Planting and Growing Dahlias
Dahlia tubers come in all shapes and sizes. The dahlia tuber size does not control the growth or performance of the dahlia plant. A dahlia tuber has a crown at the top where the sprouts or eyes will form and it has a neck and body. My dahlia tubers have been pre-sprouted, so I know I am planting a viable tuber in the ground. All you need to do is bring the tubers out of winter storage (below 50 degrees) into a room that is above 55 degrees to warm up and begin sprouting.
You can see the crown of the tuber where the eye is forming, then the neck and body of the tuber. All dahlia tubers must have an eye to grow; however, sometimes you can have a blind tuber which will grow roots but never sprout to produce a plant and bloom. As the tubers warm up, you should be able to see the eyes on these tubers. If the sprouts are too long, you can cut them back about a week before planting the dahlia. The tuber will send up more sprouts at that spot. You can place that sprout into a pot. If the sprout forms roots, you will have another dahlia plant. Pre-sprouting helps late bloomers also as they benefit from the head start on growing. Pre-sprouting allows me to plant an actively growing tuber into the ground so it can start producing feeder roots. Feeder roots may form in the crown area and at the end of the body while the tuber is waiting to be planted. I love seeing that because the tuber will start growing quicker when planted in the ground.
I do take yearly soil tests and send them to my North Carolina state agriculture department. Dahlias need a balance of nitrogen, potassium, and phosphorus. My soil tests show that I have a lot of potassium and phosphorous in my soil so I use an organic fertilizer 5-3-3 that will give my dahlias a little nitrogen. Soil tests are helpful. In North Carolina, we can send our soil samples to the NC Department of Agriculture from April to November at no charge. Local extension offices have boxes and instructions. Soil testing determines the acidity of the soil and the level of nutrients like phosphorus, potassium, calcium, etc. that are needed. Per the soil tests, my pH is around 6.6 to 6.8 which is great for dahlias and for the availability of nutrients in the soil for the plants. Since my soil pH is in the ideal range, I do not need to add any lime. My phosphorus and potassium levels are high due to all the compost I have added to my soil. Nitrogen was the only recommendation. If you have not tested your soil, you may want to do so as you could be wasting money on fertilizer your soil does not need or using too much fertilizer. The phosphorus and potassium in fertilizers can build up in your soil to the point that the dahlias cannot access the nutrients they need.
Normally, I plant my dahlias when I start the warm season vegetables in the garden like tomatoes or herbs like basil. If the soil is too cool, the dahlia tuber will just sit there waiting for the ground to warm up and it may rot due to the cool conditions. It is better if you wait and make sure you plant when the soil temperature is warm as your dahlia tuber will start growing quicker. Dahlias are tubers and not bulbs. Bulbs such as lilies, daffodils, and tulips can be planted in wet, cold soil in the fall. Gladiolus bulbs are planted in the springtime but since they are bulbs, they can tolerate wet, cold soil better than a dahlia tuber. Dahlia tubers like to be planted in warm, well-drained soil in the spring when the soil temperature is near 60 degrees. I use a digital soil thermometer that I purchased online for around $15.00. It works great! Here is a link to the digital soil thermometer. (I may earn a small commission over time from qualifying purchases as an Amazon Associate).
http://www.amazon.com/dp/B006MDEKIW/ref=nosim?tag=222208b-20
I do like to mix in compost to the dahlia beds. Compost adds organic matter to your soil and helps your soil no matter if it is sandy or has clay in it. If the dahlia beds feel compacted, I will use a rear tine tiller to break up the soil and loosen it up, so the soil is more well-drained. The dahlias are going to produce tubers, so you want that soil to be loose and not compacted. You want the soil to be well drained, so the soil does not stay waterlogged. Dahlias do not like wet damp soil as the tubers can rot so you do not want to plant dahlias in an area that stays wet.
In the spring before I plant, I put down weed block fabric since I am growing so many dahlias and this helps with weed control. The weed block is a thick 3.2 oz. woven polypropylene fabric that allows air and water to pass through to plants while conserving soil moisture and suppressing weed growth. I burn 9" holes in the weed block using a homemade template made from scrap wood. I use a handheld propane torch with a built-in regulator that allows you to hold the torch downward without the flame going out. I purchased this propane torch at my local home improvement store. In each bed, there are two rows of dahlias. I stagger the dahlias so that they are 15" apart diagonally and 18" apart going down the bed. When I am planting dahlias, I dig a 6" hole, place my dahlia tuber in the hole, and place the soil back into the hole. I also add an additional handful of compost when planting.
