March 2023 – Seasons In A School Garden

How to Successfully Start Seeds

For many years I avoided starting my own seeds. I told everyone it was because we didn’t have the space in our 1300 square foot house, but really it was because I lacked the confidence to do it right. I tried starting seeds over 20 years ago on the floor below a south facing window and within a matter of two days all the seedlings had shriveled at the base and fallen over like a tree that was chopped down. I now know that my seedlings suffered from damping off, a soil-borne fungal disease that quickly spreads through your seedlings. I could have prevented it by using the proper sterile soil (not soil that had been in a bag on my porch for a year) and making sure my equipment was sterile as well. Even with that knowledge in hand, it was enough to keep me from starting my own seeds for 15 more years. All that time, I worked alongside the experts at Denver Botanic Gardens and learned the basics of seed starting. While I wasn’t starting seeds myself, I was still teaching and inspiring others to do it. I transferred that knowledge to students in after-school garden clubs I ran where we started seeds and to teachers taking professional development classes who then successfully grew seeds in their classrooms. But I still I wasn’t ready to start my own.

Over 800 plants growing in my dining room during the Covid lockdown

It wasn’t until five years ago when I took over the garden coordinator role at a school and successfully grew over 1000 plants that first year. The fear stayed though – I would wake up in the middle of the night having dreamed that all of the seedlings had pitched over like a falling tree. The next year we started our seed starting efforts in early March, only to be interrupted two weeks later by a “three-week extended spring break” due to the Novel Coronavirus. I took a grow light shelf home along with five flats of seedlings expecting to bring them back three weeks later. Well, we all know what happened to the three-week extended spring break! The five flats ballooned into 20 and I ended up growing over 800 plants in my dining room in order to fulfill all of the orders for the school plant sale we had taken back in February. Leveraging on that success, I was able to carve out space in our small house to grow a total of five flats on top of the washing machine, dryer and adjacent windowsill three years ago. Needless to say my fears are gone! I started my fifth season of seed starting last week. I anticipate growing over 900 plants at one school, 150 at a second school and about 60 at home this spring. My excuse is gone, my confidence is solid and I’m here to share the really simple basics of seed starting with you.

Successful seed starting really comes down to making sure you have the right equipment. You can find a detailed why for each item and some suggested products here. Once you have that, it’s as simple as soil, seed, water and light. So spend some time sourcing a grow light, good seed starting soil, sterile pots and trays, humidity domes and a heat mat. And learn from my mistake 22 years ago, make sure everything is sterile. I reuse pots, trays, and humidity domes year after year, but only after they’ve been scrubbed clean, and they’ve sat in a bleach solution (1 part bleach: 9 parts water) for 10-15 minutes. You can put together a simple seed starting set up for two flats (~60 plants) for about $50. When you compare that cost to purchasing the plants from a nursery at $4-10 a piece, it really is more cost effective.

From the top down, here is how you’ll set up your system:

Grow light
Humidity dome
Pots
Tray
Heat mat

Once your equipment is set-up, you’re all set to plant, but it won’t be all at the same time. Different plants need different amounts of time to grow prior to transplant, so you’ll need to pay attention to the seed packet and follow their timing recommendations. You can calculate when to start your seeds by determining your average frost free date and then counting backwards the number of weeks from there. Here is a general breakdown of timing:

10-12 weeks before last frost: eggplant, celery, onion, rosemary, chives, snapdragons, petunia
8-10 weeks before last frost: peppers, mint, parsley, coleus, impatiens,
6-8 weeks before last frost: tomatoes, basil, calendula
4-6 weeks before last frost: kale, collard green, swiss chard, broccoli, cauliflower, basil, marigold, zinnia, cosmo, nasturtium

Also keep in mind that some plants grow best when the seed is directly planted into the garden (called direct sowing). Either they grow so quickly that they don’t need an advance start or they don’t like having their roots disturbed. These include plants like carrot, beet, turnip, radish, cucumber, melon, squash (summer and winter), zucchini, pumpkin, peas, beans, lettuce, spinach, and arugula. Don’t waste your time starting these ahead. Just wait until the soil is the right temperature and plant outside.

When it’s the appropriate time to plant your seed, follow these simple steps.

Fill the pot with soil. Use your hand to fill the pot and then two or three fingers to push the soil into the pot. The soil gets filled with lots of air space when moved into the pot and then will settle over time leaving just a small amount of soil. Pushing the soil down compresses the soil ahead of time and ensures you have a proper amount of soil. Don’t push so hard though that all of the air space is squeezed out. I like to tell students to push hard, but not superhero hard. Make sure you’re using a sterile soil that is designed for seed starting.

Plant the seeds. For small seeds, place one seed on top of the soil in the center of the pot. For large seeds, make a hole twice as deep as the seed and put the seed in the hole. Sprinkle soil on top of the seeds to cover them up. Small seeds need just ¼” of soil on top. Some seeds like petunia, coleus, snapdragons, and impatiens should not be covered up at all, but left on the surface of the soil.

Place pots in a tray. Place all of your pots in a tray without holes.

Water the tray. Using a watering can, fill your tray approximately halfway with water. The soil will absorb the water through the holes in the bottom of the pot. If you water directly on top of the soil you risk splashing the soil and seeds out of the pot. Plus it takes a lot less time to water one tray than 72 individual plants. Check on the tray after a few hours. If no water remains and the top of the soil is still dry or semi-dry, add more water. If the water is gone and the soil looks saturated at the top, leave as is.

Cover with a humidity dome. Seeds need a consistent moisture level in order to germinate. A humidity dome traps the evaporated water and then rains it back down on the soil. This keeps the seeds and soil a consistent moisture level. It also means you don’t need to water as frequently.

Heat the soil. Many seeds germinate best when the soil temperature is 70-90°. A heat mat placed under the tray will raise the temperature of the soil above the ambient temperature of the room resulting in better and quicker germination. Seeds that like it warm include: tomato, pepper, eggplant, parsley and zinnia.

Provide bright direct light. Place your seed trays underneath a grow light that is positioned 3-5” above the top of the soil. Set the grow light on a timer so that you have 14-16 hours of light per day.

Depending on the plant type, your seeds can sprout in as little as 3 days (zinnia and marigold) or it may take 3-4 weeks (eggplant, parsley, pepper, celery).

The seeds will sprout and have one or two leaves. These are called cotyledon leaves and served as the plant’s food storage for the initial growth. The next leaves and all subsequent leaves are called true leaves. The true leaves photosynthesize to make food for the plant. Once this sprouting has occurred, continue with the following:

Maintain moisture. Make sure that your soil remains moist (not saturated) until the seeds sprout. With the humidity dome on, you may not need to water at all. If there is no water collecting on the inside of the humidity dome, it is time to place 1/4”-1/2” of water in the bottom of the tray. When you water, the water should be fully absorbed within 1-2 hours. If it doesn’t all get absorbed, dump out the extra water. Once the seeds have sprouted it’s okay to let the surface of the soil dry slightly in between watering. Every few days, you will need to put 1/4”-1/2” of water in the bottom of the tray – enough for it to absorbed within a few hours.

Basil seedlings ready to be thinned to one per cell

Thin seedlings. Ideally you should have one plant growing per pot. If you ended up with more than one plant, you should remove the additional plants so each plant has enough space and access to water, nutrients and light. When thinning, try not to disturb the root system of the adjacent plant. The best way to accomplish this is to use a pair of small scissors (nail scissors work great) to cut the plant at the soil level. You can also gently pull out the seedling. Some plants like chives and alyssum are planted with multiple seeds per pot and do not need thinning.

Remove the humidity dome. Once the plant has two sets of true leaves, you can begin to wean the plants from the humidity dome. Prop up a corner of the dome and leave it propped open for a few days before you remove the humidity dome entirely.

Varying light heights on the different trays

Adjust the light height. As the plants begin to grow taller you will need to raise the height of the grow light. The light should be 3-5” above the tops of the plants at all times. If the light is too far away, the plants will grow quickly to reach the light and won’t have as strong of stems.

