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Beneficial soil amendments and fertilizers for dahlias

Posted on: May 13th, 2021 by Aimee Sherrill

Soil is the first step in a successful dahlia garden, (or any garden) because plants are only as good as the soil they are planted in.   Adding amendments like compost every year, will replace nutrients lost over the winter months, and is essential to how well your plants will produce during the growing season.  This blog post will talk about compost and beneficial fertilizers to add to your dahlia garden.

Prepare your beds: Add Compost



Add a 4″ layer of compost to your planting beds, then till or work in with a pitchfork to mix the compost and the native soil together.  This will drive the nutrition back into your soil that was depleted from last years garden.  Compost improves the soil by adding  organic matter to make the soil softer, compost allows the soil to drain better, adds oxygen to the soil by improving drainage and attracts beneficial insects, worms and micro-organisms to you soil.  The benefits of compost is endless, but too much is not a good thing either.  Your plants don’t like to grow in straight compost, they need the micro-nutrients that comes from your native soil as well.

Clay soils will become more loose in texture allowing for better drainage.

Sandy soils will hold more moisture and nutrients by adding compost.

Small gardens options vs Larger gardens options for compost:

If you have a small garden area, pick up a few bags of compost at your local garden center or nursery.  It will tell you

Dahlia Row

Dahlia Row

on the bag, how many square feet of ground 1 bag will cover.  If you have a larger garden area and a truck, buying compost by the yard is a better option.  It’ll also be cheaper than buying the same amount of bagged soil.  Nurseries in your area often have compost by the yard that they load with a tractor directly into your truck or trailer.  Take it home and put a 4″ layer all over the area you want to plant.  Turn in with a rototiller and you are good to go when it’s planting time.

Time your dahlia planting just right:

Planting your dahlias at the right time is the key to successful dahlia growing.  Timing is everything in dahlias.    Plant your dahlias when your soil has warmed to 60 degrees and is not too wet or soggy.  A good rule of thumb to follow is to not plant your dahlias until you are safe to plant your tomatoes, potatoes or corn in your area.  Planting your dahlias should be one of the last of your spring plantings.


5-10-10 Fertilizer

5-10-10 Fertilizer

Dahlias prefer a fertilizer in the ratio of 5-10-10.  Low Nitrogen (1st number) and balanced ratios of phosphorus and potassium.  Giving dahlias too much nitrogen, will produce a plant that will give you a lot of foliage and not a lot of bloom.  Add 5-10-10 30 days after you plant and again in another 30 days.  Because when you plant a dahlia, it has no roots.  By waiting a month, that gives the dahlia time to set its roots, so it can grab that fertilizer when it’s trickling through the soil.  Nutrients like nitrogen will go through the soil so quickly, that by the time the dahlias have set it’s roots and can grab it, sometimes it’s already made it’s way through the soil and is gone.

Organic fertilizers

Organics can be worked into the soil when you plant.  If you can find Espoma fertilizers, Bulb Tone is what we like to use as an organic fertilizer.  Work into the soil inside the planting hole, and in the soil you’ll use to fill

Espoma Bulb tone

Espoma Bulb tone

in after you plant.  Organics are slow release and will slowly release nutrients all season.  Look for organics with “mycorrhizae”.  A beneficial fungus that attaches to the roots, and acting like extra roots.  Taking up extra water and nutrients to feed your plant.

Other Beneficial fertilizers to add to your soil:

Iron: will cause the leaves to be greeener.  Soils that are low in iron cause the dahlias to have slight yellowing to the veins.

Epsom Salt:  added every couple years.  Soils low in magnesium can cause yellowing of the veins.  Adding epsom every couple years will also help to green up your plant.

Bone Meal:  If you prefer to grow more organically, Bone Meal is a good option.  All bulbs will appreciate bone meal.  We recommend tilling it into the soil before you plant and not laying it on top of the soil.  Every critter in the neighborhood will smell that bone meal and dig in your garden any chance they can get.  Animal instinct, by tilling it in it reduces that frustration of replanting your garden.

Soil Testing:

Basic soil test kits tell you about N-P-K, N-Nitrogen, P-Phosphorous

Soil Test kit

Soil Test kit

and K-Potassium and will measure the amount of nutrients you have in your soil.  They can also tell you where your soil pH is at.   The pH level in your soil is important because it can effect nutrient uptake by your plants.  If the plant doesn’t like where the pH level is at, the amount of fertilizer that plant will absorb can be hindered.

Why your soils pH level is important:

PH has a range from 0-14, with 7 being neutral.  You want your soil pH to be about 6.3 – 6.8 or so for dahlias because dahlias like ‘slightly acidic’ soil.  Most soils will fall between the range of 6-8.  6 would be more on the acidic side and 8 would be more on the alkaline side.  Most acid soils are found by trees that drop their needles every year and cause the soil to be more acid.   Soil test kits can be found at any home improvement store this time of year.

What to do if your soil is too acidic or too alkaline:

Too acidic:  Add Lime to raise the pH level in your soil.

Too alkaline: Add Ferrous Sulfate (iron) to lower pH

The pH level in the soil is what would cause a hydrangea to be pink or blue in color.  By adding lime to your soil and raising the pH, the hydrangea will be pink.  By adding ferrous sulfate (iron) and lowering the pH, this will cause the hydrangea to be blue.

The process of changing the pH levels won’t happen overnight.  Often it will take months up to a year to see the full results.  But knowing now, and taking steps to correct it will grow a better garden in the future.  Try to make it a yearly chore, to test your soil.

Visit our Dahlia Care page for more information on growing great dahlias.

5 Responses

  1. Just received this today. Just in time. You guys are the best. I can plant with confidence but unsure to find the eye. Which side is up?

  2. george p. smith says:

    I certainly appreciate your Dahlia care updates. Very helpful and useful. I’m ready to plant as soon as the weather warms. We had snow a few days ago, unbelievable and unusual for Ohio in May! Thanks for all you do.

  3. Roger says:

    love my dahlias. thanks

  4. Thanks for the helpful tips

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