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When you consider how dahlias burst out of the ground in spring, grow up to two metres and produce abundant flowers for four months or more plus a clump of daughter tubers in just a single season, it is no surprise that they need plenty of tucker (and water).

Because they are so hungry, give them a good start by preparing the soil in advance of planting in Spring.

Understand the soil pH

It is easy to test your soil with pH testing kits available from nurseries. Test more than one area of the proposed bed at both surface level and at a depth of about 10cm.

The ideal pH for dahlias is around 6.8 or between 6.5 and 7 (neutral). A higher or lower pH will lock up nutrients making it harder for the plant to thrive. To raise the pH (making the soil more alkaline) add dolomite lime. Gypsum also helps to break up some types of clay.

Lowering the pH of alkaline soils is more difficult. Alkaline soils have a higher buffering capacity that counteract changes in the pH. Often changes are not long-lasting so you need to keep an eye on this. The addition of sulphur will help and so will leaf litter, animal manure and most composts because they tend to be slightly acidic - but not mushroom compost which is alkaline.

To dig or not to dig?

After the tubers have been lifted, the traditional practice is to dig over the bed incorporating last year’s mulch and some new organic material. 

There is a growing move in horticulture and agriculture to leave topsoil in place and add new organic material on the top, thereby not disturbing the soil microorganisms and worms.

If you have heavy clay, you can leave it alone and build up the soil on top with compost, sand and manure dug in lightly if at all. Raised beds are ideal.

Building up the soil

Composts and animal manures are excellent for getting organic matter into the soil to improve its structure and to feed microorganisms that help break down soil.

Cow, sheep and horse manures are all good but chicken manure is the richest source of nutrients. Birds excrete both their solid and nitrogen rich wastes via a single outlet, whereas mammals tend to leave their solid droppings in a different spot to their urine so you miss the nitrogen.

If you use raw manure, incorporate it well into the soil and allow 6-8 weeks for microorganisms to break it down so it can be readily used by the new dahlias.

Planting a green manure crop

A green crop can be grown specifically to add nutrients and humus to the soil. Nitrogen-fixing legumes such as peas, fava beans and lupins can be mixed with mustard seeds which help fumigate the soil preventing things like nematodes and fungal pathogens. Plant the crop in July.

Dig it in before the crop flowers, 4-6 weeks before planting. Best cut the green crop just below the topsoil surface with a hoe. Apply a light dressing of nitrogen fertiliser, compost or both and allow seven to ten days for the soil to rest before digging it in. If you favour the no/low dig approach, this can be done lightly without turning over the soil.

Adding microbes to the soil

Much current research focuses on the relationship between plant health and soil microbiology. There are products on the market that introduce extra microbial bacteria and fungi to the soil.

Apply before planting and from time to time during the growing season. Seaweed and fish-based products promote microbial activity and serve as soil conditioners, helping plants take up nutrients.


While composts and manures will improve the soil structure, apart from chicken manure they are high in carbon but low in nitrogen and most other nutrients. Fertilisers directly provide nutrients for the plants including nitrogen, phosphorous and potassium plus other elements including iron, sulphur, calcium and magnesium.

Organic fertilisers are broken down over time by bacteria and fungi in the soil at much the same rate the plants take up nutrients as soil temperature and moisture levels rise. They can be incorporated in the soil a few weeks before planting or spread over the soil surface after planting.

Lifting and Replanting in One Hit

If your soil allows you to dig and replant your tubers in one go in Spring as some of the best growers do, most soil preparation must be done at the same time.

Well rotted composts with some added fertiliser can be incorporated. Specially formulated composts used as mulch (most garden centres stock suitable products) will also add nutrients and encourage “good” bacteria. Use liquid and/or pelleted fertiliser throughout the season.

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