Learn About Mint
Common Disease Problems
Anthracnose: Small water soaked spots on leaves and stems. Burpee Recommends: Avoid planting in the same location in the future, planting in containers can help. Prune plants to the ground in the fall.
Damping Off: This is one of the most common problems when starting plants from seed. The seedling emerges and appears healthy; then it suddenly wilts and dies for no obvious reason. Damping off is caused by a fungus that is active when there is abundant moisture and soils and air temperatures are above 68 degrees F. Typically, this indicates that the soil is too wet or contains high amounts of nitrogen fertilizer. Burpee Recommends: Keep seedlings moist but do not overwater; avoid over-fertilizing your seedlings; thin out seedlings to avoid overcrowding; make sure the plants are getting good air circulation; if you plant in containers, thoroughly wash them in soapy water and rinse in a ten per cent bleach solution after use.
Mint Rust: Small, whitish, slightly raised spots that turn reddish orange or brown. This occurs on the underside of the leaves. Burpee Recommends: Avoid overhead irrigation systems. Water early so the leaves have the whole day to dry out. Do not plant members of the mint family in the same location.
Powdery Mildew: This fungus disease occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl. Burpee Recommends: Avoid powdery mildew by providing good air circulation for the plants by good spacing and pruning. Contact your Cooperative Extension Service for fungicide recommendations.
Verticillium Wilt: Yellows begin turning yellow starting at the margin. Eventually, leaves will curl and die. Burpee Recommends: Rotate plantings, remove infected plants, do not over fertilize.
Common Pest and Cultural Problems
Aphids: Greenish, red, black or peach colored sucking insects that can spread disease as they feed on the undersides of leaves. They leave a sticky residue on foliage that attracts ants. Burpee Recommends: Introduce or attract natural predators into your garden such as lady beetles and wasps who feed on aphids. You can also wash them off with a strong spray, or use an insecticidal soap.
Cutworms: These insects cut off the seedlings at the soil level. Burpee Recommends: Place a paper cup collar (use a coffee cup with the bottom cut out) around the base of the plant. They are usually mostly a problem with young seedlings. You can also control by handpicking and controlling weeds, where they lay their eggs.
Flea Beetles: These small hopping beetles feed on plant foliage and spread diseases. Burpee Recommends: Rotate crops with plants in a different plant family. Use floating row covers to prevent damage to young foliage.
Spider Mites: These tiny spider-like pests are about the size of a grain of pepper. They may be red, black, brown or yellow. They suck on the plant juices removing chlorophyll and injecting toxins which cause white dots on the foliage. There is often webbing visible on the plant. They cause the foliage to turn yellow and become dry and stippled. They multiply quickly and thrive in dry conditions. Burpee Recommends: Spider mites may be controlled with a forceful spray every other day. Try hot pepper wax or insecticidal soap. Check with your Cooperative Extension Service for miticide recommendations.
Thrips: Thrips are tiny needle-thin insects that are black or straw colored. They suck the juices of plants and attack flower petals, leaves and stems. The plant will have a stippling, discolored flecking or silvering of the leaf surface. Thrips can spread many diseases from plant to plant. Burpee Recommends: Many thrips may be repelled by sheets of aluminum foil spread between rows of plants. Remove weeds from the bed and remove debris from the bed after frost. Check with your Cooperative Extension Service for pest controls.
Both herbs are hardy perennials, happy to grow outdoors. They may not be as well-suited to growing indoors.
Consider the following factors, one or more of which could be responsible for the black leaves:
Light: Adequate sun exposure is critical in maintaining herb health. It sounds like the mint and sage are getting enough sun, especially in the past month or so (it is late April).
Heat: However, if it’s too hot in the sunroom, the plants could be suffering from the heat. Herbs generally like temperatures of around 18-21 degrees C (65-70 degrees F) during the day. Also, the leaves should not touch the window, as they would be affected by cold winter temperatures or hot summer temperatures.
Water: Watering – either too little or too much – could be an issue. Mint likes a moist soil, and excessively dry soil could contribute to the leaves turning black. On the other hand, overwatering can also cause the leaves to rot and turn black. Between waterings, you can mist the plant or place the pot on a tray filled with pebbles, to which you add water.
As for the sage, this plant does not like soil to remain wet – after watering, let the top of the soil dry out before watering again.
