Rockwool is a commonly used substrate when growing cannabis hydroponically. Before the 1960s it was only used for home insulation (which it still is) until some folks in Denmark figured out that it is a great tool for growing plants as well.

It has excellent aeration, will only hold so much water, and deters pests from dropping eggs near your cannabis. Once your plant has an established root system the Rockwool provides a great structure for the roots to hold onto.

Some growers in the community even start their seeds in Rockwool.

Rockwool is meant to be used from seed to harvest which means it’s perfect to start your autoflower seeds. Grodan is a Rockwool manufacturer that uses little plugs to germinate your seeds. Then, transfer the plug into a larger Rockwool block and the roots will continue to grow out.

Learning to use Rockwool is a pretty easy experience and there is something pretty satisfying about watching the roots of your autoflower plant slowly taking over the cube it’s sitting in.

Why Is Rockwool A Strong Candidate For Growing Weed?

Rockwool is created by spinning molten rock into fibers that are further processed into plugs, cubes, and slabs. They are extremely lightweight when dry and due to their porous nature, have great drainage.

The interesting thing about Rockwool is that the water will gather at the base of the cube which provides water for the growing roots and the top will stay dry and provide the oxygen the plant needs.

It’s a perfect balance between moisture and aeration. This is why plants grown in Rockwool tend to explode with growth in a short time.

If you’re someone who likes to get their PH and EC down to a precise number then you’ll love growing in Rockwool. It is the best product for having control over the root zone of your autoflower plant.

Without having a soil buffer or salt buildup in your growing medium you can easily control the nutrition your plant gets. This will minimize the appearance of deficiencies or toxicities.

Growing autoflower cannabis in Rockwool takes a lot of the guesswork out of growing the plant. To get you started here’s a guide on getting your plants started in Rockwool.

How To Get Started Using Rockwool

Luckily, growing autoflower cannabis in Rockwool isn’t a difficult task and you can get started in just a few steps.

Let’s look at how to prep your Rockwool, germinate your seeds and take care of your cannabis seedlings for the first week or two.

Preparing Your Rockwool And Planting The Seed

These first few steps are pretty easy and you’ll have your seed planted in a short amount of time.

  1. Soak your Rockwool plugs in water that has a PH between 5.5-5.9. They are usually saturated within a few minutes. If you have a root stimulant that you want to use, add it to the soaking water.
  2. Remove the Rockwool plug and let the excess water drain out the bottom.
  3. The plug will still have some weight to it after the excess water is gone. Add your autoflower cannabis seed to a hole in the plug and close it up with tweezers or a toothpick.

Germination To Seedling

Keep the planted seed under a dome and spray the inside with water to generate enough humidity. If all goes well after a few days you’ll start to see it pushing its way out of the plug.

A couple of days after that you’ll see roots starting to poke their way out the bottom of the Rockwool and the growth on the top will start to speed up.

It is important that you keep an eye on the moisture level because your plant can wilt really quickly without water in Rockwool.

Vegetative And Flowering

When your autoflower cannabis starts to develop fan leaves then you know it’s time to feed your plant some nutrients. This is a fun part because you’re able to see the differences in your plant with every little tweak you make to the recipe.

If you start to see any issues with deficiencies you can quickly fix the problem in the next couple of waterings. Usually in something like soil you’ll be waiting a week or so to see any major changes.

As your plants get bigger they will drink more water so you may find yourself watering more frequently. Ensure you keep up with it because any delay with autoflower cannabis can be bad for your harvest.

Frequently Asked Questions

Here are some of the common questions growers have when they try out Rockwool for their next crop.

Why aren’t my seeds germinating in the Rockwool?

One common cause of this is that the Rockwool plug dried up or wasn’t kept damp enough. Seeds use moisture to activate the hormones that begin the growing process. Not enough water and it won’t complete what it needs to do.

To help with keeping the Rockwool moist ensure that your humidity is pretty high, around 70-80%. Using a plastic dome that’s been sprayed with water is a great way to keep the humidity and heat inside.

How often should I water the seedlings?

Since a seedling’s root system is still small it doesn’t require a lot of water to keep it going. You should check your Rockwool plugs at least once a day (most prefer twice) to make sure they are still moist.

If they dry out your plant’s delicate roots will die quickly and in most cases can’t be saved.

When should I transplant my autoflower cannabis?

Growing autoflower cannabis in Rockwool comes with its own transplant alarm system. It’s one of the best parts about growing with it; you get to see the roots develop in a way you can’t with soil.

The best time to transplant your autoflower cannabis is when the roots start to wrap around the plug. Most Rockwool products are designed to fit into the next size up in a modular format.

All you need to do is place the Rockwool plug inside a cube and the roots will grow out into its new home. Then once they’ve outgrown the cub you can then transplant it into the next size up. It’s a versatile system that makes transplanting easy.

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