Toughen Up Your Marijuana Seedlings – How-To Guide

If you plan to grow cannabis outdoors, suddenly transplanting your indoor-nurtured seedlings into colder soil could damage their chances of survival. For many growers, outdoor cultivation remains the real ‘gold standard.’

It is the ‘natural’ way of doing things. If you have the private land, and the climate, you don’t need to do much in terms of maintenance. With nutrient-rich soil already in the ground and rainfall, there is no need to feed and water the plants.

Your primary goals are to keep pests and trespassers at bay and check for infections or infestations. At the end of it all, you are generally rewarded with a bountiful harvest of potent buds.

Some gardeners like to grow the seedlings indoors or in a greenhouse before planting them outside. However, if you suddenly take your pampered seedlings and place them into colder soil, you could send them into shock.

Plants that aren’t adequately prepared for the transition don’t perform as well as those that are. Remember, in an indoor setting; everything is perfectly regulated. Your seedlings get the right amount of light, water, and nutrients because you set it all up.

Plants that grow outside face a different challenge. You generally plant your marijuana seedlings at a time when the temperature remains unpredictable. There is likely lower humidity and higher winds. It doesn’t take much for plants to lose their moisture rapidly.

As a result, it is necessary to toughen up your marijuana seedlings before bringing them outside. This process is called ‘hardening off.’ You need to complete it gradually to give your seedlings the chance to get used to their strange new environment. In this guide, we offer three different hardening off methods.

Why Is It So Important to Harden off Seedlings?

Seedlings grown indoors and in greenhouses don’t fully develop waxy cuticles that protect lower and upper leaf surfaces from drying out. Such seedlings are not exposed to sunlight or excessive wind. The overall result is faster-growing plants with long, thin cells. They also have longer stems and more fragile leaves. Seedlings not fully exposed to the sun’s UV rays also develop larger leaves than their outdoor counterparts.


Once you plant a young seedling outdoors, everything changes. Now, the plant must keep itself protected from direct sunlight and strong winds. It now needs a waxy leaf cuticle to keep water loss to a minimum. Plants cultivated outdoors generally grow more slowly but have more durable stems and leaves. This enables them to survive cold nights, gusts of wind, and droughts.

When Should I Harden-off My Seedlings?

The good news is that two weeks is usually a long enough period to ensure your seeds are toughened up sufficiently. It depends on the type of strain, current weather conditions outside, and how warm the indoor growing climate was.

Also, many growers fail to understand that sunlight’s intensity dramatically exceeds that of an artificial light source. Even cannabis plants that grow under HID lighting don’t find it easy to transition to the sun’s light.

Some growers have reported growing seedlings indoors where they had 18 hours of HID or LED lighting a day. They put these healthy seedlings outdoors for a few hours a day in the full sun. To their horror, they found that the seedlings were massively damaged.

It is sensible to gradually introduce seedlings to sunlight by slowly increasing the duration of exposure.

As far as when to begin the hardening off process, a lot depends on where you live. Weather and photoperiod (length of day) are two of the principal factors in determining a successful outdoor grow. Putting plants outside too early or too late will ensure they don’t properly develop. Placing cannabis plants outdoors before the frost has left will likely kill them!

Your location significantly impacts the photoperiod. In the Northern Hemisphere’s ‘southerly’ parts, such as America’s southeast and southwest, the longest days of the year seldom exceed 14.5 hours. In Seattle and other Pacific Northwest areas, the longest summer day is around 16 hours. In Anchorage, Alaska, it is up to 20 hours! Ideally, you will live in an area where the longest days are 18 hours.

Now, let’s look at a few ways to harden up your cannabis seedlings.

Gradual Outdoor Exposure

As the name suggests, this is a slow and steady process. Begin by placing the seedlings outdoors up to 10 days before your pre-determined transplant date. Put them under trees, so they benefit from the protection of the shade.

You can leave them in this relatively safe position for 3-4 hours a day to start with. Slowly increase by an hour or two a day. When you bring the seedlings back indoors, put them in a warm place, like a porch or heated garage at night.

