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Garden planning: starting seeds indoors

Our favourite plant and soil expert, Aaron Deacon of BIOS Nutrients, is back on the pod. This time, he’s answering questions from our fabulous gardening community about starting seeds indoors for this year’s season in the garden.

Aaron sat down with Katie to go through a stack of FAQs about starting seeds indoors — from when to get started, to what kind of potting mix to use, grow lights, how to prevent damping-off and so much more.

It’s a jam-packed episode that’s sure to help you with your garden planning this year, so press play to get all of Aaron’s (as ever) calm advice. It’s actually pretty amazing how much he and Katie cram into one 12 minute episode, if we do say so ourselves.

Show Notes

Aaron and Katie cover a lot of ground in this episode — so we’ve broken down all the questions they tackle in our show notes below.

As always, if you have a question that we haven’t covered, feel free to share it in the comments for the episode and we’ll put it on our list for next time!

When should we start seeds indoors?

Rule #1 of seed starting: refer to your seed packet! Also, know your gardening zone and the average last last frost date for your area. With that information, plus knowing how much time is needed for germination and growth (also on the seed packet), you can calculate when your seedlings can safely go outdoors.

As Aaron says, you’ll also want to consider how big you want your seedlings to be before you put them outdoors, bearing in mind that larger seedlings are generally more resistant to frost damage (because even with the best calculation, frost still happens).

One useful rule of thumb — in general, plan to start seeds 6-8 weeks before the last frost date for your area. Check out plantmaps.com for many growing-relevant dates, including average last frost date for zones around the world.

Pro tip from Aaron: Buy your seeds as soon as you can. In the last few years, seeds lately have been selling out very quickly as the popularity of gardening has been on the rise.

What’s the best kind of seed starting mix for starting vegetable seeds?

Start vegetable seeds in a soilless mix. Yes, counterintuitively, the right “soil” for starting vegetable seeds indoors has no soil in it, unlike “regular” potting soil, which may contain field soil (go here for more on the difference between potting soil and seed starting mix).

The key components of a soilless mix are, in Aaron’s words, “straight organic matter”, which is 1) low in nutrients 2) provides high water retention and 3) allows good aeration. Generally, seed starting mixes are a combination of peat moss, perlite, vermiculite and a small amount of fertilizer.

Commercial mixes designed for seed starting are convenient, because their components are sterilized and will reduce the risk of damping-off (more on that later in the episode). The ideal pH for a seed starting mix is between 6.00 and 7.00. Some growers maintain that 6.5 is optimal.

Can I make my own seed starting mix?

Yes! 100%. You can make your own seed starting mix with perlite, vermiculite and sphagnum peat moss (if you’re concerned about using peat moss, coco coir is an excellent substitute).

Aaron uses a 50:50 mixture of coco coir and perlite, and likes to bring down the pH of coco coir (he adds a little sulphur or water with a vinegar/water or lemon juice/water mix) to get his mix into the ideal 6.0 – 7.0 pH range.

Do vegetable seedlings need to be fertilized indoors before they go outdoors?

Yes. About three weeks after seeds sprout, lightly fertilize via a soluble fertilizer. You need to provide nutrition, or seedlings may start to develop deficiencies.

How do you prevent damping-off?

Aaron’s answer is simple: reduce your humidity and allow for some air flow. Make sure your seedlings have sufficient drainage, so the medium in which they’re growing is not continually wet and your plants are not suffocating.

Container size is important. Small containers help you regulate your watering. Also make sure there’s sufficient airflow for seedlings once they sprout. If you simply keep it in a very humid container, you’re likely to get damping-off and fungal disease.

What is damping-off? We thought you’d never ask. Here’s a fulsome explanation, but in essence, damping-off is a fungus or mold condition that affects plants — vulnerable seedlings in particular.

How wet should soil for seedlings be?

A good test is water-holding capacity. Aaron’s test: wet your soil, squeeze it. If just a drop comes out of it, that’s a good amount of moisture for a seedling. Just make sure it stays moist, not wet. But also that it never dries out. We know. It’s a fine balance.

You can also soak your seeds first. Aaron’s pro tip: If you’re new to seed starting or are a bit nervous about the process, you can soak your seeds in paper towel first until they germinate and then plant them into some soil.

Should you add a fan to your seed starting setup?

Yes. You should have air circulation as early as possible because you want to mimic wind with seedlings. This will help them to grow deeper, stronger roots — in essence, adding ‘wind’ will begin the hardening-off process for when it’s time to transplant indoor seedlings to the great outdoors. Air movement will also help with the prevention of fungal diseases.

Should you use a heat mat when starting seeds?

Seeds need warmth to germinate. So, either from a south-facing window, grow lights or a heat mat, give your seeds at least 24˚C (75˚F) to help them germinate quickly and properly. Once they germinate, you can turn down the heat, because you want your soil to start to lower in temperature. (Soil that’s too consistently warm is a welcome environment to disease.)

How should grow lights be used for seed starting/seedlings?

Grow lights are vital for seed starting and growing seedlings. Aaron’s recommendation is to use grow lights for at least 14 hours/day, and ideally 16 to 18 hours/day.

According to Aaron, a ratio of 18 hours on to 6 hours off is perfect for growing seedlings. As Katie suggests, you may want to invest in a timer — here’s a guide to help you choose one!

What are true leaves and how do you recognize them?

True leaves will look different from plant to plant. That being said, the first leaves of a seedling (called cotyledons) are generally recognizable for their rounded shape. The second set of leaves your seedling produces are, as a rule, their first ‘true’ leaves.

Two final pro tips from Aaron for starting seeds

Make sure you watch to the end of the episode to get two last tidbits of expert advice from Aaron:

  • give your seeds a hydrogen peroxide, yucca extract, or compost tea soak to help prevent fungal disease before you even start
  • try seed scarification to help encourage germination

The timing for starting seeds is different for everyone, depending on where and what you’re growing. If you have a question that we haven’t covered of if you’d like us to go a little more in-depth on one we have, let us know in the comments for the episode. We love to hear feedback from our fellow gardeners.

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