We know how plants are healthy when we see what’s above the ground. However, the color, feel, and smell of roots can give us better answers about each plant’s health and condition.
Healthy roots should be white or very light-colored. Unhealthy roots are often brown, black, gray-brown, reddish-brown, or in some cases purple. Unhealthy roots are often slimy and produce a foul and pungent odor. In some rare cases, unhealthy plants can have white roots but with abnormal shapes.
Without healthy roots, it’s very difficult for plants to grow. Plants depend on their gardeners, who should know how healthy roots look in comparison to sick roots.
Plant roots that grow underground don’t green with photosynthesis or bloom with flowers. So, how do healthy roots look? To summarize, root color, feel, and smell can tell you almost everything.
Healthy roots, particularly the tips, are white or off-white. In some plants, healthy roots are semi-transparent but some can even be colorless. Moreover, although older roots may be light gray or tan, they shouldn’t be dark brown or black.
Healthy roots are firm, not soft or limp to the touch. They’re thick, robust, and long, reaching into the soil for nutrients and moisture. Mature potted plants often have roots that are so long they can hold the soil in the shape of the pot.
Healthy roots smell like the soil. The smell of anything like sulphur, decay, or rotting matter, is a sign of roots sickness.
PRO TIP: When roots become soft and rotting, prune them away so that new roots can take over.
Roots that easily crumble and fall off, in colors such as brown or black, as well as with a rotting or foul odor indicate an unhealthy plant.
White roots generally indicate a healthy plant (some exceptions below). Older roots may be a bit darker, but not black or brown, or soft with oozing liquid that smells bad – that’s a sick plant.
Some common colors that indicate poor health or toxicity in roots include dark brown, black, grey, red, or pink. Here’s a short list of what these colors mean.
Root dieback is the excessive branching (forking) or stubbed roots, followed by browning and eventual blackening that starts from the top and spreads throughout the root system.
This root disease is caused by the fungi Pythium ultimum or Pythium irregulare, often seen in groundwater or in extremely wet soil. On the other hand, dieback in houseplants can also be due to the Phytophthora cactorum fungus or the Phytophthora cinnamomi, a water mold.
There’s no known treatment for dieback. However, flamethrowers were used to contain the disease with some success in Australia.
FACTOID: In potted plants, root tips that turn brown indicate a nutritional deficiency or fertilizer burn (tip burn) due to salt buildup in the soil, too much fertilizer, heavy concentrations of pet urine, or due to winter deicing road salt.
PRO TIP: If you suspect fertilizer burn, flush the soil with running water several times, and use a low NPK fertilizer.
Root necrosis is when root cells darken, wilt, die or degenerate, shown by root tips turning brown then black, and the black color gradually spreads throughout each root. Necrosis is a symptom (not a disease) that opens the plant to attacks by pests and diseases.
Root necrosis is often caused by premature tissue death (natural death of tissue is called necrobiosis) due to injury; nematode infestation; disease from a virus, fungus, or bacteria; or lack of nutrients, such as when incorrect fertilizer results in nutrient deficiency or the wrong soil pH level.
PRO TIP: Clubroot is healthy white roots that are stunted, deformed, and turn into club-shaped growths that turn black (more details follow).
Root rot is when a healthy root turns mushy brown or pulpy gray, and the root tissues soften and ooze liquids with a foul smell. This can happen when roots are in too much water where fungus such as Botrytis cinerea can thrive.
Overwatering, poor soil drainage, waterlogging, insufficient light, lack of nutrition, or dense garden soil can also lead to roots rotting. When touched, roots fall off easily. The rot can spread to the healthy roots even when the soil has dried. While root rot can happen anywhere, it is quite common in indoor plants.
Root rot is when roots are stunted, show cankers or ooze smelly sap. The more common attacks on houseplants are often caused by the Erwinia bacterium, the Rhizopus or Phytophthora cinnamomi fungus (root rot). In some cases, root rot can kill a plant within 10 days.
