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10-Step Indoor Seed Starting Guide

Do you want to successfully start your own seeds indoors?

In this guide, I will cover how I successfully start over 2000 individual plants from seed for my small nursery. Even if you are only starting just a few plants, this guide will help you select, plan, and grow your seedlings to be ready for planting outdoors.

Photo of two seed trays with seeds, labels, and a marker
Photo of two seed trays with seeds, labels, and a marker

You’ll Need Some Supplies

I have tried to keep this guide simple, yet effective. There are many guides out there that talk about putting your pots in front of a south-facing window, but this has not worked well for me personally. Rather, a small investment in the supplies I’ve recommended below will yield much greater success and healthier plants overall. Plus, you can reuse these supplies year-after-year.

Know Your (Approximate) Last Frost Date

Gardening in plant hardiness zone 4 here in Sudbury, Ontario means that we usually get our last frost in early June and our first frost in mid-September.

Depending where your garden is located, you will have differing dates and differing microclimates.

In 2021, we had an unusually warm spring and our last frost came in mid-May. I typically follow the “plant after the first full moon in June” advice, but it’s always helpful to watch the weather. There is no rush at all to get your plants in the ground outside. It’s better to err on the side of caution than to have to scramble to cover your plants.

Working backwards from your last frost date allows you to identify how many weeks ahead of time you will need to start seeds indoors. I’ve made the mistake in the past of growing pumpkins and tomatoes a little too early indoors and then they become WAY too big for the inside my house. I’ve learned from this and am sharing my best advice here 😉


What You Will Need for Seed-Starting

Here are my recommended supplies for successful seed-starting indoors:

  • A table, bookshelf, rack, or other location in your home where you can set out your seed trays – preferably away from cold drafts and pets and perhaps not on a carpet floor (unless you are very careful when watering!)
  • Seed packages of what you would like to grow
  • Plant labels – I like using plastic labels with China Markers which can be erased and reused
  • Pots or plug trays – I like Jiffy pellet trays for slow-growing plants like oregano and pots or plug trays for larger, faster-growing plants like pumpkins.
  • Potting soil if using pots or plug trays – I like using ProMix BX because it holds moisture longer than other potting soils I have tried. You will also need something to put underneath to ensure you don’t get water everywhere as these trays have holes in the bottom. You can use a simple boot tray (so long as it has sides to hold the water in) or you can get plastic plant trays that are a standard size to fit your pots in.
  • N95 Mask – to protect from dust and fungal spores when working with potting soil.
  • Watering can – I like a 1 gallon size with a long narrow spout rather than ones that have a sprinkler spout with the many holes. This allows me to direct the water to the sides of the soil around the plant to have more control and avoid damaging the seedlings
  • LED or fluorescent lights – these don’t need to be “grow lights” per-se. I have had great luck with regular shop lights so long as they are 4500k lumens or higher. Seedlings need a LOT of light! If you have a sunny south-facing window you *may* have luck with seedlings but if you notice they start to get leggy (grow tall and thin) this is a sign they need more light. Typically grow lights will be positioned about 8-12″ above seedlings to ensure they grow strong stems during this crucial time in their life cycle. If you are growing one tray of seedlings in your home and want to try a light, there is a kit you can buy that comes with a light on a stand as well as a tray of Jiffy pellets. The tray contains 72 pellets to start your plants.
  • A small fan for air movement – I like these 3-speed small desk fans. This helps minimize fungus from growing in your trays and simulates the air movement they would experience in the outdoors. With some very gentle wind, their stems will become stronger and they’ll be more adapted this when they are planted outside.
  • Bonus: A smart plug for your lights so you can set a daily on/off schedule – I like using the Globe brand smart plugs. You install an app on your phone, set the schedule, and your lights will automatically turn on in the morning and off in the evening. Seedlings need a minimum of 10 hours of bright light per day for the best growth. I like to set mine for 12 hours (on at 7am, off at 7pm). My smart plugs control the lights so I don’t have to worry about this!
  • Organic or natural fertilizer – such as worm castings or cattle manure if you will be transplanting your seedlings and/or for when you are ready to plant outdoors.

