For most flowering houseplants, the sources read for this guide agree on the basics: use a potting mix made for containers, never garden soil, and make sure it drains and holds air. WSU Extension: Houseplants (PNW Gardeners Handbook) says the mix should be porous enough for aeration and drainage yet hold water and nutrients, and states plainly that garden soil is not appropriate for indoor plants. Clemson HGIC: Indoor Plants, Soil Mixes says a good mix is dense enough to support the plant, holds nutrients, lets water and air pass through, and is free of insects, diseases and weed seeds.
Beyond that, the sources diverge or go quiet. None of them gives a single recipe for flowering plants as a group. This page sets out what each source says about mix types, drainage, pH and feeding, then marks where a plant-specific answer, such as for gardenia or hoya, has to come from a plant-specific page.
What a potting mix has to do
- Support the plant: the Clemson page says the mix should be “dense enough to support the plant”, and the RHS says top-heavy plants suit a peat-free, loam-based compost because it is heavier and more stable.
- Hold water and nutrients while letting air through. WSU describes the ideal as porous for aeration and drainage yet able to hold water and nutrients.
- Be clean. Clemson lists freedom from insects, diseases and weed seeds as a requirement, which is why it advises against raw garden soil.
Types of mix, compared
| Type | What the sources say | Source |
|---|---|---|
| Peat-based or peat-lite | Peat mixed with perlite or vermiculite. Lighter than soil-based mixes but harder to rewet once dry. | WSU Extension: Houseplants (PNW Gardeners Handbook) and Clemson HGIC: Indoor Plants, Soil Mixes |
| Coconut coir | A lower-cost, more sustainable alternative to peat moss. Rinse thoroughly before use to avoid salinity problems. | Clemson HGIC: Indoor Plants, Soil Mixes |
| Loam-based, peat-free | Heavier and more stable; suits top-heavy plants. | RHS: How to grow houseplants |
| Garden soil | Not appropriate indoors (WSU); too many bacteria for containers unless pasteurised (Clemson). | WSU Extension: Houseplants (PNW Gardeners Handbook) and Clemson HGIC: Indoor Plants, Soil Mixes |
| Soilless mixes in general | Sterile, lightweight and easy to handle, but usually very low in trace elements. | Clemson HGIC: Indoor Plants, Soil Mixes and WSU Extension: Houseplants (PNW Gardeners Handbook) |
Both US pages treat peat and coir as the base, with perlite, vermiculite, sand or bark added for structure. The RHS comes from a UK context and points to peat-free, loam-based compost. These are different starting points, so a reader should match the mix to what is sold locally and to the plant, not copy one recipe.
Drainage: the part everyone agrees on
WSU and the RHS both say containers need drainage holes, and the RHS explains why: if a pot sits in water for long periods, the air spaces fill with water and roots can drown. Both also say to empty saucers after the plant has drained. WSU adds a warning that surprises many people: putting gravel at the bottom of a pot can increase the risk of overwatering, because water perches in the finer soil above the gravel and keeps the root zone wetter. Bud drop from watering errors is covered in Overwatering or Underwatering: Which Drops Flower Buds?.
For extra drainage the RHS suggests adding grit or sharp sand. WSU says self-watering pots need a light mix that is at least half perlite. The WSU page also notes that perlite can cause fluoride burn on some foliage plants and suggests a higher lime rate to prevent it.
pH: what is and is not stated
WSU gives pH figures for components, not for finished mixes: most sphagnum peat moss is acidic at pH 4.0 to 5.0, vermiculite is usually 6.5 to 7.2 and perlite usually 7.0 to 7.5. It gives no pH target for a finished mix. Clemson's own mix is adjusted to about pH 6.0 with dolomitic limestone. The RHS says acid-loving plants need peat-free ericaceous compost with a pH below 7, tells readers to check whether a plant prefers acid or alkaline conditions first, and says ericaceous plants do best with rainwater because hard tap water is alkaline.
This matters for scented plants because the RHS Gardenia page and the Clemson gardenia factsheet are the places where acid-loving needs are described for that plant. See RHS: Gardenia growing guide. The general pages above do not name gardenia, jasmine or hoya as acid-loving.