Soil: Both herbs need to be in a well-draining soil, or they will be may be more susceptible to rot or diseases/pests. To increase the draining capacity of the soil, you could add sand or perlite to the soil mix. Consider repotting the plants if you’ve had them in the same soil for a few years. Remember too that clay pots permit better soil drainage than plastic or glass pots.
If the surface of the soil has areas that are white in colour, this is salt crystals, which the herbs do not like. Salts can contribute to turning leaf tips brown and dry/crisp. Fertilizer should help with this issue. As well, flushing the soil 4-5 times with water, permitting it to fully drain between flushes, will help get rid of the salts. I sometimes also remove the top few centimetres of soil, to get rid of the crusty salts, and replace this with fresh potting soil.
Fertilizer: Now that it’s spring, and the plants are actively growing, fertilize once every 1-2 weeks with organic fish emulsion or liquid fertilizer (e.g., 12-12-12). This should keep them healthy and less susceptible to diseases/pests.
Diseases/Pests: From your description, it does not sound like your plants are affected by insect pests. However, fungus could be a problem. Fungal blight affects weakened mint plants, causing stems or entire leaves to turn black and fall off. SF Gate’s Indoor Mint Leaves With Black/Brown Spots discusses additional diseases and pests that can strike mint. Note that fungal leaf spots can affect sage. If either plant is affected by fungus, discard the plant and soil (don’t compost it, as fungal spores may live on!) and to start afresh with new herbs.
Finally, keep the plants several inches apart, in case whatever affects one spreads to the other!
See also Missouri Botanical Garden’s Herb problems indoors.
Please write back and tell us how your herbs fare!
Mints belong to the genus Mentha and comprise approximately 20 species in the plant family Lamiaceae and are growm for their leaves which are widely used as a flavoring. Mint plants are mainly aromatic perennials and they possess erect, branching stems and oblong to ovate or lanceolate leaves arranged in opposing pairs on the stems. The leaves are often covered in tiny hairs and have a serrated margin. Mint plants produce a terminal flower spike and the flowers can be white or purple in color depending on variety. Mint plants are fast growing and can become very invasive. They can reach heights of 60–90 cm and will cpntinue to grow for many years once established. Mint may also be referred to by species and these include, but are not limited to peppermint, spearmint, water mint and Japanese mint.
Peppermint flowers ‹ ×
Mint leaves are used fresh or dried to make teas, jams and desserts. Essentila oil can be extracted from the leaves and is used as a flavoring.
Basic requirements Mint is a rapidly growing plant which is very easy to grow. It is best grown in partial shade to full sun and is generally very hardy, tolerating temperatures down to -29°C (-20°F). Care should be taken with variegated varieties which may scorch in full sun. Mint is very fast growing which can lead to it invading gardens quickly unless controlled. The best soils for planting mint are rich and moist with a slightly acidic pH between 6.0 and 7.0. Propagation Mint is readily propagated from seeds, cuttings or by dividing an established plant. Seeds should be planted in the Spring or in the Fall in areas that are free from frost. Seeds should be sown to a depth of 6 mm (0.25 in). Seedling should be thinned after emergence such that the plants are spaced 46 to 61 cm (18 to 24 in) apart. Established mint can be easily divided for transplanting by taking some branches along with a portion of root. Many people choose to keep mint in containers or sink the containers into the ground when planting to prevent mint from spreading uncontrollably. General care and maintenance Mint is very vigorous and should be pruned regularly to keep the plants in check. Remove any unwanted runners and pinch the tips of the plants back regularly. Mint may be fertilized in the Spring with a slow release fertilizer to supply it with nutrients throughout the growing season. Pinch of any flowers that form to conserve the flavor of the leaves. Essential oil content is reduced during bloom. In areas with mild winters, mint can be moved to a sheltered area of the garden to overwinter, otherwise the plant can be cut to the ground in the Fall. Container grown mint plants can be brought indoors. Harvest Mint leaves can be harvested as soon as the plants have reached 8 to 10 cm (3-4 in) in height. Cut leaves and stems with a sharp knife or scissors. If harvesting whole stems, cut the stem at about 2.5 cm (1 in) from the soil line.
Buckland, K. & Drost, D. (2009). MInt in the garden. Utah State Cooperative Extension. Available at: http://extension.usu.edu/files/publications/publication/Horticulture_Garden_2009-05pr.pdf. . Free to access.