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I’ve been growing dahlias for many years and I want to share with you the best fertilizer for dahlia plants, as well as ones that you should AVOID!
In order to get armloads of flowers from your dahlias, it is important to understand this, so keep reading to learn all about how you can help your dahlias become a flower production machine.
BEST FERTILIZER FOR DAHLIA PLANTS
Before I get into how to fertilize your dahlia plants, it is important to understand a crucial requirement that your dahlia plants need to bloom, namely, light.
Dahlias need at least some sun to produce flowers, and preferably full sun for best flower production. If all you have is a shady spot, fertilizing won’t be helping you much.
As long as you have a sunny spot with at least 6-8 hours of direct sunlight and can supply abundant water, you’re good to go! They can flower with less sun, but it will be reduced.
Dahlias are heavy feeders, so it’s important to fertilize regularly so you can ensure an abundance of flowers.
Now let’s discuss the types of fertilizers to use and which ones NOT to use.
AVOID NITROGEN-RICH FERTILIZERS
I know we see Miracle-Gro fertilizer everywhere, and it is a wonderful fertilizer to use for many plants, but this is not what you want for your dahlias.
I’m talking about the standard Miracle-Gro Water Soluble All Purpose Plant Food that contains a 24-8-16 NPK ratio.
What is NPK? All commercial fertilizers contain an NPK analysis on the package. The 3 letters stand for the amount of Nitrogen, Phosphorus, and Potassium (potash) in that order.
These are the 3 primary macronutrients that are essential for plant growth.
This fertilizer contains too much Nitrogen and the proportion of Nitrogen, Phosphorus and Potassium is not ideal. Too much nitrogen can result in the following in dahlias:
– An abundance of foliage with reduced flowering.
– Weak stems
– Dahlia tubers that don’t store well
While it doesn’t have to be exact, you’ll want to use a low nitrogen content fertilizer.
Look for a fertilizer where the first number (Nitrogen) is lower than the other two numbers and where it is also about half (or slightly less than half) of the other two numbers. Some examples are:
At the end of the post, I’ll describe a fertilization schedule of when I actually use the following fertilizers.
1. COW MANURE
If you can get your hands on cow manure, this is an amazing additive for your soil. Keep in mind that you must use well-rotted cow manure and never use fresh manure as this can burn your plants.
You can easily buy bags of composted cow manure at any garden center. Simply dig in some composted cow manure at planting time, before you plant your tubers, but you can also add it in throughout the growing season after your plants are already in the ground.
Dahlias can not tolerate frost and they don’t like cold soil, so be sure to plant after the danger of frost is over for your growing zone.
I like to dig in some cow manure before I plant because it will help to lighten the soil up if your garden soil has a lot of clay like in my garden.
Clay soils can be challenging to grow in, but If you amend your soil every year with cow manure, your soil quality will improve dramatically.
Proper soil preparation is important as it helps to create a well-drained soil that dahlias love, as well as a fertile soil that they need for good overall plant health, proper growth, and abundant blooming.
Even if you have a sandy soil, digging in composted cow manure will add nutrients and help water retention since sandy soils tend to have very rapid drainage (almost too good sometimes!)
Regardless of the soil type, cow manure will benefit your soil structure and your plants.
Do not use any other type of manure, such as poultry manure, because it can be too rich and end up burning your dahlia tubers. If you want to learn more about manure usage in your garden, check out this great post from the University of Wisconsis-Madison on using manure in the home garden.
2. LILLY MILLER BULB AND BLOOM FOOD
If you like using organic fertilizers, this is definitely a great one. The 4-10-10 NPK formulation is a great proportion for your dahlias and provides enough nitrogen for good growth, but also plenty of phosphorus and potassium for gorgeous flowers!
Lilly Miller Bulb and Bloom Food (link to Amazon) also contains bone meal which is rich in Phosphorus, and is a slow-release fertilizer that feeds for 4 weeks, so be sure to reapply monthly to your dahlia’s soil throughout the growing season.
If you are growing dahlias in pots, apply this fertilizer a little more frequently (every 2-3 weeks) since nutrients will also be washed out through the drainage holes when you water.
Keep in mind that Lilly Miller Bulb and Bloom Food comes in the form of dry granules that you would sprinkle into the soil, mix it in gently with a hoe, and water it in.
After each application, make sure you give your dahlia plants a good, deep watering.
3. LILLY MILLER MORCROP TOMATO & VEGETABLE FOOD
It just so happens that many tomato fertilizers are also great for dahlia flower production! The Lilly Miller Morcrop Tomato & Vegetable Food is a 5-10-10 formulation which is perfect for both your dahlias and tomato plants.
It comes in the form of dry granules that you would sprinkle into the soil, mix it in gently, and water it in.
4. ALASKA MORBLOOM
This is a great fertilizer to use starting in late summer. The reason is because there is zero nitrogen in this fertilizer, so after your plants have grown enough, this will help focus your plant’s energy on producing blooms.
Unlike the previous fertilizers in this post, Alaska Morbloom is a liquid fertilizer that you’ll have to mix in a watering can, so you’ll have to hand water your dahlias when you apply it.
This fertilizer is wonderful because it will encourage more blooms late in the season and will ensure that your plants keep producing beautiful flowers for you until frost comes.
Follow the fertilizer label for frequency of application.
DAHLIA FERTILIZER SCHEDULE
As I mentioned earlier in this post, I use combination of the fertilizers in this post.
AT PLANTING TIME
First, I like to dig in composted cow manure into the soil.
SPRING THROUGH LATE-SUMMER
About a month after planting, I will start to use either the Lilly Miller Bulb and Bloom Food or the Lilly Miller Morcrop Tomato & Vegetable Food. If your dahlias are in the ground, apply one of these fertilizers (or a similar fertilizer following my NPK guidelines earlier in this post), every 4 weeks.
If you are growing your plants in pots, you can apply every 2-3 weeks.
LATE SUMMER THROUGH FIRST FROST
Then from late summer until frost, stop using the Lilly Miller fertilizer and switch to Alaska Morbloom.
When applying fertilizer, always use the recommended amount on the label, as well as the recommended frequency of application.
OTHER DAHLIA GROWING TIPS
Be sure to check out my post on general dahlia growing tips. It goes over additional topics like:
– The best time to plant tubers
– Proper watering of your plants
– Deadheading old blooms
– When and how to prune your dahlia plants in order to get sturdier, bushier plants
– Storing tubers so that you don’t get tuber rot
I hope you’ve enjoyed this post on best fertilizing practices for dahlia plants. Do you grow dahlias? Comment below. I’d love to hear!