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Indoor Plants. Environmental factors affecting plant growth. Light - most houseplants are understory plants Water – very tricky Nutrients- match to the needs of the plant and time of year Temperature – warmer days cooler nights ( tropic to subtropic ) Air – clean, not too dry.

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Environmental factors affecting plant growth
Environmental factors affecting plant growth

  • Light - most houseplants are understory plants

  • Water – very tricky

  • Nutrients- match to the needs of the plant and time of year

  • Temperature – warmer days cooler nights ( tropic to subtropic)

  • Air – clean, not too dry

Light photosynthesis
LIGHT/ Photosynthesis

  • Light descriptors: intensity, duration, quality

  • All green living plants need light to grow

  • When grown in too little light plants will be etiolated (spindly)

  • The brighter the light the more color develops in leaves ( coleus, begonia)

  • Flowering usually depends on light levels and day length

Light intensity
Light intensity

  • Light intensity is measured in Foot Candles, Lux or Lumens. 1 foot candle is the light cast by a standard candle at the distance of 1 foot.

  • Light intensity influences:

    • Photosynthesis

    • Stem length

    • Leaf size and color

    • flowering

Light intensity1
Light Intensity

  • Factors that influence intensity of sunlight

    • Curtains, trees in the landscape, uv filters on windows, weather, season, cleanliness of the glass, low E glass, paint color of the room…

    • Too much light can sunburn a plant

Houseplant classification
Houseplant classification

  • Houseplants are listed as High, medium or low light, and some plants can be in several categories.

  • In a Duluth house, the south windows with have the strongest sunlight, E and W = 60% and N=20%

  • The southern exposure also generates the most heat followed by W, then E, then N

Light duration
Light Duration

  • Most plants need some darkness at night 3-4 hours.

  • If the light in the house is a bit low longer hours of illumination can compensate.

  • Plants are generally divided into long day, short day and day neutral when describing initiation of flowering

Light quality wavelength
Light Quality= wavelength

  • Photosynthesis required red and blue light

  • Sunlight has all the right parts of the spectrum

  • Electric lights may or may not

    • Incandescent lights lack blue, and add too much heat

    • Fluorescent “Grow lights” are more expensive than they are worth

    • Cool white fluorescent or a mix of cool white and warm white are the best deal – 12 “ above plants

Coping with low natural light
Coping with low natural light

  • Choose “low light” plants: Philodendron, Pothos, Aglaonema, Cast Iron Plant

  • Rotate your plants into the better lit areas for a few weeks to rev them up

  • Put plants outdoors in summer in a “bright indirect” light place


  • Let your plant tell you- Observe closely and the plant will let you know when it is at “incipient plasmolysis” ( the leaves turn a little dull colored)

  • Check the weight of the pot, lighter=less water in the soil.

  • Scratch the surface if you see no damp soil down ½ inch, it may need watering


  • Plant roots do not function when a plant is overwatered ( the roots need Oxygen)

  • Symptoms of over or under watering:

    • Lower leaves and scattered leaves turn yellow from root rot ( over watering)

    • Oedema ( corky bumps on the leaves) usually on succulents when the plant is overwatered

    • Adventitious roots on the soil surface = overwatering


  • Leaves will wilt if the plant is in dry soil OR in saturated soil

  • Letting plants wilt then snap back when watered can cause slow growth, flower drop and leaf drop.

  • Make sure you have drainage in the pot- a hole in the bottom. Don’t let the pot sit in water ( in a full saucer)

Watering the general rules
WateringThe general rules

  • use air temp water

  • water until excess water comes out the bottom of the pot, then dump out the saucer.

  • Don’t use softened water

  • Some plants are bottom watered, however you should remove the pot once the wetness reaches the surface of the soil

Potting soil
Potting Soil

  • Potting soil needs more air than garden soil

  • A good mix contains a combination : peat, vermiculite, perlite, compost , sand, loam

  • Soil mix may contain nutrients ( plant food) but that will be used up in a month.

  • If your mix is mostly peat, it can shrink away from the sides of the pot and be heard to re-wet.


  • Plants make their own food from photosynthesis but they need a source of essential minerals. You will learn these later.

  • Most house plant fertilizers are water soluble salts

  • The label recommendations are usually too strong so use ½ of the label directions.

  • Fertilize most houseplants in spring and summer only


  • Time release fertilizers will last for several months

  • ORGANIC fertilizers take longer to become available in the soil and less likely to “burn”.

  • INORGANIC fertilizers are immediately available to the plant but more likely to burn.

  • Only fertilize when plants are actively growing.

  • Don’t fertilize a bone dry pot

Fertilizers are salts salts can accumulate in potted plants
Fertilizers are salts.salts can accumulate in potted plants

  • If you see a whitish or yellow crust on the surface of the soil it is probably a salt buildup

  • You can scrape off the top ¼ to ½ inch of soil and replace with fresh soil.

  • You can leach the soil in the pot with fresh water to remove excess salt( run water through the pot).


  • Most house plants like people temps 75 day and 65 night, most can adapt a few degrees either way.

  • Respiration (burning food) in plants happens 24hours a day and high temps speed up the process. Light allows photosynthesis to make more food.

  • Houseplants suffer when they have low light and high temps


  • Most flowering plants will have higher qualit flowers if they have cooler nights (55-60)

  • In Duluth the temps next to a window can be quite cold on a winter night, you may have seen house plant leaves frozen to a window.

    • Move plants farther into the room and off a cold floor

Cleaning plant leaves lets in more light
Cleaning plant leaveslets in more light

  • The leaves will get dusty

  • Wash with warm water and a soft sponge

  • Don’t use wax products or mayonnaise

Air quality
Air quality

  • Plants use CO2 and O2

  • Air circulation is beneficial

  • Gas leaks can kill plants, products of incomplete combustion ( ethylene) can make houseplants grow strangely or drop flower buds

  • If all your plants twist up at the same time –

    • It could be an air quality problem

Insect pests
Insect pests

  • Observe plants closely

  • Prune the worst looking parts of the plants

  • Spray plants with products that won’t kill you or your pets, insecticidal soap, Ivory soap, alcohol

  • Keep checking the plants because insect eggs are usually un affected by sprays