PRO TIP: When possible, gently clean the roots under running water and prune all brown, soft roots with a sharp pair of scissors. Sterilize scissors and pots with 1 part bleach mixed in 3 parts water. Burn, disinfect or treat with cinnamon powder all affected roots and soil.
FACTOID: A white-rot fungus called Phellinus noxius causes brown root disease that kills plants by cutting off water and nutrients. Check out a fungus control spray such as this one on Amazon.
Root canker (or anthracnose) is an open wound infected by a virus, bacteria, fungus, or mycoplasma, usually in soil with poor drainage.
Decayed or rotting root tissue shows as moist, black, or reddish-brown lesions that are sunken and restricted by callus from adjoining tissues that are healthy.
Some cankers do not kill plants, but others can result in the death of the plant host. There is no available chemical control or cure for root canker. Recommended control strategies for plants with root canker include isolation, pruning, or being entirely destroyed by burning.
PRO TIP: To help your plants grow strong, healthy roots, make sure to provide phosphorus, potassium, and nitrogen.
Many indoor herb gardeners love to regrow onions on their window-sill. When growing red onions, it is common to overlook the pink onion root disease caused by the fungus Phoma terrestris which is more commonly found in roots that grow in soil.
Pink-colored roots make it easy to recognize this disease. The roots eventually turn purple or dark red and then begin to dry up and die. The disease can be spread by dirty water, pots or garden tools
Pink root disease is often seen in plants that are stressed by cold, drought, toxic chemicals, and pests, particularly in soils where cereals have been planted. This disease does not only attack onions; it can also destroy pepper, spinach, cereals, corn, and cucurbits.
As mentioned, the whitest roots indicate the healthiest plants. However, indicators such as root enlargement, over-bunched roots, and flattened roots mean that something is wrong.
Enlarged roots: Root enlargement are plant roots that are bigger than normal. (1) An enlarged root adjoining a constricted root area is sarcody, due to over-accumulation of food material in root crops. (2) Swelling roots can be galls (wart-like growths caused by a bacteria, fungi, or virus), knots (root knots caused by the Meloidogyne incognita, a parasitic nematode), or clubs (clubroot caused by the Plasmodiophora brassicae mold).
Overbunched roots (fasciculation): (1) When fine root hairs are more numerous than usual, it is called hairy root, shown by excessive clustering of branching rootlets. (2) Another example is called witch’s broom, which shows dense clusters of roots, like brooms.
Flattened roots (fasciation) refers to normally round roots that are broad or flat. Callus roots are formed around wounds or damage in roots (cankers). Stubbly roots are roots that are abnormally short and/or thick.
FACTOID: White root rot and shoestring root rot are caused by the Armillaria fungus.
How do healthy roots look in potted plants? In smaller containers, the roots of plants should be white or tan, numerous, long, and succulent, with no smell. Older roots can turn darker, but still white.
How can I tell if the roots of a plant are dead? Check carefully. If there’s no bad scent and there’s a little green anywhere in the roots, there’s life. On the other hand, dead roots are dry and brittle or are smelly and rotting.
Can I reuse soil that has root rot? No. Do not reuse potting mix or water drained from other plants. Soil and pots can contain fungus, bacteria, mold, and other pathogens that cause root rot. Use a commercial potting mix that’s certified as pasteurized.
Does hydrogen peroxide kill root rot? A 3% hydrogen peroxide (H2O2) solution around the base and roots of the plant can kill bacteria, aerate the soil, and help prevent root rot.
Before you go, here’s a summary of the top takeaways in this article:
Color: Darkly discolored roots usually indicate a dying or dead plant. For instance, brown roots and black roots can mean that the plant is suffering from root rot.
Smell: If the roots of a plant smell bad (like sulfur or rotten eggs), then there’s a fungal or bacterial infection, and cell or tissue deterioration. In short, you’ve got sick roots.
Feel: Roots that are thin and spindly, or are weak, crumbly, and break easily indicate that the plant is unhealthy and needs immediate attention.
You’ve just completed a thorough review of the colors of roots that indicate health or sickness. Happy gardening!
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