Indoor Plants

Most house plants require very little attention, with regular, but infrequent, watering and annual or biennial repotting.  

Watering & Humidity Requirements

The key is not to overwater as to cause root rot and plant death. Watering when soil is dry, or almost dry, is a good rule of thumb. It’s important to push your finger into the soil to check for moisture, as soil may appear dry but be wet underneath. 

It’s surprising how long certain species can survive without water. Succulents, of course, have various morphological characteristics that allow them to store water and reduce transpiration. 

You may remember epiphytes from biology class, such as lichens, which are not a single organism, but two in a symbiotic relationship. Epiphytes grow on the surface of other plants, and most do not root in soil. Instead, they absorb water from the air. Some species of orchid, including the popular Phalaenopsis, are epiphytes. One egregious example, such as some species of Tillandsia, also known as the air plant, have minimal root systems and grow on shifting desert soil. 

Even houseplants that do root in soil, such as Zamioculcas, most are drought resistant. If you are ever unsure about a plant’s watering requirements, first check a supplier’s website. You can also make assumptions based off a plant’s climate. Species from rainforests benefit from high humidity, and those from deserts low. Also, what morphological characteristics does the plant possess? A plant with lots of thin leaves will lose water faster than one with thick, leathery, waxy, or hairy leaves, and one with no leaves at all. 

You can tell when a plant is suffering from too little water as it will first droop and then wilt. Don’t give up. Just because a plant is wilting doesn’t mean it can’t be revived. All that is happening is that the cells have depressed and the plant become slack. When watered, its cells fill up and the plant becomes taut. So, give it a thorough watering and watch it spring to life. 

Plants in small pots (succulents excluded) do need monitoring as small pots have a tendency to dry quickly. It’s best to repot and when you do, ensure you don’t compress the soil. Otherwise, the soil is liable to becoming waterlogged. 

Plants can die from insufficient watering and it’s usually because of gas embolism. Normally, plants absorb water through its roots, which travels up through the stems to the leaves. In times of drought, plants will sometimes absorb air that can form a bubble, which blocks the flow of water upwards. 

Certain species, especially those from rainforests, benefit from high humidity. Humidity is important as the lower the humidity, the more water is lost to evaporation. Temperature affects the amount of water air can hold, so more water is lost at higher temperatures. 

Humidity can temporarily be increased by misting – spraying plants with water. Preferable, is putting plants on a drip tray covered in pebbles and filling the tray with water, which will evaporate at room temperature, creating a nice microclimate for your plant. If you drill a hole in your pot, it will help with drainage, and excess water will fill the tray anyway. You can also try grouping plants together and planting plants in terrariums, from which water can’t escape.

You can measure humidity by purchasing a thermo-hygrometer, which measures relative humidity. It will give a reading in as a %, because at say 75% relative humidity, the air can hold 25% more water at the same temperature. Once you have worked out each room’s humidity, you can put your plants in the room they are best suited. 

You can tell when a plant is suffering from low humidity as it’s leaves will begin to curl, as to reduce the surface area exposed to light. Plants are unlikely to suffer from high humidity, but from pests and diseases that multiply in conditions of high humidity. 

Light Requirements

Plants need light to kickstart photosynthesis – a process whereby sunlight, water and carbon dioxide is converted into glucose and oxygen. Glucose is used by the plant in numerous key functions such as metabolism and growth, and is also stored as starch. 

Most house plants are well suited to poor lighting and will suffice in substandard conditions. However, it’s best to reserve bright rooms for plants selected for flowering, fruiting or variegated foliage, as these processes will overwise be inhibited. 

Certain species are sensitive to sunlight hours. Some will only flower when the days are long and others when the days are short. Keeping these plants near windows, and away from artificial lights will help with flowering. Others are day neutral, unaffected by day length.

Poinsettias are one fascinating example of photoperiodism – the response of an organism to day length. The species has evolved to cease the production of chlorophyll and initiate the production of anthocyanins in response to shortening days, resulting in its leaves going from green to red, in the aim to attract pollinators to its tiny flowers. You can initiate this transformation yourself by putting the plant in total darkness for 3 months 14 hours a day. 

It’s important to give your plants a period of darkness (at least 8 hours). At night, plants respire, taking in gases used in photosynthesis in the day, and also break down glucose produced in the day in maintenance and growth. (In the day, plants close their openings to reduce water loss. Hence, while plants produce energy in the day, they use it at night.) 

Certain species, such as many epiphytes, aren’t adapted to direct sunlight and may pale, brown and die from too much exposure. Simply, move to a darker location. 

Plants do not absorb all wavelengths of light equally, with the main pigments involved in photosynthesis (chlorophyll) primarily absorbing blue and red light with green reflected. (This is why plants appear green to your eyes.) 

Artificial lighting can be absorbed by plants, but is composed of different wavelengths than natural light. Incandescent lighting produces mostly red light, while fluorescent and LED light varies. LED lights are often divided into cool and warm variants, with the cool emitting more blue and warm more red. 

You can grow plants exclusively under artificial light, say in a basement, but it’s important to get the colour balance right, otherwise flowering, variegation and growth will be affected. 

