Dahlias may be a relatively small genus of 49 species, but their habit of hybridizing and a whole host of cultivars have made this plant an extremely complex one to talk about.
In fact, the Royal Horticultural Society and American Dahlia Society both use a special classification system of 15 groups and recognize more than 57,000 registered cultivars!
Depending on the group your dahlia belongs to, it might have single flowers, double flowers, blooms the size of a dinner plate, flat or round blooms, etc.
This makes dahlias one of the most versatile plants out there. Of course, this also means that some dahlias prefer more water or light than others, and many other nuances of care can be slightly different from one plant to another.
However, dahlias are similar enough that there’s a median from which you can work if you aren’t sure what you’ve got (such as when someone gave you an unmarked dahlia tuber as a gift).
How To Grow And Care For Dahlia Flowers
Despite their variations, dahlias are known to be prolific bloomers and can bloom anywhere from late spring through fall, depending on the group.
Here’s what you need to know to grow these wonderful plants.
WARNING: Not All Dahlias Are The Same
We’ve mentioned it before, but it’s important to stress that not all dahlias are created equal.
The instructions in this guide SHOULD work for most dahlias out there, but you should always defer to instructions specific to the cultivar or species you’re growing when possible.
This is because there are dahlia groups that have a higher cold tolerance than others. As a result, some need a different amount of water or have slightly different fertilizer needs.
As a result, you might not get as much out of your dahlia as you could when using these instructions (but you will still have a beautiful plant overall).
A Few Basics
Depending on whether you have a dwarf or full-sized plant, a dahlia can range from 12” inches to more than 8′ feet in height.
Their flower sizes, colors, and shapes also vary greatly from one group to another.
While there are cultivars bred to be exceptions, most dahlias have unscented flowers. They are of little interest to pollinators, meaning you’ll want to group them with other plants that draw bees and other pollinators in if you wish to get your plant to seed.
However, as most cultivars are propagated through stem cuttings or tubers, this is usually not an issue.
Environmental Needs
Dahlias need a spot in your garden (or home) to get 6 to 8 hours of full sun. However, in harsher climates, they will prefer bright, indirect, or filtered light during the afternoon.
Morning or evening exposure works best, although, in colder climates where the sun never gets too harsh, you can get away with full southern exposure throughout the day.
As a general rule, dahlias don’t like cold, and most groups do best in USDA hardiness zones 8 to 11, with some being able to grow as perennials in zone six.
Any further north, they can be grown as annuals, or the tubers can be dug up and overwintered indoors.
You should never plant your dahlia tubers until the risk of frost has passed, preferably once the soil (not air) has reached a temperature of 60° degrees Fahrenheit.
For this reason, it’s a common practice to start both seeds and tubers indoors approximately 6 weeks before the expected final frost, then transplant them to the garden.
Soil and Feeding
Dahlias are quite forgiving with soil as long as it drains easily. Outdoors, consider mixing in some coarse sand, perlite, or vermiculite to improve drainage.
You may also wish to mix in some organic compost or peat or sphagnum moss (the former adds acidity while the latter reduces it) for extra nutrition if the soil isn’t very rich.
The soil pH should be slightly acidic, although many dahlias can handle a neutral soil pH as high as 7.5,
Potted specimens will do well with most potting soils or even a soil-free mix, although an African violet mix works a charm.
You will need to repot every 2 to 3 years to replace the soil and address root binding if present.
Note that most dahlias will need a stake or other form of support as they get bigger, regardless of whether they’re in a container or the garden.
Tubers won’t need to be fed for the first 30 days after planting. If you intend to use only one type of fertilizer, aim for a 5-10-10 or 10-20-20 NPK liquid soluble mix.
However, to get the most out of your dahlia, consider using a 10-10-10 fertilizer until the flower buds begin to form, then switching to a 10-30-20 blend.
This encourages fuller growth in the spring due to the higher nitrogen, with the focus shifting to phosphorus (needed for healthy blooms) to get superior blooms.
Remember to cut back on feeding in the autumn and winter.
As for dosage, be sure to follow the instructions given on your fertilizer packaging and check for the specific frequency needs of your plant, as this can vary.
However, it’s safe to start with a dose every 3 to 4 weeks and adjust based on how your dahlia reacts.
Watering
Your dahlias will need to be watered when the soil is dry 1” inch down.
This can be tested by simply sticking your finger straight down into the soil (the tip to the first knuckle is approximately an inch).
Use the soak-and-dry method for the best results, and avoid overhead watering.
Maintenance and Repotting
Dahlias don’t need a lot of pruning beyond removing damaged or diseased foliage and any preferred shaping.
Pruning away the lowest branches can also increase air circulation and reduce the risk of moisture-related problems.
However, there are two times when you’ll want to pinch your plant. The first time is when it reaches 15” inches tall.
At this point, there should be three sets of branches, and you’ll want to punch just above the top set. This encourages fuller growth.
The second time is when the flowers are blooming. Deadhead spent flowers to encourage additional blooms.
Just remember that you’ll need to leave a few flowers alone if you want to harvest their seeds.
Pests and Diseases
A range of pests will try and snack on your dahlia plants.
These include:
- Aphids
- Earwigs
- Mealybugs
- Slugs and snails
- Spider mites
- Thrips
The good news is they’re fairly resistant to disease and only run a risk of fungal infections or root rot if improperly watered.
Just note that while it’s perfectly safe for humans to eat flowers and tubers, they can be toxic to your pets.