I put down my drip irrigation underneath my weed block fabric before I start planting my dahlias. We buy our drip irrigation supplies online from DripWorks. Drip irrigation allows water to go directly to the roots of the dahlia. You could use soaker hoses. We are currently using Irritec P1 emitter tape. For the main line, we use 1/2" polyethylene mainline tubing. It is 15 mil and uses 5/8" tape fittings. The pressure range is 10-15 PSI. The drip is 8" on center. We have 2 lines down each row. You do want to test your drip irrigation before putting down the weed block fabric as this is the time to fix any leaks or issues. We do put these drip lines underneath the weed block. Another method of drip irrigation is using black poly tubing with four-way assemblies and drippers. I purchased this type of drip irrigation from Berry Hill in Virginia. We use ½” 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 four-way assemblies with Jr drippers with nipple. You snap the 4-way assemblies into the black tubing. The assemblies are twelve inches long so you can place the dripper beside each dahlia plant. We usually do not attach the 4-way assemblies until after I plant the dahlias, so the tubing does not interfere with planting. I still use the same spacing where I offset each row with fifteen inches apart going diagonally and eighteen inches apart going down the row. So far, I really like this drip line that goes on top of the weed block with the dripper beside each dahlia plant. The other drip tape that goes underneath the weed block is great for my first-year dahlia seedlings as they are planted at 9 inches apart.
We install shutoff valves in the main drip lines so that I do not water the entire dahlia field at one time. We are using our home well for the drip irrigation. There are two rows of dahlias down each bed with four beds per section so there are eight lines of drip irrigation in each section. I have around 100 dahlias in each row so there are 400 dahlias in each section. I water four rows at a time with the drip irrigation.
Dahlia tubers are planted in the spring after the risk of frost is over. They do prefer full sun (at least 6 to 8 hours of sun). Dahlias do like well-drained soil and the tubers are planted 5" to 6" deep. You can plant an unsprouted, dormant tuber in the hole. This would be quicker to plant as you would not have to be careful with the sprout. I plant a lot of dahlias, and I do not mind taking my time to plant a pre-sprouted tuber. I like knowing that it is a viable tuber. Just remember it can take 2 to 4 weeks for an unsprouted tuber to have growth coming through the soil. I usually have my pre-sprouted tubers coming through the ground within a week from planting. As you are planting your dahlia tubers around 5” or 6” deep, remember to place the tuber on its side in the hole with the sprout facing upwards. If you are not sure where the eyes are on the tuber, do not worry about it as the sprouts will find their way to the top of the surface. Dahlias are planted horizontally and not vertically.
After you have placed your dahlia tuber in the hole, you cover the tuber with soil. If you are using stakes or a trellis to support your dahlia, you should do so now, so you do not damage the tuber. You could use a tomato cage, metal rod, bamboo stakes, or a 4' or 5' wooden or metal stake. It is better to hammer in your stake in the hole to where you want it and then place the tuber in the hole with the crown or top of the tuber against the stake. This way when the dahlia sprouts, it will be near the stake so that you can easily attach the stems to the stake as the dahlia grows. You would not want to put your stake or metal rod through your dahlia tuber so install them now while you can see where the tuber is planted. If the neck of the tuber gets broken, it will not be a viable tuber anymore. If you are growing the dahlia in a tomato cage, you may have to add additional support by using a rebar (metal rod) or a metal stake to keep the cage from falling over as the plant grows.
I do label my dahlias at planting time. I use tree tag labels that I purchase online at Amazon. I use a drip irrigation pin and pin the tag next to the dahlia. I also use the tree tags labels to write notes about the dahlia and attach the tree tag label to the hortonova netting in front of the dahlia. You could also put the tree tag label around the dahlia stem once the dahlia starts growing. These labels lasts all summer for me. When I dig up the dahlia tubers in the fall, I make sure the tree tag label is attached to each dahlia clump. Here is a link to the tree tag labels that I purchase on Amazon. (I may earn a small commission over time from qualifying purchases as an Amazon Associate).
http://www.amazon.com/dp/B07QHXCN6Q/ref=nosim?tag=222208b-20
If you want you can add fertilizer when you are planting, you will want to mix it into the top 1/2" of soil. I do not put the fertilizer in the bottom of the hole as it could burn the young feeder roots. Some growers use bone meal at the bottom of the hole. I would just make sure you mix it into the soil before adding the tuber. I can find an organic fertilizer like Espoma Plant-Tone 5-3-3 at my local home improvement or gardening store so that is what I am using when I plant my dahlia tubers. 
If you are using time release fertilizers, you do want to leave that on the soil surface where it works the best and lightly work that into about 1/2" of soil so that it does not wash away. If you do have soil erosion, you can put some mulch on top to keep the fertilizer in place. Again, I would recommend you have your soil tested so you know your levels of phosphorus and potassium. The phosphorus and potassium in fertilizers can build up in your soil to the point that the dahlias cannot absorb all the nutrients they need. Usually, it is recommended to take a soil test every 2 or 3 years. If you use fertilizer later in June and July, remember that dahlia feeder roots grow along the soil surface so gently work the fertilizer into the top half inch of the soil. You are gently working it into the soil so that it does not wash away with rain or watering.
In 2024, I tried Permatill to help deal with voles coming into my dahlia field and eating my dahlia tubers. Permatill is a brand name for a type of expanded slate soil amendment. Expanded slate is a lightweight and porous material made from natural slate that undergoes a process of expansion at high temperatures. Permatill is used in gardening and landscaping to improve soil structure, aeration, and drainage. Stalite Environmental in Salisbury, NC produces this product. I found it at a local gardening center. Permatill also helps discourage voles because voles do not dig through materials that are coarse in texture. I mixed it into my soil as I planted dahlias and hopefully voles will not want to dig through my soil. I had great success in 2024 so I am using this product again this year. I also like to use Permatill has a mulch at the soil surface as it can help reduce soil borne diseases on the stems and foliage. Also, slugs do not like to crawl on Permatill. Voles can be persistent and can dig into the roots from the surface so this is extra protection.
Since I grow so many dahlias, I use stakes and hortonova netting to support the dahlias. I have 4-foot metal U-posts on hand so that is what I use throughout the dahlia field. I purchased them at my local home improvement store. If you are growing a lot of tall dahlias you may want to use 6-foot posts. I find dahlias do fine with posts around 6 to 8 feet apart. The hortonova netting is around 12” off the ground. As the plants grow, I adjust that height. There is a second netting at around 24” off the ground. The hortonova netting can be a pain to work around when harvesting. We often experience high winds during the growing season and the netting works great for me.
When planting dahlias, I do not water until I see the growth coming through the soil and the dahlia is a couple inches tall. If you live in a hot climate and your soil is very dry, then you will need to water your soil lightly. Usually, our soil is wet from spring rain. If there is too much water or wetness, the dahlia tuber can rot. Dahlias are hardy but when they are first planted they are a little fragile. They need time to grow feeder roots and get established. Dahlias are planted around 12" to 18" apart if you are planting commercially. For the home gardener, you can plant them around 2 feet apart especially if you are not digging the tubers up in the fall to store over the winter. You want to give them enough space to grow and enough light and air flow to grow properly. I do find that dahlias do grow well at 18" apart in my soil conditions and weather. If you want to remember the name of the dahlia, you will need to label it. You can attach a label to the base of the plant or write the name on a small stake. They will begin to grow quickly. You need to keep tying the plants throughout the growing season. You will need to tie the plants to the stake about every 12" to 18” using baling twine or plastic ties. Make sure you do not tie them too tight as you will damage the stalks. If you are using tomato cages, you will need to make sure the growth is staying around the cage. You may need to tie it to parts of the tomato cage.
I do pinch out the center growth tip when the dahlias have 5 sets of true leaves. I pinch the tip off and leave 3 or 4 sets of true leaves. This pinch encourages branching, and you will see the laterals at each leaf node start to grow. This is a one-time pinch. If you do not pinch out the growing tip of the dahlia, the plant will grow without many side branches and produce one terminal bud. Pinching the center growth will result in a bushier, well-balanced plant, and produce many blooms until frost. If you are exhibiting dahlias, you may want to leave 3 sets of leaves for the dahlias that produce the giant and large blooms. You can leave 4 or 5 sets of leaves for the dahlias that produce smaller blooms. In general, pinch the tip and leave 4 sets of leaves. Dahlias will bloom in about 90 days after planting depending on the variety and the weather. Early varieties bloom in 80 days and late varieties in 110 - 120 days.
Dahlias need 1" of water a week either from rain or a watering hose. If you can use drip irrigation or soaker hoses instead of overhead watering that will provide more water to the root system. When the hot temperatures do arrive in the summer, you will need to water deeply. Mulch will help the soil maintain moisture and may keep it a little cooler. Mulch will also help to keep down weeds. Pull the mulch away from the base of the plant a few inches so you do not suffocate the roots or encourage diseases due to excess moisture. The plant needs some room to breathe. Watering in the early morning is the best. They will appreciate a short burst of water in the afternoon when temperatures are over 93 degrees like 5 minutes of watering a couple times in the afternoon. They seem to benefit from a light mist of watering during a hot afternoon. Spraying them with a watering hose will cool down the dahlias. They may wilt during the day, but they perk back up in the evening. I water deeply using drip irrigation; however, I do some overhead watering when temperatures are above 90 degrees.
I cut the blooms when they are 3/4 open and cut deep around 16" to encourage long stems. I harvest the blooms either early morning or late evening during the coolest parts of the day. I take a bucket of water to my dahlia field and place the stems immediately into water. I recut the ends at an angle and place them in vases to enjoy in my home. I change the water every other day as dahlias to help vase life.
I try to give my dahlias Vigor Plant Booster once a month in June, July, and August. I purchase Vigor Plant Booster at https://www.vigorfertilizer.com. In August, I add sulfate of potash (00-00-50) as this is for root and bloom development and helps tuber production. I do not fertilize after late August as the days are becoming shorter, and the plants are producing tubers. I have read where fertilizing after August can affect how the tubers store over the winter if you dig them up, so I do not fertilize in September.
You can plant your dahlias in raised beds. If you live in an area that receives a lot of rain or your soil does not drain well, you may want to consider raised beds. You may be thinking about growing dahlias in grow bags. Grow bags are fabric-like bags that can be filled with a growing medium and used to grow flowers especially if you have limited space. You can easily lift them with their handles and move them from one spot to another spot if necessary. I do have a blog about raised beds and grow bags on my website.
I do use organza bags to protect my blooms from insects like Japanese beetles, cucumber beetles, tarnished plant bug, etc. I purchase them on Amazon. Below is the link for the 5x7 organza bags. (I may earn a small commission over time from qualifying purchases as an Amazon Associate). I use the 5x7 size for very small blooms. I mainly use the 6x9 size. I also use the 10x14 or 12x16 size for larger blooms.
http://www.amazon.com/dp/B07ZH84TV2/ref=nosim?tag=222208b-20
I like the 6x9 sizes. Here is the link for that size.
http://www.amazon.com/dp/B07K3TW2FL/ref=nosim?tag=222208b-20
Here is the link for the 10x14 organza bags.
http://www.amazon.com/dp/B0B11RBGXP/ref=nosim?tag=222208b-20
For really large blooms, I have used the 12x16 organza bags. Here is the link for that size.
http://www.amazon.com/dp/B07FSLY4FJ/ref=nosim?tag=222208b-20
Organza bags keeps the bugs away so you can have a perfect bloom. I used to sell dahlia blooms to florists and this is how I gave them beautiful blooms. I still use them now as I really like to cut some blooms and bring them inside to enjoy. I use them throughout the summer except when temperature go over 92 degrees as some blooms burn inside the organza bags especially white or yellow dahlias.
Remember to cut the first bloom and enjoy it in a vase in your home. If you cut the first bloom as soon as you can, the plants will start producing more blooms for you to enjoy until frost. It is also important to cut spent blooms (deadheading) so the plant will send up more branches and blooms to enjoy.
I do love the ARS Needle Nose Pruners. I purchase them on Amazon. Here is a link to the ARS shears. (I may earn a small commission over time from qualifying purchases as an Amazon Associate).
http://www.amazon.com/dp/B0009XAHJW/ref=nosim?tag=222208b-20
I use them for cutting blooms and when I divide dahlia tubers. I do disinfect my tools between each dahlia plant. The American Dahlia Society recommends this as you can transmit viruses. This is also recommended by many flower organizations not just dahlias. Disinfecting your tools is important. You can use a 10% bleach solution but I found that my pruners were rusting even when I rinsed them with soap and water. I now use Virkon S tablets. Virkon S is a disinfectant that kills viruses and bacteria. It is used for cleaning and disinfecting hard surfaces in livestock production facilities and transportation facilities. Ten minutes is the maximum contact time. You can use 1 tablet per 2 cups (16 oz) or 2 tablets per quart (32 oz). It is not as hard on your tools like bleach so my cutting tools are not rusting. I purchase them on Amazon so here is the link.
http://www.amazon.com/dp/B0088OKF1E/ref=nosim?tag=222208b-20
I hope all this information helps you when growing dahlias. Dahlias are like most flowers they need well drained soil, fertilizer, and weekly watering. They will produce tubers that you can store over the winter and plant again in the spring. I hope you have a beautiful dahlia garden!