Soil & potting mix
Best soil for Slack's Sundew (Drosera slackii)
Also called Slack's sundew.
More about slack's sundew
About Slack's Sundew
Drosera slackii · also called Slack's sundew · houseplant
Drosera slackii is a large, rosette-forming sundew endemic to a small area of the Western Cape, South Africa, named in honour of renowned carnivorous plant author Adrian Slack. It forms impressive rosettes of long, paddle-shaped leaves densely covered in red tentacles, making it one of the most visually striking of the African sundews for windowsill cultivation.
Preferred mix: Nutrient-free peat-perlite or peat-sand carnivorous mix
Watch for — Mineral burn from tap water: Brown leaf tips spreading inward, combined with white crust on the soil surface, indicate mineral accumulation from tap water. Flush the substrate repeatedly with large volumes of pure water and switch exclusively to rainwater or distilled water.
Why slack's sundew needs this mix
Slack's Sundew is a true acid-lover — it physically cannot take up iron above about pH 5.5, so an ericaceous mix is not optional, it is survival.
- Slack's Sundew has evolved on acidic, peaty ground and depends on soil fungi that only function in acid conditions — raise the pH and it starves even in "rich" soil.
- In a too-alkaline mix iron and manganese lock up chemically, so the youngest leaves yellow between green veins (lime-induced chlorosis) and the plant fades out.
- Its fine, shallow roots also want an open, free-draining structure, not a heavy clay or claggy compost.
For the full picture on what makes up a good mix, see our guide to the main types of soil and potting media — it explains why each ingredient above behaves the way it does.
What goes wrong with the wrong mix
The wrong soil is one of the most common reasons slack's sundew struggles, and the damage often shows up weeks later as a watering problem. For this species specifically:
- Ordinary multipurpose or garden compost is far too alkaline for slack's sundew — expect classic yellowing, weak growth and a slow decline over a season or two.
- Hard tap water slowly pushes the pH up too, undoing a good mix; rainwater is strongly preferred for watering.
- Lime, mushroom compost or wood ash anywhere near this plant is actively harmful.
Planting slack's sundew in standard compost or limey garden soil. Without an acidic (ericaceous) medium it will yellow and fail no matter how well you water and feed it.
pH — does it matter for slack's sundew?
This is the whole game: Slack's Sundew needs pH 4.5-5.5. Test it, use ericaceous compost (and an ericaceous feed), and water with rainwater where you can to keep the pH from creeping up.
If you want to check or adjust it, the soil pH guide walks through testing and the safe ways to nudge a mix more acidic or more alkaline.
DIY mix vs a bagged one
Bagged ericaceous compost is the correct, easy base for slack's sundew; just open it up with bark and grit per the ratio above. Do not try to acidify ordinary compost by guesswork — it rarely holds.
Drainage and the pot
Containers are often easier than open ground because you control the pH completely. Use a pot with good drainage and an ericaceous mix; never let it sit waterlogged.
Top up or refresh the ericaceous mix yearly and test the pH each spring — it naturally drifts upward over time, especially if watered with tap water. When the time comes, our repotting guide for slack's sundew covers the timing and technique step by step.
Slack's Sundew soil — frequently asked questions
What is the best soil mix for slack's sundew?
3 parts ericaceous (acidic) compost : 1 part composted pine bark or pine needles : 1 part perlite or coarse grit. Slack's Sundew has evolved on acidic, peaty ground and depends on soil fungi that only function in acid conditions — raise the pH and it starves even in "rich" soil.
Can I use normal potting soil for slack's sundew?
Ordinary multipurpose or garden compost is far too alkaline for slack's sundew — expect classic yellowing, weak growth and a slow decline over a season or two. Bagged ericaceous compost is the correct, easy base for slack's sundew; just open it up with bark and grit per the ratio above. Do not try to acidify ordinary compost by guesswork — it rarely holds.
Does slack's sundew need a special pH?
This is the whole game: Slack's Sundew needs pH 4.5-5.5. Test it, use ericaceous compost (and an ericaceous feed), and water with rainwater where you can to keep the pH from creeping up.
Should I buy a bagged mix or make my own for slack's sundew?
Bagged ericaceous compost is the correct, easy base for slack's sundew; just open it up with bark and grit per the ratio above. Do not try to acidify ordinary compost by guesswork — it rarely holds.
How often should I refresh the soil for slack's sundew?
Top up or refresh the ericaceous mix yearly and test the pH each spring — it naturally drifts upward over time, especially if watered with tap water. Containers are often easier than open ground because you control the pH completely. Use a pot with good drainage and an ericaceous mix; never let it sit waterlogged.
Keep reading
- Slack's Sundew care — the full brief (light, water, humidity, problems, pet safety)
- How often to water slack's sundew — the schedule the mix feeds into
- Repotting slack's sundew — when and how to refresh the mix
- Soil pH guide — test it and adjust it safely
- Root rot — how the wrong soil starts it, and how to save the plant
- Underwatered plant — signs and how to rehydrate it
- Should I water my plant? The simple check first
- Best soil for hound's tongue fern
- Best soil for pinstripe calathea
- Best soil for peacock plant
- All 6887 soil and potting-mix guides in the Growli library