Growing dahlias in the heat and high humidity
I hope you are enjoying your summer. We have had a lot of rain in my North Carolina area. My soil was so saturated by July 23, 2025, and the humidity was so high that the water was not evaporating. The dahlias received more water than they really needed. It is hard to work in the dahlia field in the afternoon with temperatures around 90 degrees and a 73 to 76 dew point. My clothes were wet from perspiration! Lol. The dahlias are not enjoying it either. I look forward to less humidity in August. We had one of the hottest July on record. Raleigh, North Carolina had the hottest June and July on record. We have broken records for so many consecutive nighttime lows over 70 degrees.
This year I was able to start planting dahlias by April 18th. Our last average frost is around April 20th. I finished planting everything by May 20th. I planted 1133 dahlias by hand in the ground, 80 dahlias in raised beds, 40 dahlias in grow bags, and 103 dahlias from cuttings for a total of 1356 dahlias. I also planted 1464 first year dahlia seedlings. I had a delay twice because of the rain so I could not plant any tubers as I waited for the soil to dry out.
I added compost to my rows in February; however, I still added a handful of compost as I am planting dahlia tubers. I also mix in some Permatill in the compost to improve the soil structure and repel voles. Permatill is a brand name for a type of expanded slate soil amendment. It is used to improve soil structure, aeration, and drainage. I purchase it at my local hardware and nursery store. I also add some Espoma Plant Tone fertilizer that I mix into the top portion of the soil.
By the time I finished planting dahlias on May 20th, my first 4 rows of dahlias that I planted at the end of April already had stakes and netting. A lot of the plants had 5 sets of true leaves so I had to start pinching back the dahlia to produce a bushier well balance plant which will produce more blooms. It was so wet in May as we had rain every week; however, I had no idea we would have 3.5 inches of rain that last week of May. Rows 5, 6, and 7 suffered the most as I had just planted the tubers. I had to replace the tubers that rotten with other dahlia tubers. It was hot and humid in June so we continued to have rain each week. The first 22 days of July we had temperatures in the 90’s with thunderstorms almost every day or every other day. In July, our North Carolina area received 9 inches of rain making it the 8th wettest July on record. With the hot weather in the 90's that last week of July I thought my soil could finally dry out; however, with the high dew point, my soil was not drying out quickly. I have amended my soil with compost, and it is well drained; however, we are dealing with so much rain. If my soil was not well drained, I would have lost a lot more tubers.
If you feel your dahlias are struggling, you may want to do a soil test. I take soil tests every year or every other year. A soil test can help you determine if your soil is lacking essential nutrients or there are any pH imbalances. In North Carolina, we can send the soil samples to the NC Department of Agriculture. Our local county extension offices have soil test boxes and instructions.
I am thankful I did plant early so that the dahlias could become established as soon as possible. The more they are established in the ground, the better they can handle the heat and humidity. When I plant later, the plants struggle to grow in the heat and humidity. Some growers like to plant later so they miss Japanese beetle season or to delay blooms for later in the summer; however, that does not seem to work for me. The dahlias that I planted in late April are doing the best of all my dahlias.
Dahlias really do not like the high humidity of the summer. We want to have blooms in early summer; however, the dahlias really do not bloom their best until August and September. In my plant hardiness zone 7b, they will bloom all the way until we have a frost sometime at the end of October. Dahlias are more of a late summer and fall flower. Dahlias like it when the weather is less than 85 degrees and nighttime lows are in the 60’s. At night, they have time to rest from the daytime heat. During the early summer, they are growing and building a nice root system. Some of the early blooms do not always look the best in the summer due to the high humidity. The dahlia starts to open so quickly in this heat and humidity that we can have some green immature centers. When it cools down, the green centers go away. You may also experience the sun burning some of the petals or the dahlias suddenly losing petals due to heat stress. So if you have been experiencing this hot and humid weather for several weeks now, your dahlias probably have slowed down as they are in survival mode. Once we cool back down to average temperatures, they will start growing and blooming again.
