Garden Mums

Each year, weather-related production problems challenge mum growers. Here are some situations that have occurred over the past few seasons and how growers responded to correct those problems. These tips may help prevent a problem or two with your crop this season.

Cool Temperatures

Understand premature budding
Chrysanthemum flower buds initiate easily and develop rapidly, especially if plants are stressed in any way. Lack of adequate water and fertilizer are two common causes for premature budding. Also, plants grown outdoors rely on natural daylength and temperature to control the timing of the crop. Chrysanthemums are short-day plants. Both flower initiation and development of the flower buds occur more rapidly under short days than under long days. However, temperature has a greater influence than daylength on flowering of garden mums. Several cool nights in a row can cause garden mums to initiate buds prematurely, which results in early flowering of the plants.

Sometimes a period of cool temperatures causes plants to begin the flowering process; however, when temperatures become warm again, flower development stops and the buds fail to develop properly. This results in “crown buds” which are distinguished by strapped, shaped leaves.

If premature budding occurs early in production, buds should be pinched off and adequate moisture and fertilizer supplied. The plants will almost always continue to grow and develop into a quality fall crop.

Prevent premature budding
Water-in freshly planted cuttings with a fertilizer solution containing 200 to 300 ppm of nitrogen immediately after planting. The cuttings will establish faster and grow more rapidly.

Do not stress the young plants during their first four to five weeks of growth. Keep plants moist, well fertilized and properly spaced, especially during the first 10 days of the crop.

Always use “hard” pinches, rather than “soft” pinches if pinching mums. Remove at least a half inch of new growth when pinching. This helps to minimize premature budding. Often, only five to seven leaves are left on the plant after the first pinch.

Choose varieties carefully. Many of today’s cultivars have been bred so that they branch naturally and do not need to be pinched. This technique is successful if planting actively growing cuttings and fertilizing them heavily, especially during the first few weeks of production. It is important to avoid water stress during this time. Water stress will result in hardened growth, fewer breaks and/or premature flower budding. If you change growing practices from two to one pinches or one to zero pinches, experiment on a small scale first. You may also want to plant a week later, since less pinching can result in more growth/plant size.

Overcome premature budding
During 2009, cool temperatures during the summer resulted in crown budding in July. Some growers responded by pinching buds off of plants and increasing the frequency of fertilizing using higher rates (350 ppm nitrogen) to encourage plants to grow larger and flower later. This did not always work for very early and early flowering varieties.

Some growers left buds on plants and fertilized with up to 400 ppm nitrogen of 20-10-20. Some growers also included ammonium forms of nitrogen such as 20-20-20 or urea in their fertilizer program. Their theory was that vegetative growth would bypass the budded growth. This worked for some growers while, for others, it resulted in uneven plants. Uneven plants were short, budded or flowering plants surrounded by some tall growth, but not enough to fill out the plant, creating two-tiered plants. The plant’s response varied by the variety.

In a normal year, mums should receive constant feed of 300 ppm nitrogen from fertilizers like 20-10-20 to promote vegetative growth. Reapply up to 400 ppm after each rain to keep sufficient nutrient levels.

Eliminate short plants
Short plants are also a result of plants setting bud too early. In chrysanthemums, flower bud initiation marks the end for vegetative growth. Garden mums initiate leaves rapidly. Cool weather early in the growing season will reduce the number of leaves formed on each shoot, thereby resulting in plants with fewer nodes and shorter plants. Another reason for smaller, harder plants may be due to plants experiencing stress due to dryness. Plants must never be allowed to wilt during the early stages of growth. Wilting in the first few weeks of growth restricts branching action and overall growth.

High temperatures and fertility
While cool temperatures prevail some years, during other years, high temperatures over an extended period of time have created problems. This weather pattern has caused controlled-release fertilizer (CFR) to release early, burn roots and weaken plants. Growing media temperatures over a period of two weeks soared in plants grown on black landscape cloth in open growing areas. Soil temperature is a primary factor affecting release of fertilizer from the prills.

Plants can also go from being over-fertilized to under-fertilized due to overwatering during hot periods. The controlled-release fertilizer releases during the heat and is leached out with excessive watering. If not detected early, plants will become nutrient deficient with short, hardened growth.

To prevent nutrient-related problems, growing media of chrysanthemum crops should be regularly tested. Controlled-release fertilizers should be used at the low rate in a combination feed program to supplement water-soluble fertilizer applications. This strategy will produce crops that maintain better color for your customers. A medium rate is best for a 100 percent CRF fertilizer program without any other supplemental fertilization under most conditions. The longevity of the CRF product should be matched with growing temperatures and desired delivery time. Choose longer-term release CRF products to extend nutrients for customers. Most mum growers in the United States achieve good results with five- to six-month or eight- to nine-month longevities.

