Any brief foray into the world of vegetable seed starting will soon reveal a task known as "potting up". This is the act of transplanting a young seedling from a small soil block or container into a larger soil block or container, as demonstrated in the short video below.
Since the task of potting up seedlings seems to add unnecessary extra work to the job of starting seeds, anyone new to gardening would surely have to wonder, "Why bother with the juggling act? If I wanted the tomato in a 4 inch soil block in the end, wouldn't it make sense to just plant it in a 4 inch block in the first place?" If this question has crossed your mind, you're not alone. It's one that I get asked often, and I have even wondered the same thing. After all, if there's a task I can avoid doing and still get great results in my garden, I'm all in!
But is this one of those cases?
To find out, we're going to dive into the logic behind the task potting up today, but before we get to that, we should first confirm that the size of soil block or potting container even matters in the first place.
Does pot size really matter?
There's one truth we have to acknowledge before we get into the finer details. That is the impact of pot size. Despite what any of your favourite plants have whispered in your ear, it's not just how you use your pot that counts, but the overall volume of your pot that will really impact the quality of the vegetable seedling you raise.
It is possible to use a small pot and raise a vegetable seedling that "looks" healthy. Any 6 pack of tomatoes that you'll find at your local greenhouse is a good example of this. The large leafy tops look like they are flourishing, but this is only because the underdeveloped roots that are crammed into the tiny plastic cells below are being spoon-fed synthetic fertilizers to sustain growth despite the limited space. Prolonged growth can continue with the roots contained in a small pot. In fact, it's common for commercial growers to use containers for mass production of greenhouse crops so they can carefully manage the moisture and nutrient levels, but this is not our game. Our vegetable seedlings are on their way out the great outdoors where their survival will depend in part on a large healthy root system.
When growing vegetable crops in the field root volume is a big deal. This research survey on the effect of container size emphasized that "when root restricted seedlings are planted in the field they are often unable to compensate for evapotranspiration ( < That's the rate of water loss from the leaves. ) even if they are well watered after transplanting." So despite our best efforts to irrigate plants with underdeveloped root systems, they may still experience unnecessary stress during transplanting which will impede growth.
How then do we produce seedlings with larger root systems? We give them more space in their root zone. A meta analysis of the effect of pot size on seedling growth shows that "on average, a doubling of the pot size increased biomass production by 43%." Does this mean that a large pot can accelerate the growth of everything you plant? No. A small, slow growing crop like an onion is not going to benefit from the increased pot size in the same way as a large, fast growing tomato. Therefore, we still need to understand the rate of growth of each of our crops and make sure they have an appropriately sized block or container at each stage of their growth. All of this detailed information is included in the Seed to Table course materials.
Anyway, these research references hopefully make it clear that you need to offer your seedlings ample space in their root zone. Now we can move on to the question of the day, if we know that we want lots of root space in the end, why not start with a large pot in the beginning? Here are the three main reasons to pot up your vegetable seedlings.
1. Make the Most of Your Seed Starting Space
Seed starting spaces are generally on the smaller side. Most home growers find themselves starting their first seeds indoors because the weather outside is not year warm enough. A typical indoor seed starting setup will employ several banks of grow lights, some heat mats, and shelves to help make the best use of space, and a setup like this has limits. There is finite amount of space under the grow lights, a finite amount of space on the heat mats, and a finite amount of space on the shelving.
If we start all of our seedlings in their final block sizes, we would require an indoor seed starting space many times larger. Our small house doesn't have that amount of space to spare, and I'd rather not add to the cost of seed starting by buying more grow lights and shelves unnecessarily.
If we start some of our largest transplants in tiny soil blocks first, we can fit many more plants into our seed starting area. Let's consider a batch of 20 pepper seedlings for example to see how much space we save in our seed starting shelves by starting with small blocks. If I start 20 peppers in 3/4 inch blocks, I'll need 12 square inches to begin with. When I pot them up to 20 2 inch blocks, I'll need 88 square inches. And finally, when they transition into their final 4 inch blocks, the same 20 peppers would require 450 square inches.
We can't get around the fact that our seedlings will eventually need their full amount of space in our seed starting shelves, but by delaying the use of this space until it's actually needed, we can use our seed starting equipment much more efficiently. When it comes time to pot up crops to larger blocks, their leaf area is larger, and they are ready to make full use of the grow lights above.
2. Select the Cream of the Crop
The task of potting up presents us with a great opportunity to insert a little check point for each of our crops. I like to run a tight ship in my seed starting area and if any plant is not performing up to standard it simply doesn't make the cut. A perfect time to make that cut is when I'm potting up a batch of seedlings into their large soil blocks.
The photo above shows trays of many close to perfect seedlings, but I can assure you that they are not this perfect because I'm a perfect grower or because every seed I plant is a perfect seed. This batch of seedlings is growing at a very high standard because the weak and wonky plants have already been discarded. This is just the cream of the crop. The others are not worth my time. I run a garden for the purpose of feeding my family. I don't run a garden as a care centre for slow growing or deformed plants that have little to give me in return.
3. Minimize the Use of Potting Mix
It takes time and money to prepare the potting mix that our seedlings will grow in for the first few weeks of their life, so it only makes sense to keep these costs to a minimum. When I start a crop in 3/4 inch blocks, I use very few resources, and I quickly get to see which seeds germinate, which plants are looking the strongest, and which should probably just be discarded. Once I have this information, I can select the best plants to pot up to the next block size as described previously in point number 2.
Now I could start seeds in 4 inch blocks and still uphold my same standards for performance by keeping only the best and tossing the rest, but if I tossed out a 4 inch block, I also lose the time and money that went into creating that block. Some of you will be thinking that we could just chuck any discarded 4 inch blocks back into our next batch of potting mix, but I've found after a soil block has been pressed, the ingredients become quite bound together and aren't nearly as workable as they once were. Even if it was easy to recycle the soil block material, I would never recover the extra time needed to produce all of those extra larger blocks.
Bonus Tip
Okay, I need to say this last bit quietly, and make sure there are not any plants around that might be looking over your shoulder! This is just between the two of us. We don't want any of our vegetable seedlings to figure out that they have started growing in a tiny pot. No pepper in its right mind would start growing in a tiny plastic pot or a 3/4 inch soil block on a basement shelf, especially when there is still snow on the ground outside! The odds of reproduction and survival in these conditions would be slim to none. So if that pepper or any other plant you start indoors senses that conditions are not suitable for continued growth, undesirable adaptations like elongation or stunting are going to be triggered. Therefore, we never want our plants to figure out that they are growing in these artificial growing environments. Always remember that your primary objective when raising your vegetable seedings is to create the illusion of an ideal and boundless growing environment.
The practice of potting up is just one of the ways we control the variable of space to give our plants the illusion that they happen to have sprung up in the ideal location, but space is just one of the 5 core variables we need to master to keep our artificial growing environment pleasing for our plants. To learn more about the 5 core variables, head to my free workshop, where I introduce each of the 5 core variables and the critical role they play in optimizing growth in your vegetable garden.