When you are dealing with a summer heat wave, shade cloth does help reduce some of the stress on the dahlias. With our heat and humidity, I am so happy I have two shade structures with 30% shade cloth. The shade cloth really helps the dahlias survive these dew points in the 70’s. The plants are not as stressed as the dahlias in the full sun. I love the shade cloth because the color is more intense on the dahlias.
Shade cloth comes in various sizes. I purchased my shade cloth from Greenhouse Megastore online. The shade cloth edges are sewn with heavy duty reinforcing tape with brass grommets installed around the perimeter to assist with installation. Our shade cloth is around 9 feet tall for more effectiveness. We have black shade cloth as that is what Greenhouse Megastore had in stock for the 20x48 size we needed. Black reflects the UV rays better than lighter colors; however, white does reflect more light so it may keep the area cooler. You can feel the difference when you are standing under the shade cloth as it is cooler. I see several companies selling shade cloth online now. If the shade cloth does not have the grommets installed for easier attachment, you can purchase a grommet kit online. We have had our shade cloth for several years now, and they are holding up very well. I do see Greenhouse Megastore is stocking a 40% white grommeted shade cloth now so if I do have to replace my shade cloth, I will probably purchase that one.
Dahlias do need and like fertilizer just like most flowers. We have had a lot of rain this summer so nitrogen can wash out of the soil, so they need fertilizer especially when they are actively growing. Nitrogen is important for overall plant growth. If you notice your dahlias are not growing and you have not fertilized them, they would probably appreciate some fertilizer. If you have not had any weekly rain, dahlias do need weekly water especially if you are experiencing hot temperatures. I use fertilizer through the beginning of August in my area. The field starts really blooming in late August. I like to spray my dahlias with fish and seaweed fertilizer early in the morning about every 2 to 3 weeks. I do not spray it during the day as it can burn the leaves with applied in the sunshine so that it why I spray it early in the morning. You could also spray the foliage in the late evening. I like to use fish and seaweed products because they improve the overall plant strength. You can also add it to the soil to increase microbial activity. I like to use Vigor Plant Booster to feed the soil as it has molasses as an ingredient. I purchase the Vigor online at https://www.vigorfertilizer.com/
Some growers do apply unsulphured molasses to the soil around their dahlias to feed the microbes. The common dilution is 1 or 2 tablespoons per 1 gallon of water. When you are using fish products or molasses you are increasing the sugar level of dahlias. When you raise the sugar level of a plant, the insects do not find the plant as attractive to feed on as they cannot process that sugar content. I am hoping by using the fish and seaweed products often that I will not have to spray for pests. I have so many different varieties of dragonflies flying around my dahlias daily. I do not want to spray something that would harm them. Dragonflies eat mosquitoes, flies, moths, beetles, ants, wasps, etc. Unfortunately, they may eat a bee, but I hope they are eating mostly all the other insects that are not beneficial. I do see some damselfly and they are so small. I see the fragile forktail damselfly when I am weeding a dahlia. Dragonflies glide effortlessly down my dahlia rows. We do have a small creek in our woods, so I am sure they go down there. I just enjoy seeing them as they bring joy to my day.
Besides the fish and seaweed products, I use Espoma Plant Tone (5-3-3) which has feather meal, poultry manure, bone meal, and other organic ingredients. I purchase it at my local hardware and garden center. I have had my soil tested, and my phosphorus and potassium levels are good since I use a compost to amend my soil in the early spring. Espoma Plant Tone is a light fertilizer that I use when planting dahlias. I try to use it again in July to give the dahlias an additional boost to help deal with the summer heat.
It can be very hard to keep dahlias happy when the nighttime lows are in the upper 70's and dahlias do not have any time to rest. Sometimes, I do lightly spray my dahlias in the early morning with an overhead sprinkler to help hydrate them for the day when the temperatures are going to be 95 degrees. I only spray them for 5 to 10 minutes in the early morning depending on my soil moisture levels.