Planning to Start Your Seeds

Make a plan of which date(s) you will start your seeds.

  • If you are only growing one tray of seeds, then plant them all at the same time (4-6 weeks prior to the last frost). This keeps it very simple and easy.
  • If you have a lot of plants you are starting from seed, I recommend staggering the planting dates. This would ensure that slow growers have more time to grow (think herbs, strawberries, perennials) and fast growers don’t take over your table (think zucchini, pumpkins, beans).

Seed Starting Spreadsheet

To help with your planting, I have created a spreadsheet with suggested seed starting dates.

Screenshot of Seed Starting Spreadsheet. Click the buttons below to view or download.
Screenshot of Seed Starting Spreadsheet. Click the buttons below to view or download.

I have inputted a formula that uses the estimated date of our last frost in zone 4 (June 10) to provide the number of weeks ahead of time to expect to plant each group of plants. If you are located in another growing zone you can input your own last frost date in the B1 cell instead. Remember this date is just an estimate and doesn’t need to be exact.

You may also wish to add additional columns to this spreadsheet for specific varieties, growing notes, etc. similar to my garden planner. You can also add, remove or hide any rows that don’t apply to you.

Click on the buttons below to copy/download either a Google Sheets or Microsoft Excel file:


10 Steps to Start Your Seeds

Once you’ve determined when you are starting your seeds, you can follow these steps below for planting:

