Pot-grown clematis can be planted anytime of year. The ideal time for planting is spring and early fall. Plant when the soil is neither frozen nor very wet, remembering that all clematis require well-drained soil.
Fertile, loamy soil in the garden is ideal for cultivation of clematis, most of which thrive on a regular supply of balanced nutrients and moisture. In the absