FAQ: Seed Starting with Soil Blocking

by Lisa Mason Ziegler | August 23, 2024

I have been soil blocking since the beginning of my flower farming career in 1998. When I discovered soil blocking in Eliot Coleman's book The New Organic Grower it solved a couple of challenges for me. I didn’t, and still don't have a greenhouse. My urban farm's city location doesn't offer much hope of ever having one, so this method offered some great options I could build on.

First, I saw how I could use trays without drainage holes which was totally liberating from all other methods. Secondly, it could be incredible space savvy. A 5" x 7" tray holding 40 snapdragon seedlings ready to go to the garden. Soil blocking is not only a viable seed-starting method for farmers, but is a perfect fit for the home gardener. The blocks are easily managed in a small space in a home because it can be self-contained. In the footprint of a tabletop grow light, 12” x 24 ” you can support 240 seedlings until they go outdoors. All of the following information is based on my experiences for my seed starting setup situation. I have tweaked the steps to make it possible for me to use an indoor room, 10’x10’, to start and support over 15,000 soil blocked seedlings at a time.

10' x 10' room equipped to support thousands of seedlings. I use stainless steel rolling racks with 3-6 adjustable shelves per rack equipped with grow lights. Each shelf holds 720 mini soil blocks on rigid reusable cafeteria trays. I have 3-4 racks in the room depending on the season. Another bonus is this room is a part of our work building that is already heated and air conditioned, so no additional expenses or concerns over weather events as with a greenhouse. Once bumped out of our seed-starting room seedling land on our carport- which is also home to our bucket washing / storage area. Because of my unique seed starting needs and space available I was forced to go out of the box on the recommended steps to follow. I've learned that it is very possible and viable to strip down the method to bare essentials and produce an amazing quality of transplant and do so indoors in a small space.

If you have questions : I have answered many suggested questions here. If you have additional questions, I will be happy to answer those posted here on my blog site so others will benefit from the conversation.

A key component to my method: is it's all about timing. Because I start 95% of our flower, vegetable, and herb seeds in the mini ¾” blocker and hardly ever transplanted up to a larger block, it is crucial to stick to a schedule with the quick growth and earlier planting age in mind. The goal is to grow a transplant to 3”-4” in 2-4 weeks and have it ready to plant at its right time and have the space ready for it to be planted in. If you have to sit on seedlings because you started too early they will become overgrown quickly. This happens sometimes because of weather and you have a week or so of wiggle room, but if you start 8 weeks before planting instead of the recommend 2-4 weeks--the plants are going to go down. So resist starting too early. 2 week old Sweet William showing off his great roots!

Why use soil blocking? Put aside the incredible space savviness of soil blocking and it just grows an amazingly good quality seedling. Because the plant is not in any kind of container it does not become root bound. The roots are air pruned by the air space in between each block. Because of the structure of the blocks and blocking mix you can handle the transplants when they are younger which leads to an established plant in the garden faster. Planting is also quicker, pick up the entire cluster of 20 seedlings and place on your palm, simply break off seedlings to plant. The plants that often perform poorly (like celosias) when they become root bound do especially well with soil blocks.

Celosia cockscomb a couple of weeks after being transplanting to the garden in 3/4" blocks.

When to start? As a general rule I slice ⅓ of the recommended growing time off for the soil blocking method. So a seed that it is recommended to start 6 weeks before planting, I start 4 weeks ahead. Seeds like zinnias that are quick growers, 2-3 weeks ahead. Tomatoes and peppers 4 weeks and so forth. Better to start later than to early.

Zinnias ready to go to the garden.

What is the potential of soil blocking production? Does a farm grow out of it? During the height of our production years my designated seed starting employee was easily starting 100,000 seedling a year using soil blocking. Note: we did that from the 10’x10’ room and an outdoor covered area that is 12’x20’. We did make the break from soil blocks to plug trays for our sunflower production. Sunflower seeds would require the larger blocker which takes up much more space and soil. In the same indoor space that we can start 240 mini blocks, only 32 of the 2” blockers fit. Considering we were doing 1000 sunflowers a week it was an easy choice. Because plug trays can be set outdoors in the weather (in rain) they would not require any of our precious indoor growing space. So my answer is that from the home gardener to the market farmer, soil blocking is a perfect fit. While a farmer may not use blocking for 100% of production, it can be an excellent boost to space savviness and seedling quality. We find it easier to use the handheld model of the ¾” small mini blocker although there is a floor model available which would require a larger in-house block making setup.

Sweet peas in the 2" block, the soil mix is what make the blocks durable for handling to plant.

Do I use a special soil mix? Using a mix that is specific for blocking is essential to making a sturdy block that will hold up for the duration until planted in the garden. Blocking mix has the opposite qualities of potting mix. Potting mix is intended to be light, loose and crumbly - which the ingredients vermiculite and perlite contribute too. Blocking mix should be more binding and compact --no vermiculite or perlite. For the homemade blocking mix recipe and tips for making click here . The nutrients in the blocking mix is greensand and rock phosphate powder. Because many gardeners have trouble finding these nutrients we offer it to add to your homemade mix here .

