Basics

Most entries in the fact sheet are scales from 1 to 5: the more symbols are filled in, the more of it the plant needs. For light, the strong symbol shows the ideal spot and the pale ones next to it what the plant can still put up with. All entries are rules of thumb – your plant itself is the best guide to whether it's doing well. What it's trying to tell you with yellow, brown or droopy leaves is in every portrait under "Symptoms & help".

The care level also sets the order in this booklet: it starts with the indestructible ones and ends with the divas. If you're just getting started, pick your first housemates from the front.

Light can be one of the most limiting factors for houseplants. Outdoors it is many times brighter than indoors¹. And with every step away from the window it quickly gets darker: just a few steps away, only a tenth of the light may arrive². Curtains, blinds, balconies and canopies swallow even more light².

The distances in the table are rules of thumb from an overview by the University of Illinois, converted from feet to metres and rounded¹. The overview names windows by compass direction, the table by their sun: a window with midday sun faces south in the northern hemisphere and north in the southern hemisphere and gets the most sun; one facing the other way gets hardly any direct sun. In winter every plant can stand as bright as possible. In summer, though, sensitive plants can get sunburn behind glass³ – then move them back a little.

The times in the table are rough guidelines for a bright, normally heated room. But never water by the calendar – check first whether the plant really needs water³. The easiest way is the finger test: push your finger into the soil up to the second knuckle. If soil sticks to your finger, there's still enough water². With a bit of practice you can also tell by the weight: dry pots are lighter³.

How to water properly. Water thoroughly until water runs out at the bottom, then tip away the water from the saucer or cachepot³. A pot without a drainage hole almost inevitably leads to waterlogging – and then the roots rot. Use room-temperature water, especially in winter³.

In winter most plants need less water, because they hardly grow in low light. In a cool room many manage one to two weeks without watering. If the plant stands somewhere warm above the radiator, though, the soil still dries out quickly³.

Heated rooms are often surprisingly dry: 10 to 30 percent relative humidity is not unusual, while many tropical plants are used to 70 to 90 percent in their homeland². A small hygrometer shows you what it's like at your place.

This is how you help plants that love humidity^3,2:

• Gravel tray: stand the pot on a tray of gravel or expanded clay, and only add enough water that the bottom of the pot doesn't sit in it.
• Grouping: several plants side by side raise the humidity between them.
• Humidifier: the most effective solution for very demanding plants.
• Bathroom: many tropical plants are best off in a steamy bathroom.

Misting can help if you do it often, but it can encourage leaf diseases in some plants^3,2.

The height in the fact sheet is the size a plant typically reaches indoors after a few years. For hanging plants it shows the shoot length instead, that is, how long the trailing stems get. Next to the symbols is the approximate value the level comes from. After that come the width – how much room the plant needs to the sides – and the growth habit: upright, bushy, as a rosette, climbing or trailing. If "est." comes before the width, no source gives a value for the houseplant; we derived it from the size outdoors or from the leaf size.

Growth shows how fast a plant puts on size in good conditions – and so also how often you'll need to repot or prune. The levels are rough classifications, not measurements.

Most houseplants are happy at a perfectly normal room temperature². Take care in winter: when it's freezing outside, leaves shouldn't touch the windowpane². Keep plants out of cold draughts, and don't leave them behind a drawn curtain at night – it can get very cold there³.

Soil

houseplant soil (peat-free)

Loose, lightly pre-fertilised all-round soil for most foliage plants. Ideally peat-free.

cactus and succulent soil

Very free-draining, with lots of sand, grit, pumice or lava rock. Holds little water and dries quickly – for cacti and succulents. To mix your own: houseplant soil with a third sand or grit.

standard potting compost

A gardener's standard compost with some clay. Heavier and more stable than ordinary houseplant soil, holds water and nutrients well – for large, top-heavy plants.

mineral substrate (e.g. expanded clay, pumice)

No humus at all, e.g. expanded clay, pumice or lava rock. Doesn't rot and hardly compacts; nutrients come only from the fertiliser.

orchid substrate

Coarse pieces of bark, very airy. For plants that grow on trees in the wild.

Soil is more than dirt in a pot: it holds water and nutrients and still has to let enough air reach the roots. Ready-made soils are made of organic parts such as compost, bark or wood fibre and mineral ones such as sand, perlite or clay². Sand and perlite improve drainage². Plants that need extra drainage benefit from grit or sharp sand (horticultural, not builders' sand) mixed into the soil³. Ideally buy peat-free soil, as the RHS advises⁴.

During the growing season, roughly from spring to autumn, plants use up nutrients, which you top up with fertiliser. The easiest is liquid fertiliser that you add to the watering can. Foliage plants do well with a fertiliser high in nitrogen; flowering plants need more phosphorus and potassium. In winter there isn't enough light to use fertiliser – then feed at most once a month or not at all³.

Never feed onto dry soil, it can burn the roots³. And better too little than too much: white crusts on the soil, brown leaf tips or wilting leaves despite moist soil point to too much fertiliser. Then flush the soil thoroughly several times and don't feed for a while².

The number of years in the fact sheet is only a rough rhythm. You can tell whether it's really time by these signs^3,2:

• When you lift it out, you see a crowded mass of roots.
• The soil dries out very quickly after watering, and the plant hardly grows.
• Algae grow on the soil – a sign that it may have gone sour.

It's best to repot in spring, when the plant roots in quickly during the growing season. The new pot should only be a few centimetres wider (see "Pot" below), otherwise a lot of soil stays wet for a long time^3,2. Afterwards, don't feed for a few weeks – fresh soil is usually pre-fertilised².

The fact sheet gives two diameters: the pot for a young plant or a cutting, and the one for the full-grown plant, followed by the shape and material where they matter. If "est." comes before them, no source gives a value. We then derived it with two rules of thumb and rounded it to common pot sizes: rooted cuttings go into pots of 8–10 cm⁵, and the pot measures about a third of the plant's height². For large plants we cap it at 30 cm: the RHS repots palms until they are in 20–30 cm pots, after which only the top layer of soil is renewed⁶ – as with figs that are too big to repot⁷.

• Drainage hole: you can't do without one³. A layer of gravel in the pot doesn't replace it⁸.
• Cachepot and saucer: put the plant in its pot with a hole inside the nice cachepot², and tip away any water still standing in it after a few hours⁸.
• Clay or plastic: clay is porous, the soil dries faster and you water more often. Plastic and glazed ceramic keep the moisture longer².
• Shape: succulents like small or shallow pots, because a little soil dries quickly⁸. The amaryllis wants a pot twice as tall as the bulb⁹.
• Clear: for orchids it shows whether the substrate is still moist¹⁰.

As a rule of thumb when repotting: only 2–5 cm more in diameter². The RHS says 5–10 cm, or as much room as the plant needs for two to three years³.

Many popular houseplants are toxic. That sounds dramatic, but it's usually harmless: the vast majority of plant poisoning cases cause mild symptoms or none at all, and only about two to three percent are moderate or severe¹¹. Children are affected most often: they eat fruit or put leaves and other plant parts in their mouths¹¹. Every fact sheet gives the toxicity for humans, cats and dogs separately. Our levels "toxic" and "highly toxic" roughly match the moderate and severe poisoning risk as classified by the German Federal Institute for Risk Assessment (BfR)¹¹.

Toxic is not the same as dangerous to touch. With some plants, though, the sap irritates the skin. Then the fact sheet also says "Sap irritates the skin", and you're best off wearing gloves when cutting and repotting.