  1. Gather the seeds you are planting – lay out the packages near you. Grab a bowl, scissors, some clear tape, and a pencil. Carefully cut the very top of your seed package and pour a few seeds into the bowl. Keep the pencil handy for poking holes in your soil. You’ll use the tape later on to tape your packages back up.
  2. If you are using Jiffy pellet trays, take the lid off and put it underneath the tray to use as extra support. Add warm water to the trays to hydrate the pellets. I usually fill these with water about halfway up the sides of the tray. This gives the pellets enough water to expand fully. Don’t re-cover the tray with the lid as this can invite fungus to grow in the trays. The pellets will take about 7-10 minutes to fully expand.
  3. If you are using pots or plug trays, place another tray underneath (one without any holes), put on your N95 mask, and fill your pots with potting soil. Don’t overfill your pots. Leave about a 1/2 inch space at the top of the pot. This will keep water from spilling out when you water them. Slowly wet the soil with warm water until the water starts seeping through the bottom holes. Stop at this point as your soil will now be saturated.
  4. Take a look at your seed packages to read how deep to plant your seeds. The general rule-of-thumb for planting seeds is to plant them at a depth that is 2x the diameter of the seed. For example, an oregano seed, which is very tiny, will be planted just under the surface, whereas a pumpkin seed, which is quite large, would be planted about 1-2″ deep. Use your pencil to poke small holes to the depth required. I recommend planting a few (2-4) seeds per Jiffy pellet or pot in case some don’t germinate. You can always trim or separate seedlings later on. I’ll put about 3-5 oregano seeds in one Jiffy pellet versus 2 pumpkin seeds in a pot. In your pots, plant seeds at opposite ends around the perimeter of the pot so you can more easily separate seedlings if all of them grow.
  5. Gently cover the seeds with the soil. Some perennial and annual flower seeds need light to germinate, so just be aware of this when you are planting. Check the seed package for details. You will not need to water the trays now since they are already moist. This allows the seeds to begin germinating where you placed them.
  6. Place your trays under your lights and keep moist. Check on them each day to ensure that the soil is not drying out. They will need to stay moist but not wet. I like Jiffy trays because I can use my narrow spout on my watering can to wet the spaces around the pellets which does not disturb the delicate seedlings. When watering your pots or plug trays, water from the bottom only for the same reason. The potting soil will soak up the water from the holes at the bottom of the pots not disturbing the roots or plants.
  7. Set up your small fan to gently blow air around the trays. If you start to notice a white or grey fungus growing in your trays or pots, this is typically harmless. You can use a butter knife to gently scrape this off the top of the soil if it becomes too much or just leave it alone. When your plants germinate, ensure that the seedlings receive just the smallest amount of movement (i.e.: not a strong wind) as this will encourage them to grow stronger and healthier stems. Too much wind could cause them to grow sideways away from the fan.
  8. Observe your seedlings as they begin to grow. You will notice that they kind of all look the same in the beginning. They will sprout from their seeds and start growing upward with these little rounded leaves that look nothing like the mature plant. This first set of rounded leaves are called “seed leaves” or cotyledons. These little leaves provide the plant with a source of food in its earliest days. As the plant continues to grow you will now start to notice the next set of leaves called the “true leaves” which will begin to resemble the actual leaves on the plant. Once your seedlings have reached this stage and are beginning to sprout additional leaves, your plants are ready for transplanting. You would move them into larger pots if you are continuing to grow them indoors (step 9) or get them ready to plant outside (step 10) if you are past the last frost date.
  9. If transplanting, use a slightly larger pot. By the first and second set of true leaves, your plants will begin to seek nutrients in the soil. If your potting soil does not have fertilizer in it already, I like to add a little bit of organic fertilizer (like worm castings or cattle manure) in the bottom of the pot and then fill the rest with potting soil. I use about 1/3 fertilizer, 2/3 potting soil. Adding this to the bottom encourages deeper roots as they will grow towards the nutrients. If you used Jiffy pellets, remove the outer netting from the pellet before transplanting. This gives roots a chance to spread out without a barrier. If you are moving plants from one pot to another, you can just make a hole in the centre of the larger pot and place the seedling with roots in the hole. At this time, take a look at your seedlings if there are a bunch growing together. Trim or separate them so they don’t overcrowd the pots. This will ensure your plants grow healthier and fuller. Always water the pots right away once you are done transplanting until you start to see a small amount coming out the bottom holes.
  10. When your seedlings are ready to go outside, you will need to first “harden them off” so they can get used to the sun and wind. When growing indoors, seeds are in an artificial environment. When they move outdoors, the harsh sunlight can scorch their leaves and the wind may be too strong for them at first. Hardening them off over a period of 5-7 days during the day when temperatures are 10 degrees or warmer helps them to get ready for the real world:
    • On day 1, place your plants in a sheltered area in the shade for a few hours, then bring them back inside.
    • On days 2-4 place them in the sheltered area again for up to 8 hours, then bring them back inside.
    • On days 5-6, move them to an area with part sun, part shade for a few hours, then bring them back into the sheltered area for several more hours, then back inside for the night.
    • Finally, on day 7, leave them in the sun for 8 hours, then in the sheltered area until evening, then back inside for the night. After day 7, your plants should be ready for planting in the soil. I will cover this in another blog post but you would typically wait for a day or two of cloud cover first to give them time to acclimate to their new home before they receive strong sunlight.
Photo of two trays of brassica and perennial seedlings
Photo of two trays of brassica and perennial seedlings

Watch a Video of Seed Starting

In my video below I cover the key steps to indoor seed starting using the supplies I’ve recommended in this post (1:19 mins).


Summary

This article covers everything I do to start seeds indoors before planting outside.

You can always try your own methods or pick and choose ideas from here. You may not want or be ready to invest in some of these supplies and that’s ok. You can always try putting your trays in a windowsill and see how it goes. It may be all you need. For my nursery and my own garden, I am sharing this advice because of the success I have experienced with this method for several years.

In the photos above, you will see one of my grow racks. This is a 5-shelf metal rack that I have set up inside my house. Each shelf has two lights suspended from chandelier chain, a fan zip-tied to the corner, and the rack has one smart power bar that I can use to independently control each plug/shelf of lights. Here is a video that I took showing how this is set up. Since I start a lot of seeds indoors for my nursery I have two of these racks in my house plus additional lights in my garage. I use these lights to start the seeds until it is warm enough to place the seedlings outside in the greenhouse. This helps me save on heating costs as the greenhouse is not insulated while running LED lights costs less than an electric heater.

You can follow me on Facebook or Instagram if you want to see how I have this all set up.

If you already have an established routine for seed starting, wonderful! Please share in the comments below how you do this in your own home. I would love to learn from you.

Enjoy your gardening adventures!


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