Understanding the Ingredients of Molly's Potting Mixes
Understanding the ingredients of Molly's potting mixes
Every Molly's mix is built from a short list of materials chosen for one job: give roots air and drainage without letting them sit in water. Here is what each material is, and what it does in the pot.
Three mixes, three different jobs. Aroid Mix is bark-led with moisture-holding coco, for tropical plants that climb. Orchid Mix is coarse bark and mineral grit, for epiphytes that grow on trees. Succulent Mix is mineral-dominant, for desert plants that want to dry out fast. All three carry the same living component: a mycorrhizae and bioactive blend that colonizes the roots and extends what they can reach.
Aroid
Bark-led, most organic content. Steady moisture without saturation.
Orchid
Coarse bark and grit. Open air around roots, drains immediately.
Succulent
Mineral-dominant and heavy. Dries out within days.
Fir bark
Douglas fir bark, screened to a medium chunk. It is the structural backbone of the Aroid Mix: large, irregular pieces that hold their shape and keep open channels through the pot, so water can pass and air can follow. Aroid roots grip bark in the wild, so the texture is familiar to them.
Bark breaks down slowly, over 12 to 18 months. When the chunks start to look dark, soft and uniform, the mix has aged and it is time to repot.
Pine bark
Aged Pinus radiata bark. Denser and slightly more uniform than fir bark, which makes it the better backbone where a mix needs to stay open for longer under frequent watering. It is the primary structural component of the Orchid and Succulent mixes, and works alongside fir bark in the Aroid Mix.
Coco chips and coco coir
Both come from coconut husk. Chips are coarse pieces that hold moisture while keeping their structure; coir is the finer fiber, and it holds more water for longer.
They do the moisture-buffering work in a mix that is otherwise mostly bark and mineral. Without them, a chunky mix dries out faster than most people water. The trade-off is that coir compacts as it ages, which is why the proportion stays small.
Perlite
Volcanic glass heated until it pops, like popcorn. The result is a lightweight white particle full of sealed air pockets. It absorbs very little water and does not break down, so it holds permanent air space in the mix. The white specks throughout the Aroid Mix are perlite.
Expanded clay pebbles
Natural clay fired at high temperature until it expands into round, porous pebbles, also sold as LECA. The pebbles wick moisture upward from the bottom of the pot and release it slowly, so roots get humidity rather than standing water. They never decompose.
Pumice
Volcanic rock, naturally full of holes. Heavier than perlite, which matters more than it sounds: weight anchors top-heavy plants and stops a gritty mix from floating when watered. Pumice holds a little water in its pores and drains the rest immediately.
Hyuga
A Japanese volcanic aggregate, pumice-like in structure but harder and more stable. Prized in bonsai for holding its shape over years without breaking down into fines. It gives the Succulent and Orchid mixes their long-term grit.
Akadama
Japanese volcanic clay, granulated. Unusual because it holds both water and nutrients while staying air-permeable, which is why bonsai growers have used it for centuries. It softens gradually with age, and that change is a useful signal that the mix is due for renewal.
Lava rock
Crushed volcanic rock in red and black, both screened to size. Sharp, porous and completely stable. It creates permanent air gaps between the finer particles, and adds the weight a mineral mix needs to stay put.
Biochar
Wood carbonized at high temperature under low oxygen. Its porous surface takes up salts and impurities from tap water and fertilizer, which keeps the mix from turning stale through repeated watering. It is chemically stable and does not break down.
Biochar is closely related to horticultural charcoal and does the same job in the pot. We use biochar.
Worm castings
Digested organic matter from earthworms. A small proportion supplies a slow, gentle nutrient base, so plants take up what they need over time rather than getting a flush of feed at once.
Deliberately absent from the Succulent Mix. Desert plants do not want a nutrient-rich medium.
Mycorrhizae and the bioactive blend
Every other material on this page is structure: it decides how water and air move through the pot. Mycorrhizae are different. They are living fungi that colonize the root system and extend it, threading far beyond where the roots themselves can reach, drawing in water and nutrients the plant would otherwise miss.
The relationship is a trade. The plant feeds the fungi sugars from photosynthesis; the fungi hand back phosphorus, micronutrients and moisture. Roots that are colonized establish faster after repotting, tolerate a missed watering better, and get more out of the fertilizer you do give them.
Alongside the fungi, the bioactive blend brings beneficial bacteria that work on the material around the roots, releasing nutrients that would otherwise stay locked up and crowding out the organisms you do not want. It is the one component that is in all three mixes, Aroid, Orchid and Succulent alike, because every root system benefits, tropical or desert.
What is not in these mixes
Just as important as what is in the bag.
Why the ingredient list is short
A growing medium is a physical structure before it is anything else. Particle size, shape and stability decide how water moves through the pot and how much air reaches the roots between waterings. A handful of well-chosen materials, screened to consistent sizes, does that job better than a long list.
Each mix is tuned by ratio rather than by adding more components. The Succulent Mix is mineral-dominant because desert plants need to dry out within days. The Orchid Mix is coarse bark and grit because epiphyte roots evolved in open air on tree bark. The Aroid Mix carries more organic content because tropical climbers want steady moisture without saturation.
Shop the three mixes
Aroid Mix
Orchid Mix
Succulent MixFrequently asked questions
What is in Molly's potting mixes?
Aroid Mix: fir bark, pine bark, coco chips, coco coir, perlite, expanded clay pebbles, worm castings and biochar. Orchid Mix: pine bark, coco chips, akadama, pumice, hyuga, lava rock and biochar. Succulent Mix: pine bark, akadama, hyuga, pumice and lava rock. All three also contain the same mycorrhizae and bioactive blend.
What do the mycorrhizae in the mix actually do?
They are living fungi that colonize the roots and extend their reach, pulling in water and nutrients, phosphorus in particular, from further out in the pot than the roots can get to on their own. In exchange the plant feeds them sugars. Colonized plants establish faster after repotting and make better use of the fertilizer you give them. All three mixes include them.
Are Molly's mixes soilless?
Yes. None of the three contains garden soil, topsoil or compost. They are built from bark, coco and mineral aggregates.
Why does the Aroid Mix have so much bark in it?
Aroids such as Monstera, Philodendron and Pothos are semi-epiphytes. In the wild their roots wrap around bark and thread through leaf litter, exposed to air. Chunky bark reproduces that structure in a pot.
Why is the Succulent Mix so heavy?
Because it is mostly mineral. Pumice, lava rock, akadama and hyuga are denser than bark or coco. The weight is doing useful work: it anchors top-heavy plants and keeps the mix from shifting when you water.
What is the difference between akadama and hyuga?
Both are Japanese volcanic aggregates used in bonsai. Akadama is a clay granule that holds more water and nutrients, and softens with age. Hyuga is harder and more stable, and keeps its structure longer.
Do the mixes contain fertilizer?
No synthetic fertilizer. The Aroid Mix includes worm castings, which release nutrients slowly, but that is not a substitute for feeding. Use a standard houseplant fertilizer through the growing season.
How long does a mix last before it needs replacing?
The mineral components do not break down at all. The bark and coco age over roughly 12 to 18 months in the Aroid and Orchid mixes. The Succulent Mix, being mostly mineral, can go two to three years. Repot when the chunky pieces have gone dark, soft and uniform.
