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Hardy Chrysanthemums (Garden Mum) Care

Hardy chrysanthemums (garden mums) are plants from the daisy Asteraceae (Compositae) family and are indigenous to Asia and Europe. Comprising over 200 different species, the genus includes perennials, annuals, and small shrubs.

These plants became a part of floriculture in North America when they were introduced during the late 18th century, by the start of the 1900s, several hardier varieties were being cultivated and after the introduction of Korean hybrids, these plants became popular garden plants. Most hardy chrysanthemums at present have been developed by these Korean hybrids.

Plant sizes vary, from 4 – 36 inches tall and 12 – 36 inches wide. The flowers come in several colors like pink, red, yellow, orange, maroon, green, purple, bronze and white. The small seeds are black and can be collected from flowers that naturally die on the plants.

These plants grow rapidly, blooming in their first growing season, depending on the amount of light they get – shorter days and longer nights in early September to mid-October trigger flowering.

While this occurs naturally when growing outdoors, this can also be induced by cultivating them in greenhouses and keeping the plants in darkness for 9 – 12 hours every day, for 8 – 12 weeks or more. The number of days for extended darkness depends on the needs of specific varieties or types. This method allows cultivators to produce flowering plants at any time of the year.

There are many choices when it comes to growing these plants. They can be cultivated in gardens and, due to their shallow rooting habits, they can easily be dug up and moved to other locations. They also grow well in containers since they can be moved to shaded or sunny locations. This is an advantage because the darkness is needed to set their flowering schedule. 

Container plants usually last for one year, while those growing outdoors in the soil in USDA zones 5 to 9 often survive for 3 or 4 years before they die. 

While these plants can be grown indoors, they need more care and maintenance. 

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Sunlight

They thrive under full sunlight but can do with afternoon shade in warmer climates. They flower profusely if they grow in full sunlight. Since they are photoperiodic plants and set buds according to light and darkness, you should avoid confusing them.

Plant them where they won’t be exposed to any bright light at night like from patios or windows or even streetlights. If you grow them indoors, they need sufficient light in the day and darkness in the night.

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Water

They need plenty of water. Keep the soil always damp when they are growing, then reduce this to twice or thrice a week when flower buds appear and the flowering season begins. Don’t let them get dry or wilt – increase watering if this happens, usually during hot summers. Water at the base of the plants and not the foliage, wet foliage can cause several diseases.

When growing them in containers, water them until excess water starts draining from the bottom. Soil should be damp yet not water-logged since this creates conditions for root rot. 

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Humidity

They prefer moderate humidity. They require good air circulation if humidity is high to avoid rot or disease.

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Temperature

Once these plants are established, most varieties are cold hardy to around -20°F and can tolerate hot temperatures up to 90°F but they will need lots of water and some shade.

However, temperature control, while important when it comes to flowering, varies among the different varieties – low temperatures below 55°F can affect some varieties while temperatures of 85°F or higher will delay flowering (heat delay).

They grow best under moderate temperatures, extreme heat will make the plants struggle, and cold areas that undergo hard winters can make the plants die from cold unless they are well-mulched. 

Plants growing in warmer climates will deal with problems from heat delay, where high temperatures even at night, can make the plant flower later than usual. Heat delay also causes deformed flower buds, inconsistent flowering, and deformed plant crowns among other growth issues. The only solution is to find heat-tolerant varieties to cultivate. 

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Soil

While they are tolerant of several different soil types, they grow well in rich and well-draining soil. As with most plants, poor soil drainage often causes rot. They also prefer a slightly acidic pH around 6.5 to 6.7. 

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Repotting

These plants should be divided after 2 years or so to prolong their longevity. Repotting newly purchased plants is necessary as most are root-bound, making it hard for the soil to retain water.

To report, select a container a little bigger than the previous container with good draining capacity. Fill 1/3 of the container with potting soil. Take the plant out from the old pot and detangle the roots if possible, but don’t damage them.

When transferring to the new pot, ensure the plant crown is one inch under the pot edge. Add soil and tamp down gently, water until water drains out from the bottom.

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Propagation

These plants are easily propagated by seeds, cuttings, and division, with the division being the more straightforward and faster method.

Propagation by seed

Seeds can be collected from existing plants, but it is better to use purchased seeds since most varieties today are hybrids and will not resemble the parent. If you don’t mind surprises, then go ahead. Start seeds indoors around 8 weeks before spring by preparing a seeding tray or several pots filled with potting soil.

Sow and lightly cover the seeds water the soil, keeping the tray or pots under appropriate temperature and lighting conditions, with enough moisture. Germination will start in 10 – 15 days. Once the plants grow 6 – 8 inches high, transfer them outdoors. Plants grown from seeds take around 4 months to flower.

Propagation by Cuttings

This is a good method to create a copy of your plant and is usually done indoors in spring. Cutaway a 4-inch stem, dust the end with rooting powder and insert it in a pot of regular potting soil. Moisten the soil and keep them under appropriate moisture, temperature, and lighting conditions. Roots will take about 4 weeks or so to form. Wait for the plant to grow another two inches before transferring it outdoors.

Propagation by Division

Only divide two-year-old established plants in spring, as younger plants will not have developed a root system strong enough to survive. Over time, the roots at the center of the plant usually become old and woody, while outer roots are younger and healthy. This is easy to spot as growth in the center will be less vigorous. 

Division should be done in spring, once the plant is at least 6 inches tall. Gently dig up the plant and break it apart into smaller sections with care. The woody center can be discarded, as it won’t grow as well as the young outer sections. Plant them in the garden or individually in containers.

Additional Care

The plants need to be fed nitrogen and potassium during their growing period before flower buds appear to induce healthy roots and bud development. A slow-release granular fertilizer (12-6-6), applied once in spring will provide the plants with nutrients for around three months. 

Prune down the plants from late spring until mid-July to promote bushy foliage, increase the production of buds and flowers, and delay flowering until autumn. Deadhead spent flowers to help new buds take their place. After the flowering season is done, prune down the plants to about six inches high and cover them well with straw or any suitable mulch to safeguard the roots during winter. 

Common Problems

These plants can get damaged by aphids, spider mites, and thrips. Signs of pest infestation include foliar and stem damage and webs. Leaf miners and earwigs can also be a problem. Blast them off with water or insecticidal soap spray. Slugs and snails can be handpicked. 

These plants grow at their best in autumn, when the weather becomes cooler and wetter than normal. These moist conditions can invite mold and fungal problems. Fungicide can help fix these problems but remove infected foliage.

Diseases including Botrytis, leaf spot, stem and/or root rots, rust, powdery mildew, aster yellows, verticillium wilt, and viruses can also attack the plants. If you notice visible damage or the plants failing to thrive, any one of these diseases could be the culprit. While powdery mildew and leaf spot are seldom fatal, plants afflicted with other diseases must be destroyed.

Some viruses like chrysanthemum smut are spread by insects. Tell-tale signs include yellow, brown, or dropping leaves and stunted growth. This cannot be cured or treated and the plant has to be destroyed to prevent it from spreading.

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