Transplant into larger pots. When the plant has 3-5 sets of true leaves, you can transplant them into a bigger pot. For many plants this act of transplanting disturbs the roots slightly and encourages the plant to grow more fine roots for absorbing water. This is why you start seeds in a small pot and move them to bigger pots. Some plants do not like to be transplanted though and should be started in a bigger pot.

As your seedlings continue to grow, monitor the moisture level and light height on a regular basis. As you get closer to your frost free date, you will need to transition the plants to the outside. This is called hardening off. Plants that go from the consistent warmth of inside to the variable nature of outside are in for a shock and need to slowly adapt to the new environment. Begin by putting your seedlings outside in the shade for a few hours then bringing them back inside. The next day put them in the sun for an hour and then move them into the shade, bringing them back inside after a few hours. Increase the length of time in full sun and the total time outside each day until they’re spending the full day in the sunshine. After 7-10 days you can leave the plants out overnight, as long as your nighttime temperatures are above 50°. If you have a cold day during this period of time, leave the plants inside.

Once your plants are hardened off, you’re past the danger of frost and nighttime temperatures are warm, its time to transplant your plants into the garden. Make a hole the same depth as the pot and twice as wide. Gently remove the plants from the pot, spread the roots apart at the base, place in the hole and backfill with the soil you removed.

While I know this seems like a lot of steps, it really isn’t that hard. Just make sure you start with clean equipment and provide adequate moisture and light. Soon you’ll be harvesting food for dinner and be filled with pride knowing that you grew this all by yourself!

Essential Seed Starting Equipment

This morning I stopped by one of the schools that I work at to take inventory of our seed starting equipment so I could purchase anything new that was needed. Thankfully we don’t need much other than soil this year!

Over the years I’ve homed in how to start seeds in a school. Even though we lack a greenhouse, we’ve made it work for many years now growing seeds in the atrium in between hallways. Last year we grew over 1100 plants at this school in a space about 12’ wide by 3’ deep! While most schools won’t be growing this many plants, there are some basics that I would consider essential for any seed starting set-up.

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Light source

Even the biggest, brightest south facing windows won’t provide enough light to produce healthy flowers, herbs and vegetables. You can certainly grow plants without supplemental light, but they will be super tall with lots of space between leaves and weak stems (these are called “leggy” plants) and likely won’t transplant well into the garden. There are many different options for light sources depending on how much you want to grow. The most important thing to consider is that the light source has a flexible height adjustment as you want your lights to be about 3-4” from the top of the plants at all times. There are so many different grow light options, here are a few to consider.

This is the setup one of my schools uses that allowed us to grow 1100 plants last year. We have several sets of 6’ tall commercial metal mesh shelves set up to have five shelves (we grow plants on four shelves and store supplies on the top shelf). Each shelf holds 4 flats, for a total of 16 flats per unit. Hanging from the underside of each shelf are two 4’ long shop lights, holding two fluorescent bulbs each. The chains on the lights attach to the shelf with an s-hook. This allows me to adjust the height of the lights. It’s cumbersome to change the height, but doable. We use just plain old fluorescent lights, rather than the more expensive grow lights. Grow lights emit light in the red and blue spectrums, while standard fluorescents contain a broader portion of the light spectrum. The exact portion of the spectrum depends on the warmth and color of the lights. I’ve used both grow lights and standard fluorescent and haven’t noticed a difference between the two, other than standard fluorescent are significantly cheaper. At some point I’d like to convert our fluorescent fixtures to LED fixtures, but for now it’s too cost prohibitive.

This clip-on LED light strip is the easiest and cheapest option and is what I use at home. Each clip on comes with four light strips. My plants are placed underneath a south window, so I am able to use just two light strips per flat and can therefore accommodate two flats per light unit. It has a built-in timer and you can turn on one, two, three or all four of the light strips.

