Dahlia Culture
There is an enormous amount of information on how to successfully grow dahlias on the internet- so much so that it can be overwhelming. There are a few general principles and many variants that work for different people depending on their specific growing and storage environment. Good resources include the American Dahlia Society https://www.dahlia.org/fundamentals-of-growing-dahlias/, the Dahlia Growers Facebook page https://www.facebook.com/groups/265949743891471 where many people show off pictures of their beautiful flowers and ask and answer useful questions about growing and storing dahlias, Books such as Connie Thompsons Dahlias: A Beginner’s Guide and the websites of specific growers such as Garden Bee Flower Farm. https://mygardenbee.com/ that have a lot of tips and great pictures. If you are just starting with dahlias, my advice is to start small with a limited number of tubers so you can test out different methods of winter storage and find one that works for your home.
Our farm is located in central Pennsylvania in Happy Valley outside of State College. The growing zone is 6a-6b. The last spring frost is generally May 15 and first fall frost is around October 15, although that varies. It gets to mid upper 80s during peak summer but cools down quite a bit in the evening and there is relatively little humidity.
Dahlia Tuber Anatomy
One dahlia tuber can produce multiple tubers by the end of the summer all attached to the central stalk or stalks. The basic structure of a dahlia tuber is a piece of the crown that contains an eye or eyes (cells that will form the new plant the next year, the neck, and the body of the tuber. Tubers come in all shapes and sizes but the key thing is if they don't have an eye they may form roots but they wont generate a new plant. Similarly if the neck is broken it will likely prevent growth of the shoot as nutrients stored in the tuber body wont get to shoot. The tuber body will survive a lot of damage, cuts, being chewed by voles etc and still produce a plant provided it is kept from rotting.
Unpacking Shipped Tubers From Spring Creek Dahlias
We presprout tubers before shipping and depending on the variety you may receive one that has eyed up or one that decided to just go for it and has a vigorous sprout that may be close to leafing out. Take care opening the baggies so as not to break the sprout, but if that happens not to worry it will regrow more at the base. We presprout in potting soil so your tuber may have alot of roots already. Depending on when your last frost date is you can hold a sort period of time in the baggie in a cool place but its probably better to put them in a pot with potting soil and in a sunny window if you have to wait several weeks before planting out. It does not have to be a big pot- I usually chose pot since based on whether the tuber will fit, not based on mature plant size. If they get leggy you can always pinch back and you will get a bushy plant with more flowers.
Presprouting and Planting Tubers in the Spring and Summer
There are two main options for spring planting depending on your plant hardiness zone and when you want flowers. In general tubers can be planted out directly in the ground about 4-6 inches deep after all danger of frost has passed and the soil has warmed up. Tubers should be placed in the hole horizontally. A good rule of thumb is plant dahlia tubers when you plant tomatoes. Do not water the tubers until you see a sprout as wet soil can cause tubers to rot. It can take several weeks for tubers with a viable eye to send up a shoot above ground.
If you live in a colder climate, it can be useful to presprout your tubers and potentially pot them up inside for several weeks before planting out. For most tubers you don’t want to start too early otherwise you will end up with leggy plants before they are ready to plant out. However, some tubers especially for dinnerplate varieties such as Café Au Lait, Breakout, Penhill Watermelon, Penhill Dark Monarch are very slow to wake up so they may have to be started earlier than the average tuber. Even just minimal presprouting in 6a before planting out in late May or early June gives flowers by the end of August.
I take tubers for presprouting out of the cold garage and lay out on a tray with a layer of potting soil on bottom and then cover with potting soil. Trays are kept in the dark ~60 degree basement for several weeks until you start to see growth from the eye. Some varieties will sprout right away, others take a lot longer. Depending on your situation you can plant the sprouted tubers or put them in pots while you wait for planting. If you pot up the tubers you should keep in dark until the shoots are above the soil and then put them in a sunny window or under grow lights so they don’t get leggy. For planting out around the 3rd week of May I started presprouting tubers around third week of March.
Care During Growing Season
1. Water
2. Fertilizer
3. Pinching and Staking
After the plant is about 1 foot tall and/or has 4 sets of leaves most experts recommend pinching out the growing tip with finger nails or snips. This will cause new stems to grow from the junction between leaves and main stem, help keep it bushy and most importantly produce more flowers. If the plant is growing strongly it wont be long before you see the new laterals growing up.
Digging Dahlias in the Fall
Unless you live in zones 8 or greater you should dig up dahlias after a killing frost and store over the winter otherwise, they will rot in the ground. I have had a few that I had missed surprise me and come back in spring, but this is a low probability. (A caveat: my mother-in-law who lived in Clearfield PA zone 5b/6a had a clump of dahlias planted next to a dryer vent that she never dug up, so the local environment matters.) Dahlias need 120 days in the ground for the new tubers "cure" to be good for storage so the killing frost is just a useful marker for when to dig.
