How And When To Transplant Cannabis Seedlings
The seedling phase is arguably the most important of the entire cannabis life cycle. Whether you grow from seed or clone, the hands-on part of cultivation begins with seedlings. Transplanting is all important. Get it wrong, and the crop could be lost. Get it right, and grow great ganja with this guide.
THE IMPORTANCE OF CARING FOR CANNABIS SEEDLINGS
Post-germination or post-cutting, your bean or sprout needs a place to take root. The prime objective for the cannabis plant during the seedling stage is the development of a healthy root zone. If your seedlings struggle now, it’s likely they may never completely recover, and even more likely they won’t reach their full potential. Worst case scenario, seedlings die off and the grow is over before it’s even started.
THE OPTIMAL CONDITIONS FOR SEEDLINGS
Cannabis seedlings need to be treated delicately. Mind your marijuana like babies. If seedlings need support, prop them up with a toothpick or a cocktail stick and some soft gardening wire.
An 18-6 light schedule is considered the sweet spot by most growers. White light is preferred. The source of light should be CFL, MH, or LED. If you have a sunny windowsill, it can work in a pinch.
Roots grow in the dark. That’s why we advocate 18-6 or 20-4 over a nonstop 24-hour light cycle. High humidity, ideally around 70% RH, and temperatures in the 20–24°C range make for a perfect seedling habitat. A propagator, thermo-hygrometer, and a spray bottle of water are the tools to help you dial it in.
Stretchy or floppy seedling growth can be due to genetics. Often, it’s an indication the grow lamp is positioned too far away from the canopy. A cool white 250W CFL can comfortably hang 15cm above a dozen or more seedlings without scorching leaves.
WHAT’S THE BEST STARTER SUBSTRATE?
The medium must be wet, not waterlogged. An effective wet-dry cycle is the goal. Remember to resist the temptation to overwater. Better to mist plants with a sprayer if you are not sure. Touch the medium to feel how dry it is and keep RH high.
Before we go into the specifics, we need to be clear that starting in one medium and switching to another can be hazardous. Keep it simple and keep it consistent. You can’t start seedlings in soil and transplant later into a DWC bucket.
CUBES, CUPS, AND SMALL CONTAINERS
Rockwool cubes and blocks are made for hydroponic growers. Start your seedling in a small cube and it couldn’t be easier to transplant later. Simply cut a cube-sized chunk out of a larger block and insert. The process can be repeated with minimal stress to plants.
Unfortunately, rockwool is a really bad idea for soil growers, even the smaller sized blocks. Invariably, the block retains more water than the surrounding soil. Green, sludge-like algae growth soon becomes a problem. Sure, sometimes you can get away with the tiny cubes, but why take the risk?
An oxygen-rich medium that drains well is the ideal mix for soil and/or coco growers. Between these two growing styles, there is the most overlap during the seedling stage. Peat or coco cubes are easy to use and relatively inexpensive. Similar to a rockwool block, you can visually monitor root development as the white spaghetti strands protrude from the cube. Growers, both organic and hydro, working with clones report the highest success rates using the cube-sized starter mediums.
Many soil and coco growers use cups or small plastic pots as starter containers, typically, anywhere in size from 0.5–5l. Lightly fertilised soil mixed with perlite or coco coir mixed with 30–50% perlite are the most common blended media for seedlings. So long as the base of the container has plenty of drainage holes and the interior can hold the volume of medium required, it can serve as a starter pot. Don’t be afraid to improvise. Poke a few holes in the base of a paper coffee cup with a pin and it becomes a pot.
WHEN TO TRANSPLANT SEEDLINGS
The right time to transplant your seedlings is just before they outgrow their current container. With cubes, you can see roots poking out telling you it’s time to pot up. When you start with a cup or small pot, you are relying on above-ground cues. Typically, when the set or sets of true leaves of the seedling have spread out to cover the circumference of the container, it’s time. Also, vertical growth will be an obvious indicator.
Don’t wait too long to transplant. Rootbound plants will take time to recover and may be permanently stunted. In general, it takes 7–10 days for a seedling to take root and outgrow small starter pots of 1l or less.
HOW TO TRANSPLANT SEEDLINGS
Cubes are the easiest to work with. Either cut a hole to size in a bigger block, or burrow a hole into the medium with your fingers and insert for a snug fit. Potting up takes a little more finesse. First, don’t fill your large container all the way to the top. Leave room so you can water properly later. If you pack the pot all the way to the top, water will mostly run off and not reach the roots.
Next, make an impression in your large container with another small pot, or the one with the plant in it if you don’t have any others. Make this impression in the medium after you have watered it. This creates a perfect imprint for the transplant.
It’s best to wait until the medium in the small pot is dry before going for a transplant. Wet soil can fall apart in chunks as you fumble with the sopping mud. Now, turn the dry plant upside down, and firmly pat the bottom. Grasp the plant stem from the base and ease the compacted medium out of the container in one piece. Some topsoil will spill, but don’t worry.
