Growing Dahlias in Containers

Text and Photos © 1995-2000 by Barbara Jenke (Do not use for any commercial use without the permission of the author) Hot Springs, South Dakota

When I first started to grow dahlias in containers, I was frustrated because I could not find anything written in the general gardening magazines or books on this subject. The first years I grew only those dahlias listed as “dwarf” or “low growing.” One year I realized that if I could grow a dwarf tree or bush in a tub, I could cultivate any height of dahlia in a container. A dwarf variety may be a bit easier to handle, but raising 4 feet or taller dahlia bushes is just as easy, and could make a nice privacy hedge on a patio or balcony. Different heights of dahlias also add variations of elevations to a deck garden as well as adding different colors and textures from the flowers’ different forms and sizes.

Over the years I’ve had to deal with a short summer growing season with gentle-or monsoon- type rains; summers of heat and drought with or without muggy humidity; or the coolest, wettest foggiest conditions; still air to refreshing breezes to wind gusts of 50-60+ MPH; cool days with cumulus clouded skies or bright sunny hot days; thunderstorms; electrical storms; or hailstorms. Through these schizophrenic weather conditions I’ve experimented with different potting soils, fertilizers, staking, how to start the tubers without having them rot, etc. It’s been a challenge, but I’ve developed a method of planting dahlias in containers which has been successful for me.

By starting the tubers indoors during the first week in April, I have had some early varieties start to flower by the last week in June. Because of weather conditions, most dahlia suppliers don’t send the tubers until at least the first week in April. I probably could start my over wintered tubers earlier than April, but I am usually too busy sowing seeds and transplanting seedlings of other annuals and perennials.

When I get the tubers out of storage or when they arrive in the mail, I inspect them for rot and look for good eyes. As I check each one against its invoice and write down any bonus tuber I may have been sent, the tuber is laid horizontally in trays with the eyes (or any growth which has already begun) facing up. I make sure that each tuber is identified either by having its name written on it or making a plant label for it and laying the tuber on the label. Tubers which look like they have no eyes or damaged eyes(sometimes stems break off in shipment) are placed in individual small trays on moist potting soil. This way I can watch them closely for any signs of sprouting and don’t waste time and energy planting a tuber which won’t sprout a new stem.

The tubers should be planted before the roots start to grow and get entangled in the flats. Separating the roots damages them. While they are forming new root hairs, it delays the plants’ upward growth and first bloom date.

I use heavy-weight, sturdy, plastic pots in which I can drill holes for the later described stake inserting procedure. The diameters of pots I usually use are 8 1/2-inches and 10 1/2-inches. I have a few 11-inch and 12-inch pots for planting really long tubers. For base stability, I prefer the azalea style of pot, meaning that a pot has a short height in relation to its diameter which makes it look short and squat, as opposed to a tall, skinny looking container. I do not match pot size to the final bloom size, because miniature flowers could be on a tall bush and giant blooms could be on a short plant. I match the pot to how tall the plant could be or the length of the tuber.

Use a clean container! A dirty pot could spread a disease to the plant or have unseen insect eggs in it. Before placing the potting mix in the container, I remove the saucer from the base of the pot. This will allow the excess water to drain and keep the soil from becoming water-logged from the daily drenching rains we can receive. If there are not enough holes or the holes are too small for fast drainage in the base of the container, drill another hole or two in the bottom to enhance drainage.

In the container, if the tuber is placed in the bottom of the pot and then the potting soil is immediately filled in up to 1-inch below the rim and then is thoroughly watered, the tuber could very likely rot. Planted higher up to prevent rotting, the tuber would become exposed to the surface and the base of the stalk would be sitting on the surface of the soil. Then the stalk could the be easily broken off from the tuber. When planted in the ground, directions usually state to dig a hole about five to six inches deep and fill in the hole with soil as the plant grows. I learned the best way to plant the container grown dahlia is to plant the tuber by following the same method: plant the tuber deeply and slowly add more potting soil as the plant grows.

The potting mix I use is a coarse soilless mix, Ball’s Growing Mix #2. It is a nice loose blend of fine bark, vermiculite, peat, and perlite. I use it for all of my container plants. Grace Sierra has a soilless mix which is like the Ball’s Mix #2 called Metro-Mix 700. I have noticed that garden centers are selling other brands of coarse soilless mixes which dahlias should like. Whatever potting mix you want to use, make sure it is very loose whether wet or dry. Make sure the potting soil does not have clay in it. Dahlias do not like to grow in clay soil. The clay turns the soil into mud when wet or into a block of cement if allowed to dry out.