After several days, you can decide to expose the plants to the full intensity of the sun. It is probably best to wait until the plants have at least five sets of real leaves. At this stage, only put the seedlings into the glare of the morning sun. Avoid exposure from 11 am to 2 pm in particular. Return your plants to the shade in the afternoon.

If the temperatures remain high during day and night (at least 50 degrees Fahrenheit), your plants should be ready after 7-10 days. Keep a close eye on the seedlings, however. Make sure the soil in their pots never goes dry, and account for any sudden changes in temperature. The best time to make the full transition is on a cloudy day. Water the seedlings well after planting.

You can make the process easier by using a wheelbarrow to bring your plants in and out of the garage. It is also essential to place the seedlings on a higher surface, such as a table. If you leave them on the ground, slugs, snails, and other animals could damage them.

Withhold Water Temporarily

This is a little on the ‘tough love’ side of things, but some gardeners do it! Around a fortnight before the transplant date, stop watering your seedlings. Wait until they show initial signs of wilting. Make sure you pay close attention. If you leave it too long, you could seriously damage your plants. The process shouldn’t take too long because there isn’t much soil in a seedling pot.

Start watering them as soon as they begin to wilt. Then, allow the young plants to wilt once more. After two weeks of this process, your seedlings should have the desired toughness. Again, make sure you complete the final transplant on a cloudy day and water the plants well.

Using a Cold Frame

A cold frame has a glass top where you grow and protect small plants without artificial heat. It is ideal if your goal is to harden up young plants. Again, this is a gradual process. When the seedlings are in the cold frame, put the device in a southwest or south-facing spot (if in the Northern Hemisphere). The design of the cold frame box means your plants are kept at lower temperatures but still enjoy a semblance of protection.

Keep the box closed for up to three days to give the seedlings time to adjust. Then, open the windows on top of the frame halfway to let some light in. You can do this during the day and at night, but only during good weather.

The best conditions are when it is dry and relatively warm. The goal is to keep the seedlings in a temperature range of between 40- and 50-degrees Fahrenheit. This is the ‘sweet spot’ where they continue to grow while adapting to the changes they face.

Keep the frame closed tight if the weather is a little colder. If the temperature drops significantly, add a layer of bubble wrap on top of the box for added insulation. If the weather gets really bad, it is a wise idea to abandon the process temporarily. During hotter days, leave the box fully open during the day. Otherwise, leave it half-open.

After a week, you can begin leaving the cold frame open during the day and night. You can consider transplanting the seedlings once you believe you can leave the frame open 24/7.

Use a Small Fan

There are other ways of conditioning your marijuana seedlings for an outdoor grow while it is still inside. One of the easiest methods involves a small fan. Use it to circulate air around the seedlings. This process can help toughen the stems and prepare the plants for the wind they encounter outside.

Make sure you don’t overdo it, however. Too much air movement could cause the seedlings to dry out or get damaged. Keep the fan from pointing directly at the seedlings. The goal is to blow air around the room, not straight at the seedlings. You can keep the seedlings’ moisture content at a steady level by using a spray bottle to water them. Consistent moisture helps early leaf development and germination.

Final Thoughts on Toughening Your Marijuana Seedlings

If you want to grow cannabis outdoors, it makes sense to harden up the seedlings first. It is much easier for marijuana growers to do it indoors. In fact, most states don’t allow outdoor cultivation unless you have a specific license.

You need to live in the ideal climate, have the land, easy access to your crop, and ample security. If you have all of that to your name, the rewards could include an enormous yield of high-quality bud.

However, you must exercise caution when transplanting seedlings from a comfortable indoor environment to a less stable outdoor location. Follow the tips above to make the transition effortless.

Seeds and Soil
Join The Discussion

By clicking "Post Comment” you agree with our Terms of Use and Privacy Policy

TOC Protection Status © 2000 - 2024 All Rights Reserved Digital Millennium Copyright Act Services Ltd. |

WayofLeaf use cookies to ensure that we give you the best experience on our website. If you continue to use this site we will assume that you are happy with it. More Information