Finally, gently slide the plant, roots first, into the large container. Replace the lost topsoil or coco with a handful or two over the top and add a little more water. That’s the secret to stress-free transplanting.
POTTING UP VS BEGINNING WITH BIG POTS
If your final container size is up to 11l, you have the option to sow seeds directly. This is only a viable option when growing from seed. Clones will not take root in such a large container. Initially, seedlings in large pots will grow more slowly than those in smaller containers. After a few weeks of vegetative growth, the difference is negligible.
If you don’t transplant, then you eliminate the risk of transplant shock. But you also limit the potential of your cannabis plants. That being said, a first and final transplant is sometimes the best option for autoflowering strains with a short life cycle.
Unless smaller plants are advantageous due to limited grow space, bigger is always better. Transplanting is not something beginner growers should avoid. The only way to learn is by doing. If you ever want to grow marijuana monsters, you need to master transplanting seedlings.
Transplanting seedlings is a critical stage in the cannabis life cycle. Get your grow off to a great start with our guide to transplanting.
5 Signs It’s Time To Repot Your Seedlings
It’s important to be able to recognize when it’s time to transplant seedlings from the seed tray to a larger pot. Transplanting seedlings a few weeks after starting should be part of your seed starting routine.
We start the seeds in smaller containers because we can control moisture and temperature much better that way, and if you’ve heard me talk much about seed starting then you know my motto is that moisture and temperature are the most important factors in getting good germination.
But once the seeds sprout, they quickly outgrow their seed starting container. It would be a bad idea to let them continue to grow in a pot that is too small. They’ll become nutrient deprived and their roots will grow round and round into a big knot.
Transplanting them into larger pots, helps them develop healthier roots and grow faster. By not restraining their growth indoors, you’re training them to grow big and strong when it’s time to put them in the garden.
But how do you know when to repot seedlings?
There are a few simple things you can look for that are dead giveaways your plants need a bigger pot.
1. They have one or two sets of true leaves
The ideal time for transplanting your seedlings is about 3 weeks after they sprout or when you have 1-2 sets of true leaves. It’s better to get them in new containers before they start to show the signs of stress listed below.
2. The cotyledons are turning yellow and falling off
Cotyledons are the first leaves that emerge from a seed. They are different than the “true leaves.” True leaves are the second and subsequent sets of leaves that grow after the cotyledons emerge.
It is normal for cotyledons to yellow and fall off, but if they’re doing it when you only have 1 or 2 sets of leaves, your plants really need to be transplanted.
3. The true leaves are turning yellow
It’s definitely time to transplant if the true leaves are yellow. This is a sure sign that your plants are starved for nutrients.
4. The roots are wound around and around the root ball
You definitely want to see roots in your growing medium when it’s time to plant, but if they’re circling around the edges of the root ball, then they’re getting too crowded.
5. They’re crowded
You don’t want to overcrowd your plants when they’re young. Some plants will grow taller than others and that will affect how much light the others get. You’ll also get the larger plants sequestering all the nutrients and that will stunt the growth of your other plants as well.
Why transplant the seedlings at all?
You might wonder why we would go to the trouble of repotting seedlings at all? Why not just give them some fertilizer, or better yet, start them in a larger container to begin with?
You’ll be much more successful germinating seeds if you start them in small containers. This allows you to have more control over the temperature and moisture in the seed starting container. We’ve found we have much better sprout rates in the smaller cell trays as compared to using other types of seed starting containers.
You can dose your plants with some fertilizer but that will stimulate growth. They’re already telling you they need more space, why stress them by making them grow bigger in the same small space?
Both of those options are viable alternatives, but you’ll have healthier plants if you transplant instead.
How to repot seedlings
Transplanting seedlings is quite easy to do. You simply need a new container and some potting mix. We recommend a container that is twice as big as what they are in now and a high quality potting mix like Fox Farm Ocean Forest.
We like to mix the potting mix and the seed starting mix in a 50:50 ratio. This is especially helpful for young seedlings who still have tender roots.
Before filling your containers, wet down your soil mixture to ensure even watering after you plant. Then nest your seedling in the new container, filling in around the base of the plant and pressing down to seat it in well and remove air pockets.
For tomatoes, bury the stem leaving only 1 or 2 sets of leaves above the soil line. For all others, plant them level or bury the stem about 1/4 inch or less.
Water your transplanted seedlings well and place them back under the light.
What about fertilizing seedlings?
You can fertilize young seedlings after transplanting as needed. Use a liquid organic fertilizer diluted to half strength. If they tolerate the half strength and seem like they need more, you can up it to full strength.
Don’t fertilize your seedlings until after you transplant them. You don’t want to stimulate growth in a space that is too small.
Do you transplant or repot your seedlings?
Tell us about your experience in the comments below!
You've conquered seed starting. Woot! But do you know when to repot seedlings? Transplanting seedlings is a step you can't skip! Learn the signs your plants are telling you they've outgrown their seed starting tray.