An important “secret” ingredient I stir into the potting soil when I plant the tuber is a product made of acrylic copolymer crystals. The copolymer crystals absorb water and release it to the roots of the plant when the soil dries out. They protect the plant from being overwatered or dying in dry soil. The products I have used are TerraSorb™ and Soil Moist™. They provide the added benefit of keeping the roots cool during a hot day, and from keeping the soil (and tubers) from freezing when the frost kills the top of the plants in the fall. I use the copolymer crystals in all my containers of non-cactus plants. What’s nice is that if I don’t have time to water on a hot day, I don’t have to worry about the plant dying.

The copolymer crystals are easier to work with in their reconstituted form (slurry). Use the amounts for each pot diameter as directed on the label. To turn Soil Moist™ crystals into slurry, add 1 cup of warm water to 1 teaspoon of the crystals and wait about 5 minutes. The slurry looks like little gelatin-looking globs. I like to make a batch at time in a 3 quart container. Left over slurry can be covered and stored for later use or allowed to dry out and be stored for reconstituting at a later time. Please watch out when used around children or pets! It is also very slippery when wet!

Planting the tuber: Place an inexpensive, biodegradable drip-coffee filter, or two if necessary, over the holes on the bottom of the flower container in order to keep the soil in and the sow(or other) bugs out. The roots will appreciate the extra space that pot shards or rocks would have used.

Fill the flower pot 1/3 full with pre-moistened potting soil. In that bottom layer, mix in an amount of copolymer slurry as recommended by the package directions for the pot’s diameter. Try not to pull the coffee filters off the bottom pot holes.(The soil and slurry can be combined outside of the pot and then put back in) Lay the tuber horizontally on top of that layer of mix and slurry. If at all possible, place the tuber so that the eye end will be in the center of the pot. It is okay if this can’t be done. Many times I’ve had to place a 6″inch tuber in an 8 1/2-inch pot. Just be sure to leave about 1/4-inch space between the root (non-eye) end of the tuber and the side of the pot to allow for roots to grow. If a sprout is already growing out of an eye, place the tuber so that the sprout is pointing upwards. Write the variety of plant and what other information you want on a plant label, and insert it in the soil next to the eye end of the tuber. This marks the spot where the stake will go. It also keeps roots from growing in that spot.

Cover the tuber with more pre-moistened soil, but just enough soil to hide it. The eyes may be exposed, if desired, to watch for growth. Using a spray bottle filled with warm water, mist the tuber until the surface is damp. Do NOT fill the container to the top with potting soil at this time. By just covering the tuber, the plant’s growth can be easily watched and prevents overwatering of the awakening tuber.

As the stalk grows, carefully add more potting soil to the container, so as to not break the stalk from the eye of the tuber. Do not cover the upper set of leaves. Do NOT add any more of the copolymer slurry. The gel rises. If it’s put in at higher soil levels, lots of little globs of gel will be sitting on the top of the soil after a heavy rain. Believe me, I know!

If started inside, place the containers under plant lights. The dahlias should have light from above to keep them from bending towards the light of a window and to grow compactly. Set the lights about six inches from the tops of the pots of newly planted tubers. Raise the lights as the plants grow. Suspending the lamps from chains on hooks makes them easier to raise and lower. I use plant gro lights or a combination of two fluorescent lamps: one cool white and one warm white fluorescent tube in a 48-inch two-lamp shop fixture. This provides the proper light spectrum to raise plants without having to pay for the expensive grow lights. I learned this from the Floralight Company when I bought some of their stands. The timers are set at sunrise to sunset times (12-14 hour days) The containers can be set near a south window. Be sure to turn the pots so that stalks will grow straight.

The stake should be inserted in the pot before the plant label is covered by soil additions. Stakes help to prevent the stalks and stems from breaking off in the wind or if/when the pot falls over. Also, when uprighting or moving the pot, the stake can be grabbed onto instead of the plant. I prefer to use steel stakes which are covered in green plastic. Besides being strong and easy to work with, they can be cleaned and disinfected at the end of the growing season for use the next year. (If you can only find bamboo stakes, then try doing what I used to do: for added strength, tie three stakes together with tape, tie wire or string.)