Some years our summers can be so dry. When it is hot and dry, I do like to water more to keep the soil more evenly moist during the summer. We have used Irritec P1 Emitter Tape which is a 15-mil tape with built-in emitters which is perfect for long rows. We are now changing over to another method of drip irrigation where you are using black poly tubing with four-way assemblies and drippers. I purchased this type of drip irrigation from Berry Hill Irrigation in Virginia. We use 1/2" x 100 black poly tubing on top of the weed block. This .85 thick tubing is thicker than the Irritec P1 emitter tape which is .015 (15 mil). You use a small hole puncher to attach the emitter. Then you snap in the 4-way assemblies with Jr drippers with nipple. You snap the 4-way assemblies into the black tubing. The assemblies are 12" long so you can place the dripper beside each dahlia plant; however, you can make custom assemblies to what length you need. Each dahlia should receive 1/2 gallon of water per hour if you were to run your drip irrigation for 1 hour. A drip irrigation system allows you to apply the water at the root zone.
Some growers in Australia use pulse watering. John Menzel talked about pulse watering in his book "Dahlias in Australia". In Australia, they deal with 100-degree weather. John Menzel used pulse watering as he felt it reduced the stress of the dahlia during the hottest part of the day. Pulse watering is a little water and often, so it provides the dahlia with water before the dahlia begins to show signs of wilting. It reduces the stress of the dahlia, and John Menzel said he had better tuber production. We always hear water deeply once or twice a week, so this is a different approach. I purchase a LinkTap Wireless Water timer this year. I purchased it on Amazon. Here is the link if you want to see this timer. (I may earn a small commission over time from qualifying purchases as an Amazon Associate).
I have 5 watering zones so I purchase the Q1 unit for 4-zones and the G2S unit for 1-zone. For the backyard grower, you would probably only need the G2S unit for 1-zone. There are so many options with the LinkTap water timer as you can set up 100 watering cycles per day with durations lasting from 3 seconds to 24 hours per cycle. Having a timer helps save me time as I can do everything from my phone. If we are on vacation, I can still water the dahlia garden. The timer has features like rain skip which is helpful and saves you water. It can also detect an approaching freeze, pipe leaks, valve failures, and more. You do need a router to connect the timer to the internet to see the app on your phone or computer, so you do need internet service. The dahlia field is 300 feet from our house. We were using garden hoses to connect our drip irrigation system; however, a couple years ago, we ran an underground water line to the dahlia field. That was a huge improvement.
I had planned to use it four times a day for a duration of eight minutes each time. When you turn on the drip irrigation system, it does take a minute for the water to go through the lines to reach each dahlia. When I open the app on my phone, I can see each watering zone as we have five zones. you can create or edit the watering plans. You can go to that zone and click instant watering to water when you want. You can input the duration of time that you want like 5 minutes, 10 minutes, etc. The timer is going to save me a lot of time as I will not have to run back and forth to the dahlia field and it will take care of itself if we were on vacation. I wanted the timer so I could use the pulse watering method to keep my dahlia beds evenly moist throughout the dahlia season when it is hot and dry so that I would have better tuber production. It has been raining since May so I have not had to use my drip irrigation this year. That is the way it goes some years!
July can be a hard month trying to weed and maintain your garden with all this heat and humidity. When you have a long stretch of consecutive days over 90 degrees, you may see dahlias drop their petals early or some of the petals burn from the sun. Flowers work harder in the heat. A lot of summer flowers do not like it when temperatures are above 90 degrees for an extended period of time. I do find that my 30% shade cloth helps a lot as those dahlias look much happier than the dahlias in full sun all day.
If you do not have shade cloth, I have used Agribon Ag-19 frost protection over my dahlias to help protect them from the sun. This may be an option for you if you only need to cover your dahlias for a few days when you are experiencing temperatures in the upper 90's. The below picture was taken 7/11/24. I was late planting the raised beds, and we had very hot weather that week. The dahlias were not liking it so I covered them with 2 layers of Agribon Ag-19 to help protect them from the full sun. When we went back to average temperatures, I removed it.
Once the dahlias are mature and about 2 feet tall, I do remove some of the lower leaves to help with air flow and reduce the risk of fungal diseases. My dahlias are planted 15 to 18 inches apart since I grow so many dahlias. I remove the lower leaves in the early morning when the temperatures are cool and the dahlia is more hydrated. When you remove some of those bottom leaves this also helps reduce areas for pests to hide.