3 week old sweet peas going in the garden in fall.

How much water do I add to the blocking mix? A kitty litter box or cement mixing tray is a good size for wetting the mix and making the blocks. In a perfect world, you would wet the blocking mix the night before making blocks. The mix seems to get smoother for lack of a better word. However, this never works out for us because we make such a larger volume of blocks at one time. Measure the amount of dry mix as you fill the tray. 21 cups of mix at a time is enough to make 2-3 trays (240 blocks each.) Approximately ⅓ of the soil volume in water will be added, 21 cups of dry mix to 7 cups water. Mixes will vary on volume and dryness due to ingredient variations- compost density, using peat moss or coco fiber, etc. So don't dump all water in at once add 5 cups and mix a minute and see where you are. If still dry add the other 2 cups and be prepared to add more if needed. You are looking for the consistency of moist mushy oatmeal with a smidge of pooling water to none in the nooks and crannies of the soil. Keep your final water measurement tally for the next time you make blocks. Storage of unused blocking mix: allow the unused portion of blocking mix to dry out and store in a breathable manner. It can be re-wet and used next time.

2 week old Stock seedlings.

What type of tray should I use? The number of plants for each variety of plants you are starting will dictate the size tray you should use. It is not recommended to put more than one variety per tray. Because sprouting time varies from seed to seed even amongst the same family of plants it can cause problems further down the chain of growth. The foam trays that are approximately 5” x7” will hold 2 sets of 20 mini blocks (40). The larger cafeteria trays hold 12 sets of 20 mini blocks (240). The benefit of using rigid trays is so you can handle and carry one tray with only one hand. Tray should have a flat bottom, no ridges and no drainage holes. Water in the ridges isn't readily available to the blocks and can encourage standing water problems. If you ever have the need to move blocks from one tray to another you can do so easily and with little injury to blocks with a pancake spatula on a flat bottom tray.

Snapdragons pictured in January, transplanted from 3/4 block in October.

Which blocker to use? We use the mini ¾” block to start 95% of our seeds. While it is obvious to use it for tiny seeds such as snaps, ageratum, lettuce, and basil we also use it for those seeds that are a little larger, such as: zinnias, marigolds, tomatoes, and peppers. When the seed is too large to be sown in the mini blocker, we either use the 2” blocker or a plug tray if a large volume of plants is needed. Seeds such as sunflowers, sweet peas, and squash are too large for the mini blocker.

240 3/4" seedlings on a tray.

How-to-make soil blocks . While the end result of a firm block is the same for both the mini and 2” blocker, the technique of making them is a little different. Tip on placement of blocks on the tray-- consider how you will water the blocks. I tend to place all the sets of blocks to one side of the tray to leave a space that I can pour water without pouring on the blocks. Leave a small space between sets for water to run freely between the sets. Mini ¾” Blocker: Hold the blockers by the stationary bar-- with both hands. Push the blocker down into a pile of soil twisting it a little bit as you go down to fill all the cracks. To release the suction from the blocker to the bottom of the tray, slide the blocker to the side a couple of inches before pulling up, push the mini blocker down a again to be sure all the chambers are full. Scrap the bottom of the blocker to remove the excess soil and make it flat. Place on tray and squeeze the plunger and give a partial pump a time or two if needed to release blocks. Large 2” Blocker: Hold the blockers by the stationary bar-- with both hands. Push down into a larger pile of soil twisting a little to fill the cracks, slide to release suction and with a side to side motion. Lifting the ends of the blocker, scoop more soil into the blocks, scrap the bottom of the blocker to make flat. Place on tray and squeeze the plunger to push out the blocks and give a partial pump a time or two if needed to release blocks. The suction on the larger blocks in the blocker may take a moment to release.

2" blocker used to start moonflower vines- a large seed.

How-to sow seeds. The seed packet should indicate if the seeds need to be left uncovered for light to sprout or to cover with soil to create darkness for sprouting. If the packet doesn't say, search online by entering “sowing instruction” and the name of the seed for guidance. Once you know, go to planting seeds. We use an aluminum seed pan to work from because it has no static. A toothpick moistened with saliva is our tool of choice for placing the seeds on the block--it goes quickly. One seed to a block--which leads to zero thinning. We do not touch the blocks at this point with our fingers which can compromise the integrity of the blocks that have no roots yet. We use three ways of sowing seeds in general. Small/tiny seeds are firmly placed on the surface of the block and not covered to give light for germination - good contact is important. Larger seeds that are going into the mini like zinnias and marigolds that need darkness to sprout are speared into the mini block, pointy end first with the tail left visible. Large seeds going into the 2” blocker are either pushed deeper into the block with the toothpick to create darkness or if the insert attachment is installed, drop the seed in the hole and place a teaspoon of soil on top.

