Dahlias are one of my all-time favorite flowering plants and I have a couple of dwarf varieties growing in my garden – one in the ground and one in a large pot. If you’re new to growing these delightful flowers you might be wondering what is the best soil for dahlias to ensure that they thrive and flower abundantly.
To help you grow the most beautiful dahlias in your own garden or even in a large pot on your patio or balcony, I’m going to discuss the absolute best soil that you should be using.
But remember too, that dahlias need plenty of full sun to produce their gorgeous blooms, an adequate amount of water and also some additional fertilizer.
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What Is The Absolute Best Soil For Dahlias?
If given the choice, dahlias would select soil that is fertile and filled with organic material. In addition, the soil should be reasonably friable, easy to water but also free-draining.
That is most likely why the dahlia I have growing in a pot grows much faster and produces more blooms than the one I have growing in the ground.
You see, with pot-grown plants, you have complete control over the type of potting mix that you use and I’m always very particular about choosing a good-quality premium mix for my potted plants.
Why Dahlia Plants Prefer A Loose And Well-Drained Soil
As you might or might not know, dahlias grow from underground tubers. These tubers store the energy needed to produce new growth in the following spring.
When grown in nice loose soil, the new growth that arises from the tubers each spring finds it easier to break through the surface of the soil.
In addition, tubers will often produce babies and multiply under the ground. This means that every few years you can dig up the clump of tubers and divide them so that you have many more plants that you can spread around your garden.
However, all of this production that happens under the ground is far more difficult if your soil is heavy or compacted.
Plus, compacted and heavy clay soils make it more difficult for the roots of the plants to extract the nutrients that they need. This is because the heavy clay particles tend to bind some of the nutrients and this makes them unavailable to the plants.
Additionally, anything other than well-drained soil may cause the tubers to rot under the ground. That’s why good drainage is so important.
Where Do Dahlias Grow Naturally?
Understanding where dahlias grow naturally will give you a good idea of the type of growing conditions that they need. Dahlias are found naturally in the mountain regions of Central America and Mexico.
This gives us a couple of indications of the soil type that these plants prefer. Firstly, mountain soil is often rich in organic matter and has a reasonably deep and loose structure. In addition, the soil in these regions will be quite warm for most parts of the year thanks to the warm climates these regions experience.
While it may not be possible to replicate the identical type of soil that dahlias grow in naturally, there are many things that we can do to improve the soil that we have in our gardens. This will make our soil more hospitable to excellent dahlia growth.
How To Improve Your Soil To Accommodate Dahlias
There are many things that you can do to improve your soil, no matter what type of soil you have.
Sandy Soil
If you have very open sandy soil, then the structure is fine and the soil should drain freely but it’s likely not to be overly fertile or will hold much moisture.
In this case, you want to incorporate lots of organic matter such as compost and aged manure into the soil. Not only will this help with some water retention so that you end up with moist but not waterlogged soil, but it will also provide tons of nutrients for your dahlias.
So, sandy soil is fairly easy to fix to ensure that it is perfect for dahlias, simply by adding lots of organic matter.
Heavy Clay Soil
On the other hand, clay soils can be a bit more challenging. One advantage of clay soils is that they are usually quite nutrient-rich. However, the major and minor nutrients are often tightly bound by the clay particles, making them unavailable for the plants to use.
In addition, most clay soils don’t tend to drain very well which can be a problem for dahlias because the underground tubers are likely to rot if they’re constantly sitting in wet soil.
The best thing you can do to amend clay soil to make it amenable to plant growth is to add a product called gypsum to it. This product works to break up the clay particles so that the soil becomes looser and has improved drainage.
However, if you go down this track, it might take a few months or even up to a year for the soil to be greatly improved.
Gypsum is a naturally occurring mineral that is made up of calcium sulfate dihydrate. It’s the same material that is used to make chalk and is also often used in construction materials such as plaster for internal walls.
