Hydrangea shrubs ((Hydrangea spp.) offer even more than lush foliage and large, frothy flowers. They also provide a fascinating chemistry lesson for the entire family. That's because which fertilizers you use on hydrangeas affect the color of their flowers by altering the pH level of the soil. And unlike most chemistry experiments, the ingredients you need to boost plant health with a homemade fertilizer, and to influence flower color if you choose, are household items or those which can be gathered in the wild.
Mountain hydrangeas (Hydrangea serrata) and bigleaf hydrangeas (Hydrangea macrophylla) produce flowers that are blue, pink or somewhere in between, depending on the pH level of the soil in which they are growing. Before you amend your soil, use a soil test kit or pH meter to determine the current pH level of the area in which you'll be planting your hydrangea. Blue flowers emerge when the pH level is below 5.5, and pink when it is above 6.5. A pH level in between is likely to produce purplish flowers.
If your soil's current pH isn't the one which will produce the flower colors you want, use amendments to adjust pH. You and your family can have fun experimenting with adding more, less or different garden amendments to get the results you want, or to change the flower colors from year to year.
For Blue Flowers
If you prefer blue flowers and your soil isn't naturally acidic, a homemade amendment can acidify the soil. Make a family outing of gathering acidic materials from nature. Pine needles, oak leaves and pine bark all contribute to the conditions that turn hydrangea flowers blue. Work as much of the materials as you can gather into the soil several months before planting the bushes. Once you've planted the bushes, mulch them with shredded leaves, pine needles or pine bark.
For Pink Flowers
Mountain and bigleaf hydrangeas produce pink flowers in alkaline soil. If you're lucky enough to own a woodstove or fireplace, gather the ashes once they have cooled completely. Working the ashes into the top several inches of the soil will raise the pH level. This is best done several months before planting the shrubs. In subsequent years, scatter wood ash on the ground near each bush. Alternatively, blend 1 tablespoon of white vinegar with 1 gallon of water and pour this acidifying solution around the base of each shrub once or twice during the growing season.
For General Health
Compost is the ultimate homemade fertilizer, enriching the garden while also improving both sandy and hard-packed soils. Unless your soil has specific, severe nutrient deficiencies, compost is often all you'll need to nourish your hydrangeas. Use compost both as an amendment for working into the soil before planting, and as a yearly fertilizer. To make compost, layer nitrogen-rich materials such as grass clippings, vegetable and fruit scraps and non-seedy weeds, with carbon-rich materials like leaves, hay or newspapers. Let the pile become at least 3 feet wide and tall, then begin turning the materials frequently. The more often you disturb the materials, the sooner they will break down into rich garden compost.
For Quick Boosts
Periods of heavy rain can encourage plant diseases on hydrangeas and leach nutrients from the soil. To give hydrangeas an instant boost, mist the leaves with homemade liquid fertilizer. Compost tea is a classic example of an amendment you can make yourself, especially if you have a compost pile. After filling about one-third of a 5-gallon bucket with finished compost, fill the rest of the bucket with water, and leave it to steep for at least 24 hours. Stir the liquid solution several times, and then strain it at the end of the steeping period. It's best to dilute the liquid compost with plain water until it is the color of weak tea. Use a spray bottle or orchard sprayer to apply the compost tea to hydrangea leaves, and use a watering can or bucket to saturate the bush's roots with the rich liquid fertilizer.