Hydrangeas thrive in friable, loam soil and soil that has a high organic content. The soil needs to hold moisture and have a light structure that drains well. If the soil is rich in nutrients you will not need to add any fertilizer to the soil.
Hydrangeas can live in acidic and alkaline soils although this will influence the colour of the flowers if it’s a bigleaf hydrangea (Hydrangea macrophylla).
Soil pH for Hydrangeas
Hydrangeas can grow in both acidic and alkaline soils and notably Hydrangea macrophylla (commonly known as bigleaf hydrangea or French hydrangea) will change colour depending on the acidity or alkalinity of the soil.
Other species of Hydrangea that have white or lime green flowers are not affected in the same way by soil pH and their flowers will remain the same colour regardless.
If the soil pH is 6.5 or lower (acidic) then the colour of the flowers of big leaf hydrangeas will be blue. If the soil is over pH 6.5 (so either slightly acidic, pH neutral or alkaline) then the flowers will be pink.
You can manipulate the colour of the flowers of bigleaf hydrangeas by adjusting the soil pH.
To turn pink flowers blue the best course of action is to mix ericaceous compost (which has an acidic pH of 5.5) into the surrounding soil as either mulch or you can fork the hydrangea out of the ground during its winter dormancy and mix the ericaceous compost directly into the soil where the hydrangea is planted.
This is a process that will take some trial and error and it will take months to see the results, so patience is key!
To turn blue flowers pink you will need an alkaline soil amendment such as lime or wood ash or it may just need some regular compost added which will be around pH 7 once fully decomposed (leaf mould and well-rotted garden compost tend to be pH 7).
You can buy lime on Amazon which will raise the soil pH from acidic to either less acidic, pH neutral or alkaline (depending on how much you use and what the soil pH is to start with) or use wood ash as an amendment (which is alkaline).
Using garden lime is the option I would recommend as there are clear instructions on exactly how much lime to add whereas it is much harder to judge exactly how much wood ash you would need.
Adding too much wood ash at one time could drastically change the condition of the soil which can affect nutrient availability in the soil so it is best to stick to garden lime and follow the instructions.
To tell the pH of your soil is very easy and requires no expert knowledge. All you will need is soil test kits which are very inexpensive on Amazon.
Ideal Soil structure for Hydrangeas
The ideal soil structure needs to be able to hold moisture and yet allow for good drainage so the roots are not in saturated ground.
Soils that contain generous amounts of organic matter are ideal for hydrangeas as they retain moisture and provide the ideal porous structure for roots to grow through and establish.
Soil with high amounts of organic content generally have a healthy soil ecosystem with beneficial bacteria, fungi and earthworms which all contribute to increasing the nutrient availability in the soil.
Earthworms in particular feed on the decaying organic matter and produce worms casts which is a concentrated form of nutrients that is very easy for the plant to uptake.
Worms will also create channels in the soil as they work their way through, which creates space for roots to grow and alleviates compaction in heavy soils. This will also increase drainage and draw pockets of air into the soil which in turn fuels the beneficial bacteria and benefits the roots.
A healthy soil with lots of worms will equate to a healthy hydrangea that should (as long as other conditions are suitable) produce lots of flowers and healthy foliage.
Sandy Soils for Growing Hydrangeas
Sandy soils are problematic when it comes to growing hydrangeas as water will infiltrate through the soil too quickly and dry out before the roots of the hydrangea will be able to take up moisture which results in a dying hydrangea.
Sandy soils also tend to be low in nutrients which are not favorable for the hydrangea in terms of producing luxuriant foliage and a plentiful flower display.
Fortunately, you can make some adjustments to the soil which, with careful maintenance will allow you to grow hydrangeas successfully.
To grow hydrangeas in sandy soil you will need to amend the planting area with plenty of organic matter. A mix of compost and manure is ideal as both materials are good at holding moisture and the manure in particular will add a much-needed boost of fertility to the soil.
Add as much organic matter as you can even if it means replacing a lot of the sandy soil. You will need to dig the added compost to a depth of at least 6 inches and ideally up to 18 inches wide as this will accommodate the roots of the hydrangea when it is at full maturity.
