This seedling’s purple stem may be telling you that the seedling is struggling. (Image courtesy of John Foster.)Cannabis seedlings are wise little gurus. You can learn a lot from them if you know how to understand the wisdom they’re trying to share.
I’m one of those growers who cultivates predominantly from seeds, sometimes germinating three to six different marijuana strains at the same time. Because I’ve studied my seedlings and taken notes about their growth patterns during many years of cultivating cannabis, I’ve now learned how to read them.
At times I’ve been impressed — or puzzled or frustrated — by seedling performance, and probably you have, too. That’s why I’m sharing with you my observations and analysis of marijuana seedling growth patterns and characteristics.
Cannabis Germination Rate & Rapidity
The earliest quality indicators your seeds give you are what percentage of them germinate, and how fast they germinate.
Germination rate and speed are influenced by the seed’s genetics, age, storage conditions, germination conditions, and overall vigor. The older marijuana seeds are, the fewer of them will germinate and the longer it will take.
A healthy, fresh marijuana seed that’s less than two years old should germinate within two to five days of placing it into germination conditions. Older or defective marijuana seeds might take longer than that. My slowest seed took 13 days to germinate.
Unless germination delay or failure is caused by faulty germination conditions, a seed that doesn’t germinate at all was probably worthless before you attempted to coax it to grow. And a seed that takes an unusually long time to germinate might be telling you one of several things:
- It’s an old seed that’s lost some of its vigor. This loss of vigor usually affects the entire life cycle of the plant, resulting in a weak plant susceptible to problems and unlikely to yield big, potent harvests.
- Its genetics programmed it for slow germination.
- The germination conditions, although optimal for other marijuana seeds, weren’t optimal for this particular seed.
If you have cannabis seedlings that aren’t thriving, first check your germination conditions and inputs to make sure everything is as it should be.
In general, if a seed takes a lot longer to germinate than other seeds started at the same time, consider it defective. Unless it’s a rare seed or for some other reason I feel compelled to nurse that seedling along, I won’t keep growing it.
Rate Of Cannabis Seedling Growth & Overall Strength
After a seed has germinated, its two baby leaves, or cotyledons, open up, collect light, and pretty soon the first set of true serrated leaves appear. Within one day of germination, a healthy seed will have raised itself from barely emergent to at least one-quarter inch or more above the soil line.
In the next two weeks, the seedling adds sets of true leaves, its stalk thickens and elongates, and its leaves enlarge. At the end of two weeks, your seedling should be at least 5–6 inches high.
The rate and quality of early seedling growth tells you a lot about the phenotype and how well a particular seedling will perform as it matures. For example, if seedling leaves are mutated and don’t display the normal pattern we expect from marijuana leaves, that’s an indication that the seedling’s genetics are compromised.
You also see strain phenotype, evidenced by looking at early seedlings. The differences in leaf shape, size, format and color are indicators of those plants’ genetics.
Watching main stalk development is a critical observation you have to make. If the young stalk is at least as thick as a pencil lead, is green-colored, and is easily able to support the leaf structures building from it, that’s a sturdy, desirable stalk.
But you sometimes see thin, discolored, spindly stalks that are too weak and/or tall to support their leaf sets. The seedlings keel over; unless you support them, they’ll almost never grow upright on their own.
Weak stalks, seedlings falling over, and seedlings failing to thrive can be a sign of hydroponic nutrients problems; root-zone pH problems; lights that are the wrong wavelength, too weak, too far away or too close to the plants; or defective grow-room temperature and/or humidity.
The most common problems that cause seedlings to decline are:
- Overwatering.
- Lights too close or not close enough.
- Lights that have the wrong wavelength for seedlings.
- Overfeeding or underfeeding.
If you have many seedlings and only a couple have defects, you can safely assume it’s those particular seedlings themselves, not the grow-room conditions or inputs, that are causing the problems. But if most of your seedlings are failing to thrive, you can assume that the problem isn’t built into the seedlings, but is something you can fix by altering your cultivation procedures, feeding, lighting, and grow-room environment so it’s less hostile to seedlings.
In regard to overall growth rate and vigor, if a seedling doesn’t have at least three to five sets of true leaves and is at least 5–6 inches tall by the end of the second week after germination, I consider it a weak seedling and am likely to pull it, unless I have a good reason to nurse it along.
Processing Nutrients, Water & Light For Cannabis Seedlings
Remember that my seedling observations are carried out in optimized grow rooms. When I witness seedling problems, I know it’s not because I’ve overfed, overlit, or in any way neglected the cannabis seedlings.
One of the most important tests of your seedlings is how they handle the inputs and conditions you’re providing.
I have a controlled, time-tested program that guides how and when I start feeding my seedlings, what kinds of nutrients I use, germination moistness and temperatures, what kinds of grow lights I use, and how far the grow lights are from the seedlings.
This program is stable and consistent, although I might alter it slightly to accommodate anticipated genetic needs or seed conditions, such as if I have old, rare cannabis seeds. If I place 24 seeds into germination conditions, and 21 of them thrive but three struggle, I get rid of those defective seedlings right away.
Marijuana Seedling Desirable Traits
What you’re looking for in your baby cannabis is:
- Seedlings that are lime green in color, including stalks and leaves. If seedlings show purple, reddish or grayish stalks or stems, this indicates poor root development and/or an inability to maximally process nutrients.
- Properly formed leaves that are parallel to the soil line or slightly upright. Leaves that curl, discolor or droop down indicate the seedling is defective and can’t handle the conditions your other seedlings are thriving in.
- Seedlings that show visible, appropriate growth in height, leaf sets and leaf size, every day.
I include seedling root development as one of the factors you should observe, but unless you’re growing in deep water culture or aeroponics, you can’t easily observe your marijuana plants’ roots over the course of your growing season.
For most of us, the density, branching and healthfulness of cannabis roots are only visible when we transplant our plants, or when we remove our plants from their containers after harvest.
If more than 10 percent of your seeds fail to germinate, or if they germinate but grow out to be weak, defective plants, ask yourself if they were defective seeds, if you perhaps have grow-room problems, or both.
The main thing I want to emphasize is that it’s best to dispose of any seedling that isn’t keeping up with its companions in terms of robustness, growth rate, height gain and leaf development.
If you feel you have to roll the dice by keeping troubled seedlings alive, you might get lucky and find that the seedlings rebound from their rough start in life to deliver a worthwhile yield of potent buds. But in the majority of cases, unhealthy seedlings grow into weak plants that need extra attention, are susceptible to diseases, have a more difficult time in stressful conditions, and might not yield as much as plants that started life as strong seedlings.
If you intend to use your cannabis seedlings in a breeding program, you want only the strongest, most resilient and adaptable plants to go forward into grow and bloom phase.
Be sure to carefully select, germinate and care for your cannabis seeds and seedlings, and even though it sometimes feels sad to have to do it, it’s best to get rid of seedlings that don’t measure up. After all, when you select for only the strongest seedlings, you get an easier grow op, with fewer problems and higher yields.
