Dahlias: Why September Can Change the Crop
Dahlia Season in the Mid-Atlantic
Late summer and early fall can be an important production period for field-grown dahlias in the Mid-Atlantic. Penn State Extension scouting updates have documented Pennsylvania dahlia fields at peak production in early September, but the timing and duration of that production window can vary considerably among farms, cultivars, and years.
By this point in the season, plants have developed substantial canopies and established root systems, and many plantings are carrying both marketable flowers and developing buds. Maintaining production through the remainder of the season requires continued attention to irrigation, crop health, pest pressure, sanitation, and changing weather conditions.
Recent field research also reinforces an important point for growers: performance late in the season is not simply a response to cooler weather. Water availability, cultivar, plant health, and environmental conditions all influence when dahlias flower and how many marketable stems they produce.
Water Demand Remains Important During Flowering
It can be tempting to reduce irrigation substantially once September brings cooler temperatures. While supplemental irrigation requirements may decline as reference evapotranspiration decreases or rainfall increases, an actively flowering dahlia crop still has significant water requirements.
Recent research from Utah State University provides useful field data on this relationship. During 2023 and 2024, researchers evaluated irrigation treatments in field-grown 'Boom Boom White' and 'Café au Lait' dahlias. The study included 500 plants each year and compared five irrigation levels while monitoring soil moisture, evapotranspiration, flowering, marketable yield, and tuber production.
Crop coefficients used in the study increased as the plants developed, from approximately 0.7 during establishment to 0.7–1.0 during vegetative growth and 1.15 during the reproductive stage. This indicates that crop water use relative to reference evapotranspiration remained substantial as the plants moved into flowering.
Water availability also affected flowering and marketable stem production. In 2023, plants receiving full irrigation flowered as much as two weeks earlier than plants receiving the lowest irrigation treatment. In 2024, the difference was smaller, with well-watered plants flowering approximately three to seven days earlier.
The effect on marketable yield was also notable. Fully irrigated plants produced approximately six more marketable stems per plant than the lowest irrigation treatment in 2023 and approximately three more stems per plant in 2024.
For growers, the takeaway is not that irrigation should remain unchanged from August through fall. Irrigation needs to follow current crop and weather conditions. Cooler temperatures and lower evapotranspiration may reduce the amount or frequency of supplemental irrigation needed, while rainfall may provide part or all of the crop's water requirement. However, reducing irrigation simply because the calendar has moved into September can create moisture stress during an important period of flower production.
Check soil moisture within the active root zone and adjust irrigation according to rainfall, soil type, drainage, crop size, and current weather conditions.
Cultivar Makes a Difference
The weather is only part of the explanation when one dahlia planting begins producing heavily while another remains relatively slow. Recent University of Maine research evaluated 14 commercially relevant dahlia cultivars under field conditions. Cultivars were selected with input from commercial growers, and the project was supported by the Association of Specialty Cut Flower Growers Research Foundation. Researchers found considerable differences in flowering time and stem characteristics among cultivars. 'Rock Run Ashley,' for example, began flowering 35 days earlier than 'Tempest' and 'Café au Lait,' the latest-flowering cultivars in the trial. Total flower production was more similar among cultivars than their flowering dates might suggest, demonstrating how differently production can be distributed across the season.
That distinction matters when evaluating crop performance in August and September. A planting that appears late compared with another field, farm, or cultivar may not necessarily indicate a production problem. Genetics can have a substantial influence on when the harvest window begins and how production is distributed through the season.
For commercial growers, keeping records of planting date, first harvest, peak production, marketable stem count, stem length, and the end of harvest by cultivar can provide valuable information for future crop planning.
Plant Health Can Affect Marketable Yield
Late-season production also depends on maintaining healthy plants. The Utah State irrigation research included observations of virus-infected plants. Infected plants produced up to four fewer marketable stems per plant and tubers weighing up to approximately one pound less than healthy plants. Interestingly, infected plants flowered as much as four days earlier. Researchers suggested that the earlier flowering may have represented a stress response rather than improved crop performance.
This is a useful reminder that earlier flowering does not necessarily equal greater productivity.
Virus diseases are an important concern in dahlias because infected plants cannot be cured and vegetative propagation can carry infection into future crops. Penn State Extension documented Tomato Spotted Wilt Virus in a Pennsylvania dahlia sample submitted to the Penn State Plant Disease Clinic in 2024. Other viruses reported in dahlias include Dahlia Mosaic Virus, Tobacco Streak Virus, and Cucumber Mosaic Virus.
