This article explains how to use sphagnum peat moss as the base of a cannabis growing mix. The worked example is Hortiflor Sphagnum Peat Moss. Hortiflor sells peat from Latvia, a country in northern Europe, in 70 L and 250 L bags. The product information does not say whether this is the loose volume or a compressed volume, so check the bag before you plan your mix.
What sphagnum peat moss is
Sphagnum peat moss is moss that grew in wet bogs and partly broke down over a very long time. It is harvested, dried and bagged for use as a growing medium. A growing medium is the material that roots grow in, used instead of garden soil.
Translator’s note: Sphagnum peat moss is not harvested in Thailand, so bags sold here are imported. Before you choose peat, compare its price per litre with locally made media such as coco coir. That way you know what the imported material costs you per pot.
Five properties, and what each one means in your pot
| Property of sphagnum peat | What it means for you | What to do |
|---|---|---|
| Naturally low pH, about 3.5–4.5 | The raw peat is far too acidic for cannabis roots. Because it starts low, you can set the final pH yourself. | Raise the pH with dolomite lime before planting (see below). |
| High water retention: holds many times its own dry weight in water | You water less often, but the mix can stay wet too long. | Mix in perlite to add air (see the mixes below). |
| Long-fibre structure | The long fibres leave gaps between them, and these gaps hold air for the roots. | Do not press the mix down hard when you fill pots. |
| Sold as free of pathogens and weeds | Bagged peat is often sold as free of disease organisms and weed seeds, usually without test results. | Open the bag and check it for mould, a sour smell or seeds. Grow a few test pots before filling a whole room. |
| Slow decomposition | The peat keeps its structure through one growing cycle. The mix does not collapse and squeeze out air during flowering. | You can plan on one full cycle from seedling to harvest in the same mix. |
“Good drainage” and “holds a lot of water” sound like opposites, but both can be true. Water clings inside and around the fibres. The larger gaps between the fibres drain and fill with air. If the peat is packed tight or crumbles into fine dust, the air gaps shrink. That is why most growers add perlite instead of using peat on its own.
Translator’s note: In Thailand, root-zone temperatures of 30–35 °C make waterlogging more dangerous. Warm roots use more oxygen, and warm water holds less dissolved oxygen. A peat mix that stays soaked for days can leave roots short of air. That is the condition in which root rot such as Pythium starts. In the rainy season, outdoor pots also take in rain and dry more slowly in humid air. Lift each pot before you water it, and water only when it feels clearly lighter. This tells you whether the roots actually need water.
Using peat on its own or in a mix
You can use sphagnum peat as the main part of your growing medium. You can also blend it with other materials to change how it holds water and air:
- Perlite: white granules of heated, expanded volcanic rock. It adds air pockets and helps water drain.
- Vermiculite: heated, expanded mineral flakes. They hold extra water and nutrients. (Both are compared in Perlite, Vermiculite or Pumice.)
- Coco coir: shredded fibre from coconut husks. It is another way to adjust water holding and air (growing media compared).
Two common starting mixes:
| Mix | Peat moss | Perlite | Vermiculite | Use |
|---|---|---|---|---|
| General purpose | 60% | 30% | 10% | Standard indoor and outdoor pots |
| High drainage | 50% | 50% | — | When you need the mix to dry faster |
In both mixes, raise the pH with dolomite lime as needed.
Translator’s note: The high-drainage 50/50 mix is the safer starting point for rainy-season outdoor pots. It is also safer for rooms that stay very humid or for growers who tend to overwater. The general-purpose mix holds more water, so it suits dry-season pots or growers in very hot, dry spots. Mix a small trial batch of about 10 L first. Water it once and note how many days it takes for the pot to feel light again. This tells you which mix matches your watering routine before you buy in bulk.
Translator’s note: Coco coir is made from coconut husk, a local crop in Thailand. It usually starts close to neutral pH, so it does not need lime. It does need its own preparation, such as rinsing and calcium buffering, which is covered in the growing-media topic linked above. If you are unsure, put one peat pot and one coco pot side by side with the same plant. Compare their watering frequency and pH over a few weeks.
