By Steven Voser


Thinning is a technique used by growers to select their best seedlings, thereby producing better harvests when all is said and done. It’s generally used by gardeners who grow vegetables, legumes, or other edible plants—but cannabis growers can take advantage of thinning too.

The process of thinning is pretty simple; once your plants have grown a few true leaves (meaning leaves other than their cotyledons), simply cut down the weakest looking seedlings, leaving behind only the strongest/healthiest.


Thinning can seem pretty counterintuitive. After all, aren’t you better off growing as many plants as possible? Well, not really.

Cannabis plants grow fast and big. Once they start developing, your plants will naturally compete with each other for light, space, airflow, nutrients, and more. If you’ve got too many plants growing together, this competition can seriously affect your plants’ ability to develop properly and produce a high-quality harvest.

Thinning also helps ensure an even canopy in your grow room. Cannabis plants all grow at different speeds, and if you let all your germinated seedlings go along and do their thing, you’ll soon be left with an uneven canopy. When growing indoors, this can be really problematic as your shorter plants will receive less light than the ones closer to the light source.

Another benefit of thinning is that it can help protect your plants against diseases and pests. If you don’t thin your seedlings, your grow room may become cramped, which will drive down airflow while increasing temperature and humidity, creating a breeding ground for powdery mildew, mold, and more.

Even Cannabis Canopy after Applying Thinning Technique


As we mentioned earlier, thinning is very popular among gardeners who grow crops using multiple seeds. But when we grow cannabis, we tend to sprout a single seed per cell or starter block. Nonetheless, thinning is still a great way to strengthen your cannabis crop.

Unfortunately, cannabis seedlings are really fragile, and even the most experienced growers will have a few seedlings that don’t make it. So, thinning can be a great way to pick out your strongest and give them more personalised attention to promote healthy, strong growth.

Once your seedlings reach their vegetative growth phase, you’ll have helped create an even canopy, minimising the competition between your plants. Not only for light, but also for nutrients, airflow, and space.


Arguably the hardest part of thinning is knowing which seedlings to thin and which ones to keep. Timing is also really important.

In general, you’ll want to keep an eye out for those seedlings that lag behind the rest. After the first 1–2 weeks, it's a good idea to thin off any seedlings that are smaller and have developed less foliage. Also consider thinning the smallest, thinnest, and leggiest looking seedlings in your crop, and instead only keep the strongest looking plants. After all, these are likely to produce the best possible yield come harvest.

At the same time, make sure to keep an eye out for seedlings that look sick or limp. Damping off is a really common disease affecting seedlings that is caused by Pythium, Botrytis, and Fusarium fungi. If you spot signs of damping off, get the affected seedlings away from the healthy ones and cull them quickly.


If you’re growing from regular seeds, thinning is a little trickier. Male cannabis plants tend to grow faster than females, so keep that in mind when thinning your plants, and maybe don’t be so hasty to cull slower-growing plants as you would when growing from feminized seeds.

Once you’ve decided which plants to thin, simply use a set of small, sharp trimming scissors to cut the seedling as close to the soil as possible. If the seedling was healthy, feel free to reuse the soil or starter block it was growing in. You can then use the cut seedlings as feed for your compost or soil as they are actually packed with nutrients.

While thinning, also consider pruning your plants a little by cutting off some foliage to encourage faster growth.


If you’re struggling to come to terms with throwing out weaker seedlings, there is an alternative; you can keep them together in a separate grow area. If you do this, you obviously shouldn't cut the seedling. Instead, you’ll want to keep it alive and grow it together with other, similar-sized seedlings in a separate area where they won’t have to compete with your bigger plants.

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