Symptoms can include chlorosis, mottling or mosaic patterns, ringspots, distortion, and stunting. Symptoms alone should not be considered a definitive diagnosis because nutrient disorders, herbicide injury, insect feeding, and other stresses can produce similar appearances. When virus infection is suspected, diagnostic testing can help determine the cause. Plants confirmed to be infected should not be retained as propagation material.
Continue Scouting for Thrips and Mites
Peak flower production is not the time to stop scouting. Thrips remain important because their feeding can directly reduce flower quality, and some species can transmit viruses, including Tomato Spotted Wilt Virus. Flowers should be inspected regularly, particularly when damage or distorted growth is observed.
Two-spotted spider mites are another important dahlia pest, particularly following periods of hot, dry weather. Populations established during summer may persist into the late-season crop.
Penn State Extension recommends examining the undersides of leaves for mites and looking for characteristic stippling or bleaching of foliage. Heavy infestations may result in webbing and reduced plant quality. A hand lens is useful for scouting, and foliage can also be shaken over a white surface to help detect moving mites.
Scouting should include the lower and interior canopy, not just the flowers being harvested. Problems are often easier to detect there before symptoms become obvious throughout the plant.
Flower-Feeding Insects Can Still Affect Marketability
Late-season insect activity can also affect flower quality. Penn State Extension documented brown marmorated stink bug feeding in Pennsylvania dahlias during September 2024. Feeding injury resulted in ragged petals and reduced marketability, with apparent differences in feeding among cultivars. Penn State's September 2025 scouting observations also identified several insects capable of feeding on dahlia flowers, including Japanese beetles, rose chafers, Asiatic garden beetles, and stink bugs. For cut-flower growers, the economic threshold for flower-feeding insects can be low because even relatively minor petal injury can make an otherwise marketable stem unusable. Continue inspecting flowers during harvest rather than assuming insect pressure has ended as summer temperatures moderate.
Changing Weather Means Changing Management
September management should not simply be an extension of the August program. Temperature, rainfall, evapotranspiration, day length, canopy density, and the duration of overnight leaf wetness are all changing. The crop itself is also changing as plants mature and flower production increases.
Irrigation schedules should be reassessed rather than continued automatically. Maintain adequate soil moisture while avoiding prolonged saturation. Where possible, irrigation timing and delivery should minimize unnecessary foliar wetness.
Dense mature canopies also deserve attention. Scout inside the planting and examine lower foliage for disease symptoms, declining leaves, insect and mite activity, and areas that remain wet for extended periods. Remove declining flowers and diseased or senescent plant material when appropriate to improve sanitation and make scouting easier.
Good recordkeeping becomes particularly valuable at this point in the season. Note which cultivars are producing heavily, which began flowering late, which maintain useful stem length, and which show repeated problems with insects, disease, or flower quality. Those observations can be as useful for next year's cultivar selection as the appearance of the flowers themselves.
Make the Most of the Remaining Harvest
Late summer and fall can represent a substantial portion of the annual dahlia harvest, as it can also be one of the most used florals of weddings and events. Penn State Extension's recent Pennsylvania scouting observations, together with field research from Utah State and the University of Maine, show why growers should continue actively managing the crop rather than treating September as the beginning of shutdown.
Water availability can affect flowering time and marketable stem production. Cultivars can differ by several weeks in their onset of flowering. Virus infection can reduce marketable yield. Thrips, spider mites, stink bugs, and other flower-feeding insects can remain economically important while the crop is still being harvested.
Harvest frequently while stem quality remains acceptable. Continue scouting both flowers and foliage. Adjust irrigation according to actual soil moisture, rainfall, evapotranspiration, and crop demand. Remove declining plant material and monitor plantings for symptoms that warrant further diagnosis.
As fall progresses, monitor local forecasts closely. The timing of damaging cold varies considerably with location, elevation, topography, and individual farm microclimates. Short-term forecasts are therefore more useful for harvest decisions than relying on a single regional "first frost" date.
The goal during these final production weeks is straightforward: maintain plant health and flower quality, respond to changing conditions, and capture as many marketable stems as the crop and weather allow before the damaging cold ends the season.
