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.
Waiting to Transplant
Mark Macdonald | March 29, 2020
Many plants benefit from a head start by sowing indoors during late winter and early spring. For a few crops, notably peppers and tomatoes , this indoor start is an absolute requirement if growing from seed. These tender, tropical plants will be killed outright by frost, and will show immediate signs of distress if exposed to cold spring weather. So the gardener’s strategy is to make an educated guess about when it will be warm enough to transplant them outdoors, and work backwards from that date according to which crop is involved.
Tomatoes, peppers, and many perennial flowers require a good six to eight weeks of indoor growing before even considering peeking outside. But that’s a long time for plants to grow, so here are some strategies to consider while you are waiting to transplant outdoors.
Just about from the time the seeds are first placed into (or onto) the soil, bright overhead light is essential. With insufficiently strong light, seedlings will begin to grow tall and leggy from the very start. The seedlings are stretching their stem tissues, literally straining to get their leaves higher and closer to any light source so they can begin to photosynthesize and produce food for themselves. All seedlings do this, from tomatoes to palm trees.
If sufficient light is supplied, the seedlings have no need to strain and stretch, and they will remain stout and compact, with good colour and overall health. How does one provide sufficient light? Well, every grower has access to different tools. A heated greenhouse would be perfect for most seedlings, but these are expensive and few of us have access to them. So seedling lights are a smart option. Inexpensive T5 fluorescent tubes are available in several sizes. They produce full spectrum light in the frequency plants need for foliar growth. Even with a good double (or multiple) tube set up, it’s recommended that the tubes be kept 10cm (4″) above the tops of the seedlings. That may seem very bright, but one cannot over-apply light in this setting. The Growlight Garden is a self-watering kit with an adjustable hood that can be raised as the seedlings grow. And there lots of other ways to use the lights with adjustable stands . A superb, super-low energy alternative to T5 tubes is the recently developed LED light strips that fit most grow light fixtures.
Seedling Warmers do an amazing job of speeding up germination. They work with “bottom heat” which gently heats the soil above the ambient room temperature. This stimulates growth and really helps get plants started. But the gardener’s strategy is to keep the seedlings small and compact during this early indoor stage, so the mats should be removed or unplugged once germination occurs. Otherwise, they will continue to encourage fast growth, and the seedlings may become too large for their containers, or take up too much space indoors.
Even for heat-loving tomatoes and peppers, a warm growing space is not required during this nursery stage. Given ample light and a cool environment of around 18°C (64°F), the plants should grow slowly, but steadily, producing the healthiest transplants.
Seedlings will nearly always benefit from some movement of air indoors. This will help reduce excess moisture buildup and the possible mould and mildew problems that result from it. If their leaves and stems are subject to even slight movement, seedlings will develop stronger cell walls and be better prepared for the harsh elements of the great outdoors. If seedlings were started under domes, it’s a good idea to remove the domes after germination so that air can move freely and excess moisture can evaporate from the soil and trays. A very basic table fan is all that is needed to improve air movement for the benefit of seedlings.
There are numerous reasons for encouraging compact growth while waiting to transplant seedlings outdoors. As seedlings grow, they begin to compete with their neighbours for light, and if they are planted together, for nutrients and moisture. The gardener’s strategy here is to prevent unnecessary competition between seedlings. So lots of light and a cool environment will help. But plants continue to grow beneath the soil just as quickly as they do above.
This is a good reason to not fertilize seedlings prior to transplant. Fertilizer produces strong, fast growth, which is not wanted at this early stage. Seeds contain enough food to produce the initial cotyledon or first pair of leaves. These are then used by the plants to produce their own food, through photosynthesis, to allow for the growth of new tissues. Until they need to really go to work at transplant time (and after), the plants need no further nutritional help.
The phrase “potting on” describes the gradual transition from seedling tray to small pot, and from small pot to slightly larger pot, as needed, as the seedlings grow. If cold weather persists outdoors, transplanting may be delayed by weeks. And even with the light, space, and environment described above, most seedlings will eventually out-grow their root space.
Most plants are not bothered by potting on, but it should be done with great care not to damage the delicate root system, and without bruising the leaves and stem. Handling seedlings by the root ball is often safest. Refer to specific instructions about each type of plant in question.
Some plants respond very poorly to transplanting, so if they absolutely must be started indoors, it’s a good strategy to use peat , coir , or newspaper pots , or Cow Pots , that can be transplanted, pot and all, into a larger container, or into the garden row. This prevents the seedling from having its roots disturbed, and they will eventually penetrate the pot as it biodegrades in the soil. Soil Blockers are a fantastic alternative for small farms or nurseries, or wherever large numbers of seedlings need to wait for transplanting.
When to Transplant?
The question of when to transplant seedlings is absolutely tied to regionality. The last average frost date in a given region is a very general tool for estimating how many weeks later is appropriate for transplanting. A basic plan can be used by employing our Regional Planting Charts , but it takes careful management to get this right. For peppers, tomatoes, and most tender seedlings, a good rule of thumb is to wait until night time temperatures are steadily at (or above) 10°C 50°F before even contemplating transplants outdoors. It may work earlier with the help of a greenhouse, cloche cover, or cold frame, but that’s another subject.
All seedlings will benefit from hardening off – the process of gently acclimatizing to direct sunlight, cool temperatures, wind, and night/day temperature fluctuations. These can all cause transplant stress, so hardening off is a key step to success.
I like to think of the indoor seedling stage as an artificial holding area. We want the seedlings to be at their peak possible health once we transplant them. Before that, though, they’re still young. They’re still in school. Only when they actually get transplanted do we put them to work. It’s that key point when they’ll benefit from organic fertilizer to give them the push into the proper growing season. After transplanting is the real time to help these plants accomplish their goal, to mature, and to produce the leaves and fruits that make all this work worth while.
Learn some expert gardening advice on when to transplant seedlings, along with some key organic gardening strategies for producing the healthiest seedlings.