Recipes the sources actually publish
| Recipe | Ingredients (by volume) | Source |
|---|---|---|
| Clemson Mix | 2 parts pine bark (soil conditioning grade), 1 part peat, 1 part sand; pH adjusted to about 6.0 with dolomitic limestone; feed with water-soluble 20-20-20 every seventh watering | Clemson HGIC: Indoor Plants, Soil Mixes |
| Foliage mix examples | 2 peat : 1 perlite : 1 coarse sand; or 1 peat : 1 pine bark : 1 perlite; and two other combinations | Clemson HGIC: Indoor Plants, Soil Mixes |
| Cornell Epiphytic Mix | One third finely ground Douglas fir bark, one third shredded sphagnum peat, one third medium-fine perlite, plus lime and fertiliser | Clemson HGIC: Indoor Plants, Soil Mixes |
| WSU soilless mix | About 2.25 quarts peat or coir, 1.25 quarts vermiculite, 1.25 quarts perlite, 2.5 tablespoons limestone and 1.25 teaspoons superphosphate | WSU Extension: Houseplants (PNW Gardeners Handbook) |
The Clemson page lists the Cornell Epiphytic Mix as suited to plants such as bromeliads. It does not describe its recipes as made for flowering plants, and it does not mention flowering or acid-loving plants as a category. The epiphytic recipe is relevant to hoya, an epiphyte, only as an inference, and no opened source says so. The RHS hoya page is where that plant is described: RHS: Hoya growing guide.
Feeding: a mix is not a fertiliser plan
Clemson says good commercial mixes usually contain slow-release fertiliser that lasts a few months but are low in trace elements. After that, feeding has to come from you. The sources differ in detail:
| Point | WSU | RHS |
|---|---|---|
| Nutrient balance | Foliage plants need more nitrogen; flowering plants respond best to higher phosphate | Flowering houseplants need fertiliser higher in phosphorus (P) and potassium (K) |
| Season | More regularly March to September; little or none in winter | Growing season March to October; in winter no more than once a month |
| Liquid feed | Do not exceed label rates | Once or twice a month, or quarter strength with every watering |
| Dry soil | Do not feed a wilted plant; water first | Do not feed dry soil; it can burn roots |
WSU adds that plants should be flushed every four to six months using water equal to twice the pot volume, which in its example means about 20 cups for a six-inch pot. The comparison between slow-release and liquid feeds is on its own page: Slow-Release vs Liquid Fertilizer for Flowering Houseplants.
When the mix is spent
The RHS says old compost compacts and holds less air, which can encourage disease, and recommends potting on every two to three years in spring, choosing a pot 5 to 10 cm (2 to 4 in) larger. WSU gives a smaller step of about two inches larger in diameter, and says young, actively growing plants need repotting more often than mature ones. Both mention the signs: roots filling the pot and growing out of the bottom, or a plant that dries out quickly. Do the work at the right time with Spring Repotting Checklist for Flowering Houseplants.
What the sources do not cover
None of the opened pages tests potting mixes against flowering. None says which mix makes jasmine, hoya or citrus bloom more, none gives a mix ratio for flowering plants as a group, and none compares peat-free brands. The recipes above are published for foliage or epiphytic plants, not flowering houseplants as a category, so treat them as starting points.
Frequently asked questions
Can I use garden soil for a flowering houseplant?
The sources say no. WSU says garden soil is not appropriate for indoor plants, and Clemson says it has too many bacteria for containers unless it is pasteurised.
Is peat or coir better?
Clemson describes coir as a lower-cost, more sustainable alternative to peat and says to rinse it before use. Neither source ranks them for flowering.
Should I put gravel in the bottom of the pot?
WSU says it can increase overwatering risk because water perches above the gravel. Use drainage holes instead.
Does a mix with fertiliser already in it need extra feeding?
Clemson says the built-in slow-release fertiliser lasts a few months and the mix is low in trace elements.
Sources read for this page
- WSU Extension: Houseplants (PNW Gardeners Handbook)
- Clemson HGIC: Indoor Plants, Soil Mixes
- RHS: How to grow houseplants
- RHS: Gardenia growing guide
- RHS: Hoya growing guide
Last reviewed 25 October 2026. Written by Md. Firozur Rahman. This page gives general guidance and cannot diagnose a plant from a description; see the Disclaimer. Browse more in Getting Blooms Indoors.
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