High intensity blue light will promote the flowering of long day plants and inhibit that of short day plants. Likewise, even low intensities of red light exposure at night will inhibit the flowering of short day plants and promote the flowering of long day plants. 

Blue light is important in promoting leaf colouration. Varieties with purple leaves may go green in its absence. A dearth of red light will result in spindly growth and a dearth of blue light stumpy growth. 

Light intensity decreases as the distance from the source increases. It is for this reason window direction affects the intensity of natural light received. Windows facing east and west receive about 60% of the intensity of southern exposure. Northern exposure a mere 20% of southern exposure. 

A room’s light intensity is affected by the colour of a room’s surfaces (with lighter colours reflecting light and producing brighter rooms), and the transmission of natural light, whether it’s blocked by outside objects, curtains/blinds or dirt on windows. 

Light intensity varies throughout the seasons. In summer, window sills are bathed in light, as the sun rises quickly and is mostly high in the sky. In winter, natural light actually penetrates interiors further as the sun stays low in the sky. 

Insufficient sunlight leads to etiolation, whereby a plant grows spindly with long gaps between nodes – areas of the stem where leaves are located – and chlorosis, whereby leaves turn yellow, caused by the death of chlorophyll. Both these effects make evolutionary sense, allowing the plant to stretch in search light and forgo wasting energy on useless leaves.

It is possible to measure light levels, but you’ll need a camera. The process is detailed here.

Temperature 

Temperature speeds up respiration, causing plants to break down glucose faster. Glucose is produced from photosynthesis, which is a function of light intensity, duration and composition. 

First, glucose is used in maintenance and then, if there is some spare, growth. Hence, growth increases with more light and higher temperatures, and slows with less light and lower temperatures.

If a plant has insufficient glucose for maintenance, it’s cells break down and it eventually dies. To combat this, you can either raise light levels to increase glucose production or reduce night time temperatures to reduce glucose consumption.

Cooler night time temperatures benefit many plants, as it is in this period they exchange gases and release water. If temperatures are too high, a plant may fail to open its pores, causing it to suffocate/overheat. Switching your heating off at night will benefit most plants.

House plants from tropical and subtropical climates, where there are small variations in day/night temperatures, and equatorial climates, where there is no seasonal fluctuations, but not those from deserts, where the nights are very cold, are liable to chilling injury. These plants are simply not adapted to low temperatures and can’t change their respiration rates. 

Sudden exposure to cold is worse than gradual and can lead to rapid chilling injury, so try to keep temperatures consistent. Keep your plants away from draughts and place in rooms where temperatures fluctuate the least. Don’t water tropicals with cold water. 

Nutrition 

Nutrition is important, but if you have a problem with your plant, it’s likely down to sub-optimal lighting, humidity, watering, or temperature. 

Diagnosing nutrient deficiencies from visual examination is challenging and soil testing is necessary for certainty. There are five symptoms caused by nutrient deficiencies: stunted growth, chlorosis (yellowing of the leaves), interveinal chlorosis, purple-red colouring and necrosis, although these are also caused by other factors including nutrient toxicity. 

The development of symptoms help rule out certain factors. Some nutrients are immobile and can’t travel from old to new growth, while others can. Thus, if the symptoms are localised, it will be an immobile nutrient, but generalised a mobile one. Montana University have developed a useful identification key from such information, on pages 6 and 7. 

Just like with watering, you can over-fertilise, as so nutrients become toxic. Build up appears as a whitish crust on the soil’s surface and around the rim of the container. (Watering with tap water will cause this also.) To remedy, flush the soil by gently watering, ensuring water flows out of the bottom. Like with deficiencies, the symptoms of toxicity are examined on page 13. 

Due to the complexities of diagnosing nutrient deficiencies, it’s best to follow rules of thumb. For example, if you want to grow your plant, you’ll want to apply fertiliser, but also put in a well lit area, as plants produce most of their energy from photosynthesis. Vice versa plants in well lit areas will need fertilisation. 

You’ll want to apply fertiliser in or before the growing season, depending on whether or not the fertiliser is slow release. During or before dormancy, fertiliser is unnecessary. Fertilisation should be based on the amount of soil and size of the pot and the species. Fertilising slow growing succulents is unnecessary, but is useful for fruit maturation (i.e. with chillies). Fertilise after watering and never before, as water will wash away nutrients, but is necessary, as most nutrients absorbed is dissolved. 

Repotting

Key is not to compact the soil as compaction will stop air and moisture reaching plants’ roots. Contrary to popular belief, you don’t need specialist compost, although it’s preferable. Different materials exert different effects on plants. Terracotta is famously porous and drains faster than non-porous materials. Recently, I planted my chillies in a mix of terracotta and plastic pots, and made the mistake of watering them at the same time. The terracotta pots’ soil would be bone dry and the plastic pots’ moist. 

Acclimatisation 

Every plant will adapt to its conditions, but adaptation takes time. Commonly, house plants are moved outside in the summer months, put on a flush of growth, and are then moved back indoors, and start to drop leaves. The plant simply can’t produce the energy to support the sun leaves and will produce less energy intensive shade leaves. Changing light levels is rarely fatal, but changes in temperature is. Outside winter temperatures are too low for many house plants, so ensure you house doesn’t get too cold. 

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Jorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

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