How To Grow Dahlias

Dahlias have an interesting history. The first tubers arrived in Europe at the end of the 18th century, sent over to Madrid by the Spanish settlers in Mexico. Andreas Dahl (after who the plant is named) regarded it as a vegetable rather than a garden flower, but interest switched from the edible tubers to the blooms when the first varieties with large, double flowers were bred in Belgium in 1815. Within a few years, nearly every colour we now admire had been introduced and Victorian catalogues listed hundreds of varieties.

The favourites in those days were the Ball and Small Decorative Dahlias. Today it is the Large Decorative and Cactus varieties which capture the public fancy. Fashions change but the popularity of this late summer flower continues to increase. The reasons for this devotion to the Dahlia are fairly obvious. First of all the skill of the breeders in America, Australia, Germany, Holland, and England has produced a range of sizes and colours unmatched in the world of garden flowers. Plants ranging from dwarf bedding (twelve inches high) to giants taller than a man. Flowers range in size from an inch to the largest dinner plate.

Equally important is the time of flowering. From the end of July to the first frosts, Dahlias provide large amounts of colour when so many flowers are past their best. Above all the Dahlia is an accommodating plant. It likes a good loam but will grow almost anywhere. It relishes sunshine, but can still do well in partial shade. A bed just for Dahlias is the ideal way of growing them, but they are quite at home in the herbaceous border or even the rockery for dwarf bedding varieties.


Dahlias will grow in almost any location and almost any soil. However, to have outstanding plants and flowers, you must be selective of the planting placement. Dahlia roots, (tubers), need a sunny location to thrive. They should receive at least a half-day of sun and even more is preferable.

Select a site for your dahlia garden that is away from trees, sunny, and yet sheltered from direct wind. Dahlia tubers are surface feeders.  Since they don’t send down a tap root or long feeder roots the plants will easily be blown over by the wind. Staking the plants is essential and will be covered later with planting instructions. A further important consideration is the condition of the soil. In most cases, good everyday garden soil is adequate. But good soil drainage is vital for dahlia plants. If the soil holds surface water for more than several hours after a rain, the likelihood is that it should be augmented with organic matter. Humus, peat moss, sand, or well-rotted manure will work well. A mixture of equal parts of all of the above makes an excellent addition to heavy soil.


If at all possible, choose the planting site in the autumn. Dig or till your plot and start working in compost, peat moss, sand, and rotted manure. Keep the site as weed-free as possible during the winter months. This will make your spring work and planting much easier. Then as spring comes, the area will need a further digging or tilling to a depth of at least six inches but eight to ten inches is better.

If you choose to use a commercial fertiliser, be sure to keep the nitrogen (the first of the three content numbers) to a low number. For example, a 5-20-20 would be adequate. This of course should be well worked into the area in a ratio of 3 to 5 pounds per 100 square feet. Further fertilisation should not be needed although some people apply a second mid-season application of the above formula to their dahlias or use a similar ratio in a liquid form.


The storage of dahlia tubers prior to planting is critical. Tubers must not be allowed to freeze or to be placed in a room that is heated above 50 degrees F. A temperature of around 40 degrees F is preferred. The tubers must be stored in a dark location, high in humidity. We store the tubers in a concrete built building in racks to leave an airflow around the racks. Continue to check your dahlia roots (tubers) weekly for rot or mould. As you handle the dahlia tubers be careful not to damage the growing point known as an ‘eye’. 


The dahlia tuber is unlike many other bulbs in that it wants to be planted in warm soil compared to say, tulips. A rule of thumb for planting time is plant dahlias when you would plant other root type vegetables such as carrots. In other words, spring should be well on its way with the longer and warmer days.


Now that the area for planting your dahlia tubers is well prepared and your stock of tubers is in hand, it is time to prepare a garden layout plan. Because certain varieties grow considerably taller than others, you should plot where you want tall plants and where the shorter than average should go. Also, if the colour mass is important, then get these details laid out before you actually begin to plant. 
Many commercial dahlia suppliers indicate the approximate height of the plants in their catalogues. Using this information can be a help in formulating your layout plans.

The layout plan will also need to take into account the number of varieties that you plan to plant. The average planting space between plants is 18 to 24 inches, especially for the large flowering varieties. The shorter varieties can be planted closer together, but remember, when you dig those clumps in the  autumn, you don’t want them intertwined with their neighbour.

Plan for the rows to be three to five feet apart, depending on the size of the plant. 
When the rows are two to three feet apart, the plants will generally grow taller as they ‘reach’ for light and your access up and down the rows becomes more difficult.
Close planting also shuts down air circulation to the lower leaves, encouraging powdery mildew. If you plan to use a hand tiller between the rows, then plan your rows according to its width and be sure to leave extra width so as not to till too close to the plants and damage those new tubers.


Prepared holes for planting should be 5 to 6 inches deep. If you plan to stake the plants, NOW is the time to do this and not later when you may damage the tuber by running a stake through it. Pound a sturdy stake, 4 – 5 feet tall into the ground beside the tuber hole. Some have found tomato cages to be satisfactory in supporting the plants, but these too have wire spears and so they should be put in place when you can see exactly where the tuber is to be located. 

Tomato cages are generally only satisfactory for the smaller plants. With the stake or tomato cage in place and a planting hole on one or both sides of the stake, place the tuber in the hole laying longwise on its side, with the sprout or eye facing up. 
If the tuber has a sprout an inch long or more, care should be given not to damage the fragile shoot. However, if this does happen, and it is very easily done, then don’t despair, there are auxiliary eyes at the base of the broken shoot and they will grow, but you will have lost some advanced growth in your future plant.

Do not add fertiliser to the hole as this may damage the new tender root system.
Cover the tuber with 4-5 inches of dirt. Some gardeners have found it helpful to hill the plants as they grow to provide support to the stems, but often this is not adequate in wind prone areas. Tie a name-tag on your stake so you will know later which plant is growing there.


Unless it is a very dry spring, it should not be necessary to water at the time of planting. The tubers will begin growing with the warmth and moisture in the soil.
It is vital that they form a root system early in their planted life to assure a strong and healthy plant. Watering at the time of planting may encourage rot causing you to wonder why that prized variety is not growing. When you carefully investigate the problem, you may not even be able to find the tuber or you will find a lump of rotten muck. Not a pretty sight! Once the plant begins to grow you can begin to water every few days. 

Watering will be necessary in most areas throughout the summer months. Water dahlias at the root level using a drip system. Deep watering, or in other words, a good soaking is better than passing a spray on the plants for a short period. 
In fact, it is much preferred in order to prevent disease on the foliage and to conserve water. Many growers find the soaker hose in its various forms to be the best and then the water goes in the soil where the plant needs it.


Once the shoot is above the soil surface, the first slug within a city block will ‘smell’ it. Be prepared!! Get out that slug bait and spread it liberally everywhere, or the slimy pests will devour every tender morsel for lunch! You may have other methods of taking care of these hungry critters, but I have found the slug pellets to be my favourite as I can broadcast them over an area very quickly and in most cases with good results.


Now that you have gone to all that work, you will want to care for your plants and see them produce beautiful prize-winning blooms. Keep the area weed-free, or at least as much as your back will tolerate. Also, remove any broken or damaged foliage. Good air circulation, especially near the ground is needed by the plants to prevent powdery mildew. Once the plants are several feet high the lower leaves can be removed to increase air circulation.


When the plants get to be a foot tall, be sure to begin tying them to the stakes. 
A wind will lay your plants flat from here on and may even break the stalk from the tuber. (Then you will be sad!) You will need to continue to tie them to the stake every 18 to 24 inches. If you grow 4 or more plants of one variety, you may find it easy to run garden twine the full length of the row. This can easily be done down each side of the plants using the existing name stakes. The twine should be spaced every 8 – 10 ” up the stake and can be secured on the stakes just with a single wrap around each stake as you go down each side of the row. This fast method works well until your partner snips the string while cutting a bouquet of flowers.  


In some cases, there will be three small buds and one large bud at the tip of a lateral. When this occurs, the third bud will be attached to the stem of the central bud together with a small leaf. The 1st and 2nd side buds will be in their usual place on their respective stems. When the buds are first visible, all four are firmly packed together and hard to differentiate. All three of the smaller buds should be removed.

If a plant is not disbudded, all the buds will develop. The picture on the right shows a central bud and two side buds whose stems are lengthening. Eventually, the stems will be 6 to 10 inches long and all three buds will bloom. The three blooms will be approximately the same size but smaller than the central bloom would be if the two side buds were removed. Ideally, when the blossom is mature, the length of the stem of the central bud should be 1 and 1/2 times the diameter of the bloom. Technically, the removal of the side buds and the third small bud, if it exists, constitutes disbudding. However, in practice, the activity of disbudding also includes removing some laterals.

When the small side buds have been removed, two questions need to be asked and acted upon when answered.

The first question is:

  • Will the stem for this bloom be long enough for the purpose it is to be used?

Blooms are generally cut just above a node. In the picture on the right, two side buds were removed from the first node below the central bud. The next node down has two laterals (sometimes two buds instead of two laterals) and two leaves. In the picture, the distance from the bud to just above the second node is about three inches. By the time the bud matures into a full bloom, that distance may be estimated at 9 to 12 inches – not really long enough to ensure good placement in a competition type vase. Therefore, the two laterals at the second node would be carefully removed – do not remove or injure the leaves. Later the bloom would be cut just above the third node and the stem would be 18 to 20 inches long.

Alternatively, if the distance from the bloom to just above the second node was estimated to be 15 to 20 inches, the two laterals at the second node would be retained and the bloom stem would be cut just above the second node.
The picture on the right is a similar view of another plant. It is presented in the hopes that one of the two pictures will be helpful in understanding disbudding and removing laterals.

The second question is:

  • Do we want this plant to be on the tall side, the short side, or medium height for its type?

If it is desired that the plant be taller than the average, do not remove any more laterals than is required to get the desired stem length. 

If a shorter plant is desired, remove the laterals at the next node down. 
Some people remove laterals two and three nodes below the node where the side buds were removed. In the picture, the laterals at the second node are currently about waist high. They will grow considerably. A little imagination suggests that leaving the laterals at the second node would eventually result in a very tall plant.

Removing laterals is an art, not a science. The beginner is advised to disbud and remove the laterals at the next node down if they are laterals. If the growth at that node appears to be additional buds, they should be removed. Beginners will not be beginners for long. They will very quickly develop their own disbudding and lateral removing strategies once they see the results.