For a small set-up or a dispersed set-up, the Jump Start Grow Light System is super easy to use. This is what I recommended schools use for many years before LED lights became more readily available and they are still a great option. This is also a nice option when you want to spread out the seeds among numerous classrooms. The lights are very easy to adjust in height and the whole system easily comes apart for storage in its original box. You can fit four flats underneath the 4’ model, giving you space to grow almost 300 plants, depending on what size pots you are using. The 2’ model fits two flats and up to 150 plants.

Pots

Seed starting and nursery pots come in a variety of sizes. It’s best to start your seeds in small pots like a small four-pack. You can also use six-packs, but I like the flexibility that four-packs provide. Then when they have at least two sets of true leaves, you can transplant them into a larger pot. The act of transplanting helps to stimulate finer roots to grow and results in healthier plants. For a school’s needs, you typically don’t need to transplant any larger than a 2.5” pot. As part of our plant sale, we offer flowers in four-packs. In this case we start and finish the seeds (no transplanting) in a large four-pack. Some seeds (squash family, root crops, celery, corn) don’t like their roots being disturbed during transplanting into the garden. I prefer to direct seed these plants into the garden, but if you must start them inside it is best to use peat pots where you can plant the entire pot to avoid root disturbance. I use peat pots only when necessary as they are more difficult to work with. You can typically buy all of these pots in small amounts at a local nursery or in bulk from a greenhouse supply store. We reuse the pots from year to year, cleaning them out and bleaching them after use so they are sterile for the next year.

Soil

If you’ve ever gone to buy soil by the bag, it can be a bit overwhelming as there are many different kinds of mixes available, each with a different purpose in mind. For seed starting you absolutely want a mix designed for seed starting. These mixes are made with a variety of fine ingredients that allow for fine roots to easily grow, contain organic material to feed young plants, and optimize water retention without becoming soggy. Oddly enough, these mixes don’t actually contain any soil, which by definition is decomposed rock particles, but have a variety of organic materials. The kids always ask what the white stuff is – it is called perlite and is a volcanic rock. Perlite has low water absorption and helps prevent compaction of the soil. I like to use ProMix Organic Seed Starting Mix. This works great and it doesn’t contain any chemical fertilizers for students hands to come in contact with (our state child care licensing prohibits this for preschool programs). This is also available in large compressed bales from greenhouse supply stores, sold as Pro-Mix MP Mycorrhizae Organik.

Flats

Using greenhouse flats without holes is critical to a seed starting system where water cannot drip on the floor. They not only act as a saucer to keep water from spilling on the floor, they also serve as the method in which to water your plants. I always bottom water the seeds filling the tray up with about one inch of water every 2-3 days or whenever the soil starts to dry out. This keeps newly planted seeds from washing away and is much easier than individually watering 1000 plants! This type of tray is available in varying thicknesses. The thinnest ones are flimsy and are hard to move around when there is water in them – they would work well in a stationary environment, but don’t hold up when you’re sliding trays in and out of shelves. The thickest ones are very sturdy, but they are more prone to breaking (and leaking). I’ve found the middle of the road ones (1.8 mil thick) to be the best. These TO Plastics 1020 trays are an industry standard.

Humidity Domes

Humidity domes are a key component of success when growing seeds outside of a greenhouse. Once a seed gets wet, if the seed dries out, it can no longer grow. So, maintaining consistent moisture prior to germination is critical. Humidity domes act like a miniature atmosphere trapping the water that has transpired (i.e. evaporated) out of the plants and then dropping it back down onto the plants. After I plant seeds and water from below until the soil is wet, the humidity dome traps most of that water and I don’t have to water very frequently. I typically keep the domes on until the plants have 3-4 sets of true leaves. The humidity domes shown are 2” tall and perfect for this use case. They also sell 7” tall domes which are great for if you have to leave your seedlings for an extended period of time without water (i.e. Spring Break). Once the seedlings are big enough to remove the domes, I slowly wean them away from the high humidity environment by propping up the edges along one side for several days. I’ve found Duplo blocks work perfect for this.

