The pH Needs of Clematis

Soil Requirements

• Clematis can be difficult to grow if it isn't provided with the proper conditions, including soil requirements, and it won't do well if the soil is either too sour or sweet. Look for a pH of roughly 7.0, or neutral, when performing a soil test. This is also a good time to check for phosphorus levels in your soil, as clematis needs larger amounts of this nutrient for root formation and to encourage blooming. Phosphorus is represented by the letter P in the N-P-K nutrient designation listed on fertilizer packaging. When purchasing fertilizer for your clematis, look for a blend with a higher P rating, such as 5-10-5, which means that the phosphorus content is 10 percent of the entire nutrient formula.

Soil Amendments

• Preparing the site for your clematis plants should include working in lots of organic material such as compost, which loosens, feeds and aerates the soil. If your soil is acidic, or below the 7.0 pH range, this is also a good time to add lime, which sweetens soil. Soil that is too acidic affects how many plants absorb certain nutrients, while soil that is too sweet affects the absorption of others. Nutrients such as phosphorus, nitrogen and potassium are less available in soil with a low pH, while other nutrients such as chloride and manganese are more difficult to obtain in soil with a very high pH above 7.0.

Soil pH Considerations

• If your soil is too acidic, which is generally when it registers a pH of 6.5 or less, the directions on the lime packaging will tell you how much to add to bring it closer to 7.0 and make it suitable for growing clematis. A simply home soil test usually does not provide data on how much lime is needed, while a detailed soil test performed through your county's extension will provide that information, usually at low to no cost. Lime is best added separately from commercial fertilizers, as they may counteract each other, but it is safe to add with compost or other organic soil amendments.

Other Growth Requirements

• In addition to a neutral soil pH, clematis needs at least six hours of full sunlight per day, but can tolerate some shading, particularly during the hottest part of the day, which is usually afternoon in a south-facing orientation. Keep clematis roots cool and moist by using mulch or by planting low-growing perennials or annuals to provide shade around the base of the plants. Plant large-flowered varieties in a more easterly area to minimize color fading of the blossoms, and prune regularly to encourage more blossom formation each year.