We’re thinking ahead about mum production in the heat of summer, which is always a challenge for growers.

PROBLEM: Every year, it seems like different regions in North America experience some extremely high temperatures in summer and that time is coming soon. It goes without saying that high heat can spell trouble for garden mums. Soon enough, someone reading this newsletter will feel 90F daytime temperatures, so this week’s tip is for any grower who might be wondering what to do to when the thermometer approaches triple digits. Consider sharing this with your production team now, so they are better prepared when the heat kicks in.

NICK’S TIP: A lot of research has focused on flowering heat delay, but let’s dig into other production concerns and discuss the pros and cons of strategies to manage these in extreme heat scenarios.

Quick Plant Physiology Review

All plants have optimal temperature ranges for growth and development, both of which slow or can be altered when temps are outside of this range. For garden mums, the optimum range for vegetative growth is about 65 to 75F (18.3 to 23.9C). When temps are below the low end of this range, vegetative growth slows, and flowering induces (this is why crown budding occurs in summer after a few cold nights). Above the maximum in this temp range, flowering delay (a.k.a. heat delay) can occur, but vegetative growth slows or stops completely once temps reach a certain threshold.

Growth & Development Concerns

Above about 86F (30C) is where growth and development of mums really start to grind to a halt. Stomata (gas exchange sites on leaves) close, and chlorophyll, enzymes, and other organelles can start to degrade. This means the internal “bio-machinery” of your crop is getting damaged and shutting down. So when temps exceed the upper 80s (30 to 32C) for extended periods be sure to take the following steps:

- Shade your mums (not to be confused with the term “shaded mums” – i.e., inducing early flowering using blackcloth) if growing in a greenhouse. Aluminized shadecloth will help reflect the heat load and help reduce the chances of physiological injury. Black shade netting over the top of your greenhouse or applying a white shading compound will also help reduce temps. If growing mums outdoors, shading options are limited. Shadecloth above the crop will be helpful, but black shade netting siting right on top of plants will likely increase the temperature due to the black material absorbing heat AND restrict air flow.

- Consider doing a few canopy “cool-down” sprays during the day. While there isn’t really any research that supports the effectiveness of this strategy in mum production, the physics behind evaporative cooling (phase change of water from liquid to gas absorbs heat) are undeniably true. If shading is not an option for your crop, trying this may be a better option than doing nothing and letting your mums cook in the extreme heat.

• If you attempt this strategy, spray down plants with cool water using the finest droplet size you can. Smaller droplets will evaporate more readily—even if humidity is high—and reduce addition of water to the growing media.
• If it has rained recently, additional misting will not be very effective and may just increase the risk of foliar disease. So only attempt this on dry, sunny days.
• Because increased foliar wetness increases the risk of disease, be sure to reapply foliar protectants at appropriate labeled intervals when the air temp cools down.
• Elevated temperature also means water in the growing media will be taken up and/or evaporate more quickly. Avoid letting plants dry down to the “flagging” or true wilting point under extremely high temps, or roots may be damaged and recovery delayed.

- Time your irrigations carefully. Mums do not like having soggy soil, so while you may feel watering three or more times per day is helping keep your crop cool, you may be causing more problems by starving the rootzone of oxygen. However, waiting until soil is at a moisture level 2 (on the 1–5 scale; 1 = air dry, 5 = saturated) or lower under extreme heat may make it impossible to keep up with the plant's demand plus evaporated water loss and result in irreparable wilting. Start on the wetter side and carefully increase time between irrigations to allow some dry-down. For example, wait until plants are at least at a moisture level 3 before watering again, rather than re-watering at level 4 or higher.

Disease Concerns

Stressed plants are always at higher risk for infection by various pathogens. Most diseases that impact garden mums prefer moderate to warm temperatures, but a couple of pathogens can cause major problems under extreme heat conditions. Also, damage incurred during extreme heat events can prime your crop for infection once temperatures return to normal, so be prepared.

Bacterial soft rots such as Dickeya and Erwinia/Pectobacterium often prefer hot temperatures and high moisture levels. If you start to see leaves and shoots browning and collapsing quickly, and symptomatic tissue has a strong, nasty odor, one of these bacteria is likely the culprit. Remove affected plants ASAP (throw away media, containers and all: Do not compost them!) and apply a registered bactericide to reduce spread.