Another solution that I’ve had success with in the past is to add a thick layer of wood chip mulch over the heavy clay soil. Over time, the mulch will break down into the soil and improve it quite dramatically.
Of course, this method will also take some time to affect the overall structure of the soil but will often produce the best results.
In this instance, you might be better off growing some dahlias in large pots while you’re waiting for the soil to improve.
Loamy Soils
If you’re lucky enough to have loamy soil, whether it’s a sandy loam or even a clay loam, you can grow dahlias quite successfully without too many extra amendments.
I would still recommend that you add some organic matter to the soil such as compost but there’s really no need to dig this in heavily. Generally, all I do is throw some compost over the soil surface and then just rake it in a little.
When it rains or the soil is watered, all the particles will mix together naturally anyway. This is actually a much more sustainable way to nurture the soil. This is because you’re not pulverising it completely and destroying the basic structure.
Don’t Overlook The pH Level Of The Soil
When providing the best soil for your dahlias, it’s important not to overlook the pH level of the soil. Dahlias prefer a fairly neutral soil pH of between 6.5 and 7.
You can easily test the pH level of your soil with a soil testing kit. This is readily available either online or at your local garden center or hardware store.
If you find that your soil pH is too high, you can easily add more compost to it to make it more acidic. However, if the soil pH is too low, consider adding some lime to increase the pH level.
What Soil Is Best For Dahlias Grown In Pots?
When growing any plants in a pot, it’s important to remember to never ever use garden soil. Garden soil is far too heavy for use in pots. It will not sustain good plant growth as it will compact fairly rapidly.
Always remember to use a good quality premium potting soil such as Miracle Gro or something similar for your potted plants. This has the added benefit of fertilizers. These are designed to sustain plant growth for up to six months. The added nutrition will give your dahlias a good head start
You can even make your own potting mix by combining homemade compost with fine bark chips and perlite. The bark chips and perlite will help with drainage and will provide plenty of air pockets for the tuberous roots. The compost will keep the soil moist and provide nutrients for the growing plants.
Another good addition is coco coir as this will also help with moisture retention. Don’t use peat moss because many gardeners now avoid this product. This is due to the destruction of the peat bogs that have resulted from the harvesting of peat moss.
Frequently Asked Questions
Dahlias can grow equally well in pots as they do in the ground. However, what I found is that the dahlia I have growing in a pot will sprout faster in spring and produce more beautiful blooms. This is compared to the one growing in the ground. I believe that this is because the soil in the pot warms up faster.
The best way to encourage dahlias to bloom more is to deadhead them regularly. An annual application of organic fertilizer that has an adequate amount of phosphorus and potassium and not too much nitrogen will also help to produce more blooms.
The best time to plant dahlias is from early spring to early summer after the last frost date has passed. Remember to grow them in direct sunlight to get plenty of blooms. Ensure that the soil temperature has warmed up sufficiently as well.
Keep an eye out for spider mites. These can easily be controlled using an organic oil spray such as eco oil or neem. Powdery mildew can also be a problem. This is best dealt with by spraying your plants with a mixture of milk and water.
If you want your dahlias to be bushier, you have to pinch out the main growing tips. Use a sharp pair of scissors. Just cut the tip an inch or two from the top. Make the cut down to a pair of leaves. This will encourage your plants to produce more lateral stems and result in bushier plants.
Final Thoughts
In order for your dahlias to thrive and produce the most beautiful flowers, you have to supply them with the right foundation. This consists of right the soil that they’re growing in.
Essentially, dahlias will thrive in richly fertile soil that is loose and drains well. Sandy loam is perfect but clay loam will also work well. If you don’t have this type of soil, there are plenty of amendments that you can make. This will ensure that your dahlias have the nutrient-rich soil that they need.
When given the right conditions, you’ll be rewarded with the most stunning blooms. These also make excellent cut flowers to display inside your home.