I cannot say this has been my favorite summer as the dew point has been off the charts in July! June was so hot also. With all the rain and high dew points, it felt so tropical that I felt I was living further south. No wonder a lot of dahlia growers cut their dahlias back in the summer and wait for fall blooms. In the southern states, they do not have to dig up their dahlias, so they have early blooms in May and then they cut their dahlias back like you were cutting deeply for longer stems. They probably leave 2 or 3 sets of true leaves on each stem of the dahlia and then enjoy the dahlias when they start blooming in the fall when it is cooler. With these really high dew points, my dahlias were not blooming much. When we have an average summer with temperatures around 88 degrees, nighttime lows around 70 degrees, and dew points around 69 and 70, I had blooms throughout the summer especially from my dahlias from seed. I have had the least amount of dahlia blooms that I have ever had this month due to the high dew point of 75 and 76. Whatever blooms I did have were not the best blooms. The dahlias were not able to rest at night with temperatures in the upper 70's.
I have been growing my own dahlias from seeds that I saved from my dahlia garden. I will be selling tubers of Paula’s Doris Jane in December. She is an early bloomer! By the end of June, I had so many blooms it looked like a Paula’s Doris Jane convention. Lol.
The plants are about 30 inches tall so they can be on the short side. I am still able to cut 15-inch stems to enjoy inside my house. If you disbud, the stem length will be longer. If you cut deeply, you will also have longer stems. When I cut deeply for long stems, I leave 2 sets of true leaves on that stem.
Paula’s Doris Jane does a great job dealing with the heat. When we do have temperatures in the 90’s for several weeks with a high dew point in the 70’s, she can have a small little center where her center does not totally close. Dahlias can only handle that humidity for so many weeks before they do get tired of it. I do like to cut the blooms when the center is almost closed and the bloom is 3/4 of the way open. It will continue to mature in a vase in my house. I love the darker color in the center when you cut the blooms early and the vase life is longer.
Paula’s Miss Confident did start to bloom in July. The bloom size is around 3 inches. She has good petal count and the petals flow back to the stem so beautifully. I grow her mostly under the 30% shade cloth as the color will be more intense than in the full sun. I love her bright pink color.
Paula’s Paul Wix was blooming in July. Paula’s Paul Wix has petals that are stacked like shingles. He has a nice symmetrical bloom. The color is normally a bronze; however, some of these early blooms have a pinkish blush which is probably due to our humidity. I have so many pictures of the back of the bloom. He has nice hair! lol. The bloom size at maturity is 4.25 inches across.
Paula’s Mollie Sue was starting to bloom in July. If you do not like surprises, you probably will not like Paula’s Mollie Sue. Sometimes she can have a solid red bloom or a mix of red and white petals.
Usually, she is white with red stripes. She is very photogenic, and I love to take pictures of her. She could use a little more petal count in the heat of the summer; however, I really enjoy her cheerful blooms.
In August, it looks like we will be back in the 80’s and the nighttime lows in the 60’s. I hope so because the dahlias will start to recover from all this heat and humidity and I will be so busy in the dahlia field evaluating seedlings. I do post daily on my Instagram stories and show pictures of the dahlias from seed that are blooming.
So if your dahlias are not blooming yet, you are not doing anything wrong as they do slow down in the heat of the summer. They really are a late summer and fall flower so be patient as they will soon be rewarding you with beautiful blooms all the way to frost!
I hope to have my tuber sale in December. The tubers will ship in March or April depending on the weather and where you live in the U.S. They will be limited this year as this is my first tuber sale for my Paula's Dahlias. I hope the plants produce some nice tubers this year. We did use a rear tine tiller this year to mix in the compost and fluff up the soil. I had been no till for 4 years; however, I felt the soil needed to be tilled to loosen it up more. My soil is well drained; however, with all the rain this year, I did lose some tubers. We do have some tropical storms and/or hurricanes in the fall. I will know more when I dig up the tubers in November. Thank you for all your support. I appreciate it!