and place it in a separate container. Don’t forget to label the new transplant and water as before!

What Does “Hardening Off” Mean?

Hardening off your plants is a simple process of moving them outside for a few hours every day. You're essentially letting them adjust to conditions such as sunshine, wind, and temps a little at a time.

Increase the time they're outside each day by a couple of hours until they're ready to stay outside full-time.

I recommend to always give your plants a chance to acclimate to the outdoors before planting in the garden. This helps reduce the risk of shock to the plant and helps mitigate stalling the plant's growth or killing the plant altogether.

Should I Soak Seeds Before Planting Indoors?

Soaking seeds isn't generally necessary, however soaking larger seeds with thick coatings for 12-24 hours before planting can help give them a head start on the germination process.

Do not soak small seeds! They will gel together making it somewhat impossible to separate out for planting.

I hope this post helps you realize starting seeds indoors is really a very simple process and nothing to be intimidated by.

Here's to extending our growing seasons and getting our hands in soil a little before the outside temperatures are ready for us!

A woman with a handful of carrot seeds.

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