Heat Mat

Some plants prefer cool temperatures for germination, while others prefer it to be warm. Plants in the nightshade family (tomatoes, peppers, eggplants), squash family (pumpkin, winter squash, melons) and certain herbs (basil, parsley, rosemary) prefer soil temperatures of 75°-90° for germination. Unless your school is really warm (and I’ve been in classrooms that are 80° year-round), you will likely need a heat mat to achieve decent germination with these plants. These are easy to use, you simply plug them in and put the flat on top of it. They raise the soil temperature 10°-15°. You can’t control the exact temperature, but I’ve never found that to be a problem. Alternatively, if you’re in an old school with a radiator, you can sometimes place your plant trays on top of the radiator panel to achieve this increase in temperature.

Water

I love having students involved in watering the plants, which means having a watering can that they can easily handle. Ikea’s PS 2002 watering can is the perfect size for students. It easily fits under a water bottle filing station or sink and is light enough for students to carry and maneuver. They do have an open top though, so students have to walk slowly to avoid spilling water. When I water alone, I choose a 2-3 gallon watering can with a small spout on it.

As I mentioned before, I always bottom water filling the tray underneath the pots up with water rather than pouring the water on top of the soil. The soil absorbs the water and moves it up through the pot through osmosis. We water every Monday, Wednesday and Friday, adding water to trays that are dry. I like to draw a silver line about 0.75” up from the bottom of the tray to mark where the water should stop. One of the Ikea watering cans is typically enough for one tray.

With this equipment on hand, it is possible to grow just about everything for your garden from seed. It can be expensive making the initial investment in the equipment, but most of it is reusable and your cost in subsequent years is just the cost of seeds. This year I spent $175 on seeds in order to grow 900 plants from seed and over 500 plants that we will direct sow in the garden. If our garden produces 500 pounds of food like last year, this equates to $0.35 per pound in plants (the cost of labor and other equipment is not calculated into this). Not bad for a small school garden!

Seed Catalogs as a Learning Tool

As I child I loved to look through seed catalogs. I would leaf through them and imagine what I would put in my garden. Sometimes I would even be allowed to cut out the pictures and I could glue them together to make a garden picture. Now, it is my own two daughters who lunge for the seed catalogs when they arrive. They flip through oohing and aahing over all of the different fruits and vegetables, dreaming about the what ifs….

“Mom, can we have a lingonberry, it’s in our zone. Then we could make our own lingonberry jam”.

“Can we please have the red yard long beans. They look so cool!”

“Ooh, white pumpkins! Wouldn’t those be fun to have?”

Seed catalogs are a great tool to engage students in planning the garden while also gaining practical experience in reading, writing, and math. I always save my seed catalogues each year so that students can use them, but I’ve also had success calling up the some of the bigger seed catalogs and having them send me a classroom set. Students love looking through the catalogs just as much as my daughters and I do. You can harness their excitement over the plants to practice academic skills while aIso planning your garden. Here are some possible ways you can use seed catalogs in the classroom.

Descriptive Writing – Read the descriptions for various plants. Identify the adjectives used to describe the plant. Which adjectives capture your attention more than others. Have students write their own description for a plant to help market the plant as a choice in your garden.

Riddles – Students can choose a variety and then write a riddle that describes the variety. Have other students try to guess what it was.

Compare and Contrast – Have students select two or three varieties of a plant. Read through the description of each one. How are they the same and different. Create a Venn Diagram to capture the attributes of the selected varieties.

Persuasive writing – Choose a plant and write a persuasive essay on why that plant should be chosen for the garden.

Money – Have students pick out plants for a dream garden. Using the prices listed in the catalog, calculate how much the garden would cost. Have students calculate sales tax to get some practice with percents and decimals. Build a budget for the garden.

Measurement – Using the width and height listed for each plant, create a life size drawing of the plant. Put all varieties of a certain species in order from smallest to tallest.

These are just a few of the possibilities to utilize seed catalogs as a teaching tool. The next time you need a way to teach or reinforce a new concept, pull out your seed catalogs. Perhaps they’ll provide some curriculum inspiration in addition to garden inspiration!