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Here’s a concern that’s come up during one of Nick Flax’s numerous calls with growers across North America: How to beat the heat — or at least manage the problems it can cause — in your garden mum crop.

Problem: For the last couple of years, North American garden mum growers have gotten hit by heat waves, and it seems this year is no exception! With 100°F temperatures hitting more half of the United States, anyone with a local forecast that looks like this needs to pay close attention to their mums over the next week or so.

Nick’s Tip
Extreme temperatures create a host of challenges and stressors for mums—particularly in the rootzone. While symptoms of stress in the canopy are more obvious at first glance, it can be difficult to know what’s happening below the soil line without regular scouting and paying close attention to detail. Keep the following items on your radar in the coming days and act quickly to keep your crop happy through the worst of this weather.  

Q: New Mums Are Wilting … Is It a Root- or Shoot Concern?

Extreme heat and the physiological stress that comes with it can cause plants to wilt. The thing to pay attention to is: How severely are your mums wilting and how quickly do they recover?

Newly transplanted mum liners that still lack robust root systems are particularly vulnerable in this situation. Incipient wilting (sometimes called “flagging”), where plants appear to just barely start to wilt, is not uncommon for fresh transplants struggling to keep up with the weather.

If you see incipient wilting in a newly transplanted mum, be sure to check the roots and crown right at the soil line for symptoms of disease. If roots are discolored or you discover any lesions on the stem near the soil line, act accordingly. Check out some of our quick guides below on common diseases in garden mums and how to manage them appropriately.

·      Pythium

·      Fusarium

·      Botrytis

·      Bacterial leaf spot

If plants are fully turgid in the morning, show signs of incipient wilting midday or in the late afternoon, recover fully at dusk, and lack disease symptoms, they should be just fine once we’re past the extreme heat.

If your production infrastructure allows it, consider “syringing” the canopy (quenching the canopy with a fine mist) a few times per day to help reduce high heat-related stress. 

Keep in mind that prolonged leaf wetness can increase the chances of foliar disease, so it’s critical that water droplets are very fine and foliage dries within an hour. Repeat as necessary during the hottest part of the day.

Some varieties are more sensitive to high heat than others, and it’s not unusual to see mild discoloration or necrosis along the leaf margin under extreme heat stress. 

If you see a high percentage of one or two varieties showing these types of symptoms but roots look fine, consider making a foliar fungicide application after dark when the crop cools off a bit. 

This will help protect the canopy from opportunistic pathogens that may try to jump onto compromised leaf and/or stem tissue.

Q: Are Roots Only Growing On One Side of the Pot?

In the early, hottest part of the crop cycle, mum roots often grow on one side of the pot and avoid the other. A couple of factors can affect this but, ultimately, the underlying reason is the same: HEAT!

In outdoor production systems, the south-facing side of the container will almost always have fewer and/or less-robust roots. In North America, this side of the pot has the most direct sun exposure in summertime, and radiant heat from the sun soaks into black plastic pots like a sponge.

If soil temperatures in black plastic pots can reach 126°F in northern Minnesota in the summertime, rest assured—soil temps can get even hotter the further south you are. Stress and damage to roots can start to occur above 86°F in many crops, and excessively high soil temps are often responsible for these “voids” in our mum rootzones.

While it can be labor-intensive (especially if you grow thousands of mums), a periodic rotation of pots can help minimize this effect. This keeps one side of the plant from getting pummeled by high soil temps for the whole crop cycle and allows roots to grow back into the voided area.

Another consideration is drip-irrigation emitter placement. If drippers can be placed strategically in a more southerly location in the pot, delivery of water to this area of the rootzone first whenever you water can help combat excessively high soil temps.

Watch Your Soil EC

Mums are high-feed crops and grow best at higher substrate EC (electrical conductivity; a general measure of how much fertilizer/soluble salts are available to plants). In general, it’s best to target a substrate EC of about 3.0 mS/cm or higher during the first half to two thirds of the production cycle to achieve optimal growth. As such, many growers provide constant liquid feed (fertilize every irrigation at around 200-250 ppm N) or alternate fertilizer at high concentrations (more than 300 ppm N) with clear water applications.

However, under extreme heat, uptake of nutrients from the soil often slows and media dries down more rapidly. Both factors can lead to fertilizer salt buildup in the rootzone, which can, in turn, damage roots and open the door for soilborne pathogens. Be sure to spot-check substrate EC levels in your mums and add extra clear water irrigation or two as needed to ensure your roots stay safe from harm!

CHECK OUT THIS VIDEO I made a while back on how to conduct a pour-thru if you aren’t sure what’s the quickest and easiest way to check soil EC.

Have more questions for our technical services team? Reach out to us!