Adding a layer of mulch every spring will continue to improve the moisture-retaining properties of the soil and add fertility.
Apply a two-inch layer of compost to the soil surrounding the hydrangea, leaving a gap of 6 inches between the mulch and the wood of the hydrangea to avoid exposing it to persistently moist material which can lead to rot.
As long as the soil is amended with good garden compost, leaf mould or manure then the ground will be able to retain enough water and provide the right balance of nutrients for hydrangeas to grow and flower successfully.
Although I would always water hydrangeas in gardens with sandy soils more diligently. During the growing season, a hydrangea that is in full sun will need water every other day with 2 gallons of water (9 litres) each time.
Growing Hydrangeas in Clay Soil
If you have loam soil which contains approximately 20% clay (the rest is composed of sand 40% and silt 20%) then this is ideal for growing hydrangeas. The clay is able to retain moisture whilst the silt and sand will allow for drainage and provide enough of a porous texture for roots to grow and establish in the soil easily.
However, if you have heavy clay soil then this can cause problems for hydrangeas. In heavy clay soil, water will either drain slowly or even form puddles around the roots for extended periods of time which will lead to root rot.
Also, slick heavy clay is a difficult material for roots to push their way through and establish properly.
Heavy clay soils that are slow draining must be amended before planting and growing hydrangeas.
The process is largely similar to amending the soil for sandy soils. You will need to dig out the heavy clay and replace it with organic material such as compost or manure, which will provide good drainage and be light enough in texture so that the roots can easily become established.
Make sure that you remove the clay to a depth of around 10 inches and add some sand to the compost mixture to make sure that water does not collect underneath the plant.
Digging a hole that is 18 inches wide will ensure that the roots of your hydrangea can grow through the soil and establish easily without being restricted by lumps of clay.
Fortunately, clay soil does tend to have good fertility which suits hydrangeas well and it is unlikely that you will need any additional plant feed for the hydrangeas to put on a good display for flowers every year.
As clay soil is naturally water-retaining, you may only need to water your hydrangea twice a week in the growing season, although this will depend on the climate.
Soil Nutrients for Hydrangeas
If you have enriched the soil with organic matter before planting the hydrangea then you are off to a good start in terms of providing all the right nutrients to produce a healthy plant with lots of flowers.
Adding a yearly application of mulch around your hydrangeas is also a great way to continually add some fertility to the soil and stimulate the beneficial soil ecosystem.
Under these conditions, your hydrangea should be healthy and have strong blooms.
However, if you have naturally poor soil or the roots of the hydrangea face competition for nutrients from other plants and trees (such as pine trees) then it is a good idea to add fertilizer to the soil to ensure the hydrangea gets all the nutrients it needs.
Hydrangeas will do well with a general shrub fertilizer but there are specific fertilizers that are made for hydrangeas which I would use if there is a lot of competition for nutrients from other roots in the soil.
Special granulated hydrangea feeds contain additional magnesium and iron to boost flowering as well as provide all nitrogen, phosphorous and potassium to meet their requirements.
Conclusion
Hydrangeas do best in loam soils or soils that have been amended with organic matter. The soil needs to be friable and porous in texture for good drainage and to allow for the roots to be established.
You can make some adjustments to poor soils to make them suitable for growing hydrangeas by adding amendments such as compost and manure which have exceptional water-retaining properties and a good concentration of nutrients.
Slow-draining soils that contain a lot of clay will also need amending with plenty of organic matter (such as compost) to improve drainage and allow the roots to grow freely and establish.
Heavy clay will likely need sand added to the compost mixture to ensure water does not pool in the soil and rot the roots of the hydrangea.
A regular annual mulch is very important to maintain the ideal soil conditions for hydrangeas.
Hydrangeas do not necessarily need additional fertilizer if the soil is healthy, however, if the hydrangea has to compete for nutrients with a lot of other plants and trees in close proximity then some general plant fertilizer twice per year is advisable.