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Soil Building 101 for Dahlia Gardens

Posted on: March 29th, 2018 by Aimee Sherrill

Soil is the first step in a successful garden, because plants are only as good as the soil they are planted in.   Replacing nutrients lost over the winter months is essential to how well your plants will produce during the growing season.  Growing great gardens takes a lot out of the soil from year to year, and it’s important that we put that nutrients back into the soil from year after year.

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

Working our compost pile

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.

 Soil Testing:

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

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 plants.

7 Responses

  1. Sunny says:

    We have moved to a retirement residence with a large balcony and I’m wondering if I can successfully grow 3-4 dahlias in a large grow bag? ( I used the bag for potatoes so it’s quite large)

  2. Roger Michalski says:

    Dahlias are my favorite. Love all the good tips on growing them.

  3. Mimi Hendel says:

    We live in SW Minnesota. We had a cool spring. I planted my dahlia tubers on May 25,2022. Today is August 19, 2022.
    The plants are 2 1/2 feet or smaller. A few have flower buds, only 2 out of 40 have bloomed. I use Scott’s Bloom food every 2 weeks. My soil is clay. I mixed in coffee grounds and Peat Moss, but maybe not enough. I water every other day when its hot.
    I’m disappointed in my dahlia’s performance.
    What else can I do? Thanks.

    • Clay soil is never good for dahlias. When you plant again, replace the soil with bagged soil or a top soil delivered from a local source. They will only rot in clay soils. The Scott’s Bloom food may have been too high in nitrogen, too much nitrogen you’ll only get foliage and no blooms. Look for a granular 5-10-10 fertilizer. Switch to a soaker hose next season too, hook your hose up to it for about 45 minutes to an hour to deep soak the area. Hand watering is never enough. Thank you!

  4. Betsy says:

    Moving last years tubers into raised garden beds this year. I have Garden Mix being delivered but it doesn’t have any vermiculite/perlite. We get too much rain in Seattle, and I am going to try leave tubers in bed for winter. I will put 14″ diameter black nursery plant container with drain holes upside down to cover each clump of tubers with a brick on top to keep in place from wind. Hope this helps keep excess water away from tubers and allows for air circulation from the holes. (saw a garden like this and he has 100’s of dahlias) I’ll just divide tubers every 2-3 yrs. So now to my question. Should I be adding perlite to help with drainage instead of vermiculite, or both? What is the rule of thought for how much perlite/vermiculite per square yard of garden mix? Thanks for any suggestions.

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