Don’t overwater
Fall mums will often exhibit signs of wilting during extended periods of 90-plus degree temperatures. The solution may not be as easy as turning on the irrigation. Fall mums wilt when the soil is dry, but wilting will also occur in hot weather when plants are stressed, or if the roots are damaged from a root disease such as Pythium, even if the soil is saturated with water. When the roots stop functioning normally, the plants will show signs of stress by wilting.

Fall mums can survive excess amounts of fertilizer and resulting soluble salts when temperatures are normal and soil moisture is maintained. Problems occur when the growing medium is dry and the salts become more concentrated, resulting in root damage. The plants will react first by wilting and then collapsing.

Take precautions to avoid overwatering garden mums, especially during extended periods of hot weather.  Lift the pots and check the roots periodically. If the pots feel heavy and the soil is saturated, do not irrigate, even if it is hot. However, do not allow plants to dry to wilt prior to watering.

If plants are wilting on a regular basis and excess amounts of fertilizer have been used, the roots may be damaged and root disease will occur. Check the plant roots, especially with slower growing varieties. The roots tell a lot about a plant’s health, often before the top growth shows symptoms.

Signs of poor root health are blackened or rotted roots or lack of roots. Diseased roots will not take up fertilizer compounding the problem with high soluble salts.

Pythium aphanidermatum is the species of Pythium most commonly causing root rot losses to mums in recent years. The pathogen thrives in hot, summer weather. Avoid puddling around containers, deep planting or overwatering. Provide even moisture and avoid high EC values. Root stress is likely to open the plant up to Pythium attack. To manage Pythium, utilize etridiazole-containing fungicides (Truban, Terrazole, Banol) at monthly intervals. An occasional treatment with mefenoxam (e.g. SubdueMAXX) is advisable in rotation with the etridiazole, but beware of using mefenoxam exclusively because of widespread fungicide resistance issues with that active ingredient.

Other Factors

Combat misshapened plants
Although not weather related, misshapen finished plants are a problem that can be prevented during production. Garden mums grown too close together will be tall, but poorly shaped, and often flat on one side. Space so plants barely touch each other when flowering. Ball’s guideline recommends 8-inch pots have a final spacing of 9 inches to 18 inches on center. Larger plants to be marketed in garden centers may require 24-inch spacing. Garden mums grown too close together may also develop a “stove pipe” appearance.

Combat chrysanthemum white rust
Chrysanthemum white rust (CWR) is a quarantine significant pest in the United States; therefore, occurrence of this disease leads to state and federal regulatory action. CWR has been an issue for growers for the last few years. The weather conditions in the Northeast and Western regions of the United States in certain years have been ideal for the development of CWR on the hardy mum crop. The symptoms of chrysanthemum white rust are very distinct; light green to yellow spots up to 5mm in diameter appear on the upper surface of the leaf. These spots become brown and necrotic with age. Raised beige to pink pustules form on the underside of leaves beneath the spots. Pustules become white with age and are most common on young leaves and flower bracts but may form on any green tissue or the petals.

Infected plants may look normal until correct environmental conditions encourage symptom development. Hot and dry weather may delay onset of symptoms for up to eight weeks. Symptoms usually occur during cool, wet weather. Basidiospores are only produced and released during periods of high relative humidity (96 to 100 percent) and temperatures from 40°F to 73°F (optimum 63°F). A film of free water is required for infection and penetration.

In the fall of 2009, the Massachusetts Flower Growers’ Association and the Connecticut Greenhouse Growers’ Association proposed changes in CWR regulations to top administrators in USDA/APHIS. The process moved slowly but progressed to where a meeting was held in winter 2012 to have growers discuss and review a proposal to change CWR regulations to a Regulated Non-Quarantine Pest or (RNQP). This change would allow growers to deal with CWR as a common disease pest. CWR regulations would continue to stay in place for propagators and imported plant material would still be subject to inspection at the U.S. borders.

Paul Lopes, from the University of Massachusetts, and a grower attended the meeting representing the Massachusetts Flower Growers’ Association and its members. As a result of the meeting, USDA’s Animal and Plant Health Inspection Service (APHIS) is expected to proceed with a notice in the Federal Register of the proposed changes in CWR regulations. The notice will invite public comment on the quarantine status of chrysanthemum white rust.

In addition to asking for public comment on all regulatory options, APHIS will continue communicating with its trading partners, including Mexico and Canada, as discussions proceed. Any actions taken will likely be lengthy and time-consuming. It will be important for interested growers to participate in the request for public comment and make their opinions known concerning CWR.

Tina Smith, extension floriculture specialist, UMass Extension Greenhouse Crops and Floriculture Program, (413) 545-5306; tsmith@umext.umass.edu.