Sowing zinnia seeds in 3/4" blocks.

Do I really need to use a heat mat, germination chamber, or domes? As a general rule, most seeds germinate at 75-85 degrees. When the sown seeds get warm, and stay warm (even at night), and are moist they will sprout quicker and more of them sprout. This sets up for uniform plants growing together on the tray. Why you need a heat source: If the air temperature is 70 degrees in the seed starting area, then the mass of soil the seeds are planted in is 15-20 degrees cooler than the surrounding air temperature, plus it gets even cooler at night. So, soil in a 70 degree area is 55 degrees which explains poor germination. Cool soil leads to poor germination and damping off problems in seedlings. Seedling heat mats that have a built-in thermostat warm the soil to 15-20 degrees warmer than the surrounding room temperature. Larger commercial heat mats required a separate thermostat which are costly but worth it in a commercial setting. I do not find it necessary to use domes on heat mats. A germination chamber is a closed-in unit with a heat source in water (works like a water heater) and creates a kind of steam bath that warms and moistens many trays at once. We use both heat mats and a germination chamber. Many growers with greenhouses build warmer tables filled with sand and place heater cables in the sand. The table can be hooped to create a warm moist environment. Trays can stay on/in the heat source typically for 2 to 10 days depending on the variety and it's germination time. Once 50% of the seeds show signs of sprouting, remove from the heat source and move to light.

Feverfew ready to go to the garden.

How do I water blocks and how often? . The room we grow seedlings in is southeast facing and is glass on two sides. It gets ridiculously hot on sunny days,100 degrees or more not uncommon. I water thoroughly once a day in the morning. To water, use a non-sprinkling water can or a gentle nozzle on a sink hose. I pour water into the tray in the area I left free of blocks. I always put a little more water than I think it will take to wet the blocks all the way through to be sure the middle of the blocks get wet. After I have watered all the trays, I go back to where I started and look to be sure all blocks moistened and if there is any water left standing. If so I pour off any excess water. Do not leave standing water in trays. The most common concern I hear is that the blocks dry out during the day. This can happen when the blocks are not thoroughly watered. Trays with mature seedlings that can be prone to drying out are moved outdoors where the conditions aren't so intense. I have never used capillary mats and cannot comment but to say I’ve not found it necessary to use.

2 weeks of our sunflower production in outdoor plug trays.

Are grow lights necessary? 16 hours of light a day is required to grow stocky, well-branched, healthy seedlings in the shortest amount of time. A timer is recommended to ensure timely on/off. When plants grow tall and leggy they are reaching for sunlight. Depending on the type of grow light bulb will dictate the distance from the light to the seedlings. For fixtures with bulbs that don't generate excessive heat, lights should be 2”-3” above the seedlings. For the new T-5 fixtures that put out much heat I've found that 15”-20” inches above works well. Plants stay under the light until they go outdoors.

Yarrow on the verge of becoming overgrown in the tray if the weather doesn't get with it.

Should you fertilize seedlings? I like Neptune’s Harvest organic liquid fertilizer to feed our seedlings. I begin using it weekly after the seeds sprout following the recommended mix. I also use the mix in a spray bottle to spritz newly sown seeds. It is said to enhance germination and I have found it to be true.

Excellent germination. Empty blocks usually occur because the sower got distracted and miss a block... Notice the dry set of blocks that were missed during watering--it happens.

When and how to transplant up to a larger blocker. In northern regions with shorter growing seasons, it is of great benefit to grow a larger transplant to go to the garden. Inserts can be installed on the 2” blocker that makes a perfect ¾” hole to receive the mini block. The perfect time to transplant is when the ¾” block has sprouted and has nice roots hitting the edge of the block. Drop the ¾” block in the hole and give a gentle push to snug it in. The plant then continues to grow on for 2-3 more weeks. This produces perhaps the most beautiful seedlings I have ever seen. Be sure to back up your seed starting time if you intend to grow this additional time.
What is hardening off? To adapt seedlings to the outdoor elements before planting in the garden I move them from the seedling room to a covered carport area. They get acclimated to wind and sun gradually. The covered porch provides sunlight protection but also protection from rain that can make a quick mess of blocks. They are on the carport for at least 3-4 days and sometimes longer until they go to the garden to be planted.

Trays emptied by a morning of planting blocks, ready to be washes and reused with Babs standing guard.

How do I plant blocks? I like to fertilize with Neptune’s Harvest the day of planting. For easy planting, gather a set of 20 blocks in your hand and place on your palm. Break off blocks as you plant. Plant the block just deep enough to cover the block lightly with soil. Water the transplants and provide the recommended 1” or more of water a week--especially the first 2 weeks. To create a safe and encouraging environment in the garden, cover transplants immediately after planting with a floating row cover to protect from rabbits, birds, squirrels, and deer. Whether to use hoops or not depends on the growing situation.

Fall planted transplants protected from deer and wind during the winter.