Acid-loving Plant Care

How to amend the soil and care for acid-loving plants, plus a fantastic mulch recipe from Sheena Adams!

How to amend the soil and care for acid-loving plants, plus a fantastic mulch recipe from Sheena Adams

All plants react to soil pH, which is a measure of acidity or alkalinity on a scale between 0 and 14. The lower the number, the more acidic the soil; as you move up the scale the soil becomes more alkaline or “sweet.” When the soil is partnered with a plant suited to its pH level, the plant will be able to accept the nutrients in the soil, allowing it to grow and be healthy.

Most plants grow best in a pH range close to neutral, with a slight touch of acidity, or 6.5 on the pH scale. Some plants, such as lavender and rosemary, prefer a sweeter soil, a pH above 7. Others prefer more acidic conditions, about 5.5; these plants are referred to as “acid-loving plants,” as they require an acidic soil in order to take up the necessary nutrients that make them grow, bloom and produce fruit.

Popular acid-loving plants include rhododendrons, azaleas, blueberries, potatoes, strawberries and heathers. When planting and caring for acid-lovers it is important that their soil pH desires and acid-humus diet are met. There are several easy ways to accomplish this.

First, determine your soil’s pH using a pH thermometer. A more elaborate way to test pH is at a soil lab, which is beneficial as the test also measures nutrient levels, and will quite often include handy tips to improve any deficiencies.

If the pH is higher than 6.5, it’s time to acidify the soil. Pine needles, peat moss, bark mulch and leaf mould are all organic components that will lower pH. You can add these to the soil when preparing your garden beds, or use them once a year as a mulch to maintain soil acidity.

Sulphur is another natural way to lower pH. To reduce the level by one full point, you’ll need to add a little more than a kilogram of sulphur per 30 square metres (reduce this amount by one half if you have extremely sandy soil).

When watering is also the perfect time to maintain your acidic soil. Three times a year, water your plants with leaf mould tea, which will not only lower the pH, but also add potassium, promoting strong root growth, disease resistance and abundant flowering.

Save your coffee grounds, which are rich in nutrients and tannic acid, and sprinkle them lightly under your shrubs to help keep the pH down.

Another tip from the kitchen is to recycle leftover pickle juice (vinegar-based), mixing it with 20 parts water and pouring it over the soil. Or simply put all pickle juices, tea bags and coffee grounds into the compost – and be sure to use it often.

A common sign that plants are planted in too sweet a soil is when leaves or needles take on a yellow-green hue, a condition referred to as chlorotic. Plants become chlorotic when they cannot access nutrients (usually iron or manganese) due to the soil’s high pH level. If a plant struggles too long, it may suffer from death of leaves, dieback of branches and lack of vigour. Left untreated for several seasons, the plant could die. To correct the problem simply lower the pH level and fertilize.

Acid-loving trees and shrubs often have shallow root systems. When planting, remember to dig a wide and deep hole, and amend heavy clay soils thoroughly. It is also important not to over-fertilize as the roots, being so close to the surface, will easily burn. To encourage long feet, water infrequently and deeply; the plant will reach farther through the cool soil to seek its water.

The perfect soil for acid-loving plants is an organically enriched, well-draining garden loam. If the soil becomes waterlogged and the plants sit with wet feet, they develop leaf scorch, resulting in wilted leaves with brown patches. Amend heavy soil with sand and organic matter, and if the problem is severe, planting in a raised bed could be helpful.

Generally the soil in gardens planted near forest is naturally acidic from years of decaying forest debris. It is also common for these gardens to be invaded by weevils, as they appear to migrate from the forest into our gardens and are much attracted to acid-loving plants, particularly rhododendrons and primulas. An annual application of beneficial nematodes will help control their population.

Plants that prefer a lower pH will require similar care when it comes to feeding, mulching and pest control. When designing your garden, plan to keep the acid-lovers together in the garden bed, which will make their special care easier to provide.

Each spring begin your gardening with a simple pH test of your soil and plan your soil amendment around the results. Your rhodos, azaleas and camellias will thank you with bright-green leaves and huge, colourful blooms.

5 vital tips for acid-loving plants

Do not apply mushroom compost/manure to acid-loving plants. This material helps to balance acidic soil conditions, which is useful for roses, but not rhododendrons.

• Since most acid-loving plants are shallow rooted, be sure to add five centimetres of mulch in late fall to protect plant feet from winter conditions.

An acid soil will have a pH level below 6.5; anything sweeter and your acid-loving plants will struggle to get nutrients.

Leftover coffee grounds are perfect for sprinkling around your acid-loving plants. Three times per year, lightly sprinkle the grounds under the base of all your acid-loving plants. If you need extra, Starbucks and many local coffee shops are happy to provide them at no cost.

Keep lime away from acid-loving plants; an application of three kilograms over 30 square metres will raise the point by one (for example, from 6 to 7, a no-no for acid-loving plants).

Nutritious acid-loving plant mulch

1 bag fish compost, 30 litres
1 kg glacial rock dust
1 kg green sand
250 mL coffee grinds
125 mL Epsom salts

Mix thoroughly and apply a layer 7 centimetres thick and 30 centimetres wide around the base. Apply in early spring and again in late summer.

Fish compost

Fish compost is an excellent product for your acid-loving garden. Full of nutrients, it is perfect to use at planting time, as mulch, or as a major part of your soil amendment program. Fish compost contains nitrogen, potassium and phosphorus, as well as trace elements and calcium, all of which will help your plants develop good root systems and colourful blooms.

The product is extremely useful as spring mulch or garden bed filler, since it is free of weed seed, meaning no weedy surprises come late summer. I have prepared perennial beds, rose and rhodo beds with straight fish compost and they have performed beautifully. The dark colour shows off the plants, the compost holds just the right amount of moisture, and of course, no weeds. With a pH of about 6, it is just the right level to use with any acid-loving plant.

Made from composted fish and wood waste, fish compost is an innovative way to recycle the waste from local businesses, which means a major reduction of waste in our landfills. This ecological product can be found at local nurseries and garden centres. For a certified organic high-quality fish compost, look for Sea Soil, a brand made on Vancouver Island.

Weevil control

The root weevil is a common pest of rhododendrons, laurels, heathers and many other acid-loving plants. If you have noticed small chew marks around the outer side of your leaves, or plants being attacked at the root zone, then a root weevil has most likely been at work. It is a nocturnal pest, which devastatingly attacks the leaves and roots of plants.

The easiest way to rid the pest from an infected area is by applying beneficial nematodes. These microscopic wireworms source the pest larvae using heat and carbon dioxide sensors, and then enter the larvae and destroy it from the inside out. They are easy to apply; they are commonly supplied on a sponge, one million at a time. To apply, soak the sponge in a bucket of cool water and water the plant’s infected areas. It is important to apply the solution to the soil and then water it in well, and later in the evening is best.

If you can keep the area from drying out for at least a week, this would definitely help. A package of one million will do 900 square metres, and should be applied once a year. Use on your rhodo gardens, compost piles, and directly on your lawn. Nematodes should be applied when the soil is warm and damp, in late spring or early fall.