Raising the pH with dolomite lime
Dolomite lime is ground rock made of calcium and magnesium carbonate. It raises the pH of the mix slowly and also adds some calcium and magnesium. You need it because raw peat at pH 3.5–4.5 is too acidic for cannabis roots to take up nutrients properly. More on why pH matters is in pH & EC: The Nutritional Foundation.
There is no fixed liming rate. Work out your own rate with a small test batch instead:
- Measure 10 L of your finished mix (peat plus perlite and the other parts).
- Add a weighed amount of dolomite lime. In a US university study of peat and peat-perlite mixes (Owen and colleagues, HortTechnology, 2020), about 5–9 g per litre of mix was needed.
- Moisten the batch, mix it well and leave it covered. Dolomite works slowly, so the pH keeps changing for some time. Measure after about one week, then again a week later. Use the result once two readings are about the same.
- Measure the pH. Translator’s note: A simple method is to stir 1 part mix into 2 parts distilled water, wait, then measure the water with a pH meter.
- If the pH is still too low, add a little more lime, wait and test again. A common target for crops in peat-based mixes is about pH 5.8–6.2.
Once you know how many grams worked for 10 L, multiply by the volume you actually have. A 70 L bag of loose mix needs seven times the test amount. If your bag is compressed and expands when opened, use the loose volume after fluffing, not the number on the bag. Write down the grams per litre and the pH you reached so you can repeat it next time.
Translator’s note: If the dolomite or Cal-Mag you buy is sold as a fertiliser, check that it is registered in Thailand. How to check a registration is explained in Verifying a Fertiliser Is Legally in Thailand.
Before planting: soaking and storage
Pre-soak the peat before use. The aim is for the peat to absorb water evenly all the way through. Dry peat pushes water away at first, so water runs down the side of the pot and leaves dry pockets around the roots.
Translator’s note: A quick check is to squeeze a handful after soaking. A few drops should come out, and the inside should be dark and evenly moist with no dry, light-brown patches.
Store bags in a dry place, out of direct sunlight. This keeps the peat in the condition it arrived in.
Translator’s note: In the rainy season, keep bags off the floor and under a roof. Close opened bags tightly so the peat does not absorb humid air and grow mould. Check an opened bag for mould before you use it in a new batch.
Feeding plants grown in a peat mix
Peat contains few minerals of its own, so two rules apply:
- Use a calcium-magnesium (Cal-Mag) supplement. It replaces the calcium and magnesium the peat lacks. Follow the dose on the product label. Learn the deficiency signs in Reading Your Leaves, so you can tell whether the dose is enough.
- Monitor and adjust the pH of your nutrient solution. The nutrient solution is the water with fertiliser mixed in. Checking it every time keeps the root zone in the range you set with the lime.
Translator’s note: Also measure the pH of the runoff (the water that drains out of the pot) about once a week, and write it down. A runoff pH that keeps falling shows the mix is drifting back towards acidic. That is your signal to correct the pH of your feed before the leaves show deficiency.
Where to get it
For Hortiflor peat or similar products, ask the official distributor in Thailand. You can also ask other growers about their experience in the Equipment and Supplies section.
Checklist before you plant
- Check the bag for mould, smell and seeds, and confirm whether it is compressed.
- Mix a 10 L test batch: 60/30/10 for general use or 50/50 peat and perlite for high drainage.
- Find your dolomite rate on the test batch, record the grams per litre and the pH, then scale up.
- Pre-soak until a squeezed handful releases a few drops.
- Store bags dry, shaded and off the floor.
- Add Cal-Mag as the label says, check the feed pH every time and the runoff pH weekly.
- In hot or rainy weather, water by pot weight, not by calendar.
If you have used sphagnum peat in Thai conditions, post your mix ratio, your lime rate and your results below this topic. Questions are welcome here too.