If the average height of the variety is known use that length of stake. If not known, judge what length to use by the height of the trunk when the stake is inserted. Otherwise, use a 4 ft stake, because most dahlia plants seem to have an average height of 4 feet. To keep the stake upright, tie the stake to the container using plant tie-wire (or string, if preferred). Drill 4 holes in an “X” or “+” position (depending on the plant’s growth) in the sides of the pot near the rim. Cut a piece of plant tie-wire at least four inches longer than the diameter of the pot. Fold the wire in half and wrap the middle of the wire around the stake at the same level with the height of the pot’s rim and twist the tie wire to the stake. Remove the plant label which is near the eye end of the tuber and replace it with the stake. Next, thread one wire end through one of the pot’s holes and twist to secure it, then thread the other wire through the opposite hole and pull the wire taught until the stake stands upright next to the stalk of the plant. The base of the stake should touch the bottom of the container. Repeat with the other set of holes. Occasionally, two stakes may be needed if there are two main stalks growing from the tuber. Tie the stalk(s) to the stake(s). Return the plant label to its pot. (To keep the labels from becoming separated from their pots, mainly due to curious baby raccoons, I am going to experiment and use those aluminum name tags which have tie-wires thread through holes in them and attach them to the drilled holes in the pots.)

After being staked, when the plant has grown taller than the top of the container, add the rest of the potting mix to within one-inch of the top rim of the container. It is okay to cover the leaves below the soil line.

When the plants get at least three or four sets of leaves, pinch out the growth tip of the stalk. Pinching helps to make a bushier, sturdier plant. It does not delay the blooming time of the plant, but the plant does make more flowers. When I don’t pinch, I usually get tall, skinny plants. My husband and I like larger as opposed to more flowers; so, the side buds get pinched when they start to develop. The miniature flowered buds don’t get pinched.

Watering: While the plant is developing roots, let the soil almost dry out before watering again. The copolymer crystals will prevent the tubers from drying out. If the soil is kept too wet before the roots and top growth get a good start, the tuber may rot. Water the plants after adding more soil to the pot. The city’s water here is hard with a pH about 7.4 and contains lots of calcium sulfate, magnesium, and other minerals. I understand that dahlias like a soil towards the alkaline side, so this water’s pH does not seem to bother them (they grow and bloom well!).

The indoor water goes through a water softener which replaces the calcium with a sodium salt. While they are inside, all my plants get watered with softened tap water. I use tepid water because the cold water that comes from our faucets is really cold. The softened water does not seem to harm any of my plants. While inside the pots will need to be placed on a saucer to protect the floor. But do not attach the saucer to the pot.

Outside, all the container plants get watered daily either from rain showers or the hard unsoftened city water directly from the garden hose. If the the soil is moist one inch down from the surface, do not give the plant any water. Because of the copolymer crystals, on cool days plants may not need to be watered. In August, the roots have usually filled the pots, so in the evening after a day in the 90’s or more, I check the soil in the containers to see if they need more water.

On hot, dry, sunny days, I will take the garden hose and mist the plants and the deck, so that the evaporation of the water will help cool the plants. I will do this in the hot mid-day sun if they look like they need it. Contrary to some people’s beliefs, the drops of water on the plants will not leave little magnification burn marks on them. Don’t you feel cooler after getting sprayed by a sprinkler? My fertilizer of choice is Ra·pid·Gro Bloom Builder (19-24-18 with micro nutrients). I’ve tried other formulas with the soilless mix, but so far, prefer this one. I like the foliage, blooms and good tuber production I get with this formula. The plants get fed every 7-10 days. I mix the fertilizer and water in a watering can and pour it into the individual containers until the liquid runs out of the bottoms. I stop feeding the plants at the end of August, because we usually get our first killing frost in the first weeks of September.

I spray the combination fungicide and pesticide, Orthene III, in the early evening (if the air is still and cool) at the first sign of powdery mildew, spider mites, or thrips. The first time I used it, I thought that I would wake up the next morning to find that all the plants’ leaves were burned or the plants dead. Instead, the plants looked happy and healthy!