Before you dig and preferably while the plants are flowering it is a really good idea to attach some sort of tag with the variety name around the base on the stem. Garden markers are much better than sharpies since the writing doesn't fade over the season. I've used cut up window blinds with a twist tie through the hole to attach to stem, and currently use fluorescent trail marking tape that you can write variety name on. This is important if you want to keep track of the varieties as you process the tubers for storage. It is very easy to mix them up if you have a lot of different ones.
After cutting the vegetative growth many people suggest waiting 2 weeks to dig, supposedly because the eyes are more visible and the outer skin changes so tubers store better. I think this is basically the natural response of a plant after removal of top growth which is to put up new shoots. But other experts cut and dig on the same day. I don’t think there is a hard and fast rule and if you have a lot digging to do it is usually stretched over several days/weeks so choosing to do when not uncomfortably cold, and wet makes the task less onerous.
One important point about cutting down the vegetative growth. It is crucial to bring a bucket with bleach/or detergent to dunk the cutting tools in after each plant, or each variety, as well as when digging up tubers. This should significantly reduce the possibility of transferring any viral or bacterial pathogen from one plant to another. Since infection by these pathogens ultimately leads to poor performing plants, and can be spread through open wounds this is a good practice to follow. Any plants that exhibit evidence of viral infection during the growing season or presence of either crown or leafy gall (caused by bacterial pathogens) after digging should be discarded in trash not compost pile, and tools definitely sterilized with bleach.
Dig around the perimeter of the clump/stalk. Each variety produces different size tubers so this can determine how far away from the main stalk you want to start digging. You want to be careful not to slice through tubers and to be mindful of varieties with very long thin necks as these tend to break during the digging process if you’re not careful. A garden fork seems to work better than a regular shovel. For instance, Teddy has very large, long tubers that form on relatively thin necks connecting to the main stalk. It is easy to either slice through or break off the connection to the main stalk and potentially the eye. However it is not necessarily a catastrophe if you slice through tuber especially large ones, the cut ends just have to be treated with cinnamon or other antifungal agent before storage.
After lifting the tuber clump, if possible, rub off extra dirt. Minimize shaking clump because that can break thin tuber necks. Here is where the storage methods diverge, and it is important to figure out what works best for you. However for all storage methods stable temperatures of 40-50 degrees and a relative humidity around 60% that doesn’t cause drying or rot is best.
1) Dirt clump method. For this you simply let the clump dry in most of the dirt that you dug it up with for several days, then store in a container of your choice or black contractors trash bag. Multiple clumps can be stored together if they are appropriately tagged. Open or shut the container or bag to achieve desire humidity. This is a good method if you don’t have a lot of dahlias as the dirt clumps take up a lot of room.
2) Standard single tuber method.
Here, you wash all excess dirt off of tuber clumps (I usually do it outside with some type of screen to hold dahlia clumps and rinse with hose). After all the dirt is removed and the clumps are dried to the touch, they can be divided into individual tubers with an eye, or small clumps of tubers. If the tuber doesn’t have an eye, it won’t sprout the next spring. I use a Hawksbill knife and clippers to divide. Before or immediately after I divide, I use a Sharpie pen to label each tuber with a code, so I know which variety it is. Again, when dividing tuber clumps it is a good idea to use a clorox wipe for tools in between each clump. I usually dip my tubers in a copper sulfate anti-fungal solution and let them dry again, some people use cinnamon which has anti-fungal properties on cut edges. I haven’t compared either method or whether it is helpful compared to not doing anything.
From this point there are two further strategies
A) Storage of separated tubers or clumps in some type of solid media. Vermiculite, peat moss, shredded newspaper, wood chip bedding are different types of media that people use to store tubers. I’ve had success with wood chip bedding used for small animals in grocery store plastic bags within a plastic storage container. The trick again is to monitor temperature and humidity by looking at the tubers during storage: drying out, spritz with water and keep container closed to increase humidity; tubers soft with mold growing - too much humidity. Decrease humidity by opening up lid of container and getting rid of any tubers from clumps that are rotting, although simply wiping off mold and using a clorox wipe can solve the issue.
B) Saran Wrap method. 5-10 Individual tubers depending on size are wrapped in saran wrap with the saran wrap. Each tuber is place on saran wrap then rolled to cover. And next tuber added. The idea is to not let tubers touch each other. The final package ends are sealed with masking tape, labelled, and then stored in garage. I use an old dresser. This works well with very few losses over the winter.