The plants get hardened off to the outside weather conditions when the nighttime temperatures rise into the high 40’s F, usually in the first weeks of May. The containers are set on the southeast facing deck under the front porch stairs. At night I cover the plants with a reemay fabric blanket and/or a giant piece of bubble wrap which are both made for covering vegetable gardens. All the pots get taken back inside if there is a frost warning. The first week of June, the sun loving plants get hauled upstairs to the top deck.

Until the end of August, I add more potting soil to the container when the soil level looks like it has dropped, exposing the roots. When the lower leaves start dying, I cut them off. In August, outer green leaves get thinned out to allow the inner branches to receive light so they can grow and make more flowers.

Some dahlias are affected by sudden drops in temperature(within 20 minutes) from the 80’s into the 50’s(F) due to a thunderstorm (with rain or hail)or by an electrical storm (lightning and thunder only). These situations seem to trigger a winter season shut-down even though it’s the middle of the summer. The leaves droop as if the plant doesn’t have enough water, even though a check of the soil proves that it is moist. Let the soil in the pot dry out before watering again. Many times this will get the plant to regenerate and start growing again and flower nicely. Sometimes only the buds will continue to develop and bloom. Sometimes the plant will die.

Keeping a growth record of each dahlia and taking a photograph of the plant and flower is a good idea for future year’s reference. Besides the usual classification data such as bloom type, color and size, I include pot size, height, first bloom date, supplier, and how well it did. I also write a summary on what the season’s growing conditions were like, so that I can see why a plant may not have done very well that summer or did exceptionally well.

The best thing about container gardening is that the plants can be moved! The ones in flower can be turned around or moved to the front for the best flower show, the ones growing too tall go to the back, or the sun stressed ones can be moved to a shadier location. When there’s a hailstorm warning or dark thunderstorm clouds are seen moving in our direction, I move the plants next to the house, under the eaves, until the storm passes or until the next morning. In early September, when there is an early frost or snowfall warning, and we are home, my husband and I make a mad dash out and bring inside the dahlias which are in full bloom or have a lot of promising buds on them. After that first frigid spell, the temperatures usually warm up again; the plants go outside and give us another month of beautiful dahlia flowers.

This way of growing container dahlias is not written in stone. This is a starting point to help develop your own system of growing contained dahlias. I saw some very nicely grown container plants when I was at the ADS National Show in Kalispell, Montana, so if you have found a method which works for your dahlias, by all means continue to use it!

Growing Dahlias in ContainersThis article was originally published in the Bulletin of the American Dahlia Society;

Author’s update:If you are growing potted dahlias on a cement patio, put the container in the shade during the hottest part of the day or the plant could get too hot and burn from the reflected heat of the cement. Morning sun is the best, especially, if you live in a hot summer area.

Check the label of ingredients on the potting mix. Some of today’s mixes already have the copolymer crystals added to the mix. If so, do not add any of the crystals to the pot.

Some mixes already have fertilizer added to the mix. While the roots are filling the pot, you do not need to add any more fertilizer, but once the plant starts really growing, it consumes fertilizer rapidly, so you will need to give it additional plant food.

Schultz brand potting mix with fertilizer for Roses is a nice potting mix for dahlias. If you have an Eagle Hardware Store, they have a Cole’s brand planting mix which grows dahlias well, too. I do not recommend Peter’s brand potting mix for dahlias, because it has too much water retaining peat moss and a wetting agent which keeps the soil too wet for the tubers.

Growing Dahlias in ContainersPHOTO NO. 1: The potting mix just covered the dahlia tuber when it was placed on the bottom 1/3 mixture of soil and copolymer slurry. The eye of the root was left exposed. The plant has grown beyond the top of the pot and has been staked, so it is ready to be completely filled in with potting soil. The lower leaves will be covered with soil.

PHOTO NO 2: The staked dahlia’s pot has been filled in with potting mix. The top growing tip has been pinched off. The leaves below the soil line were covered over with the mix. The plant is ready to be tied to the stake and then watered.

Hot Springs is located in the southern Black Hills of South Dakota in the southwestern part of the state, and has an altitude of 3800 ft above sea level. Please refer to Barbara’s Letter to the Editor in the June, 1993, issue of the ADS The Bulletin. Photos of the deck gardens appear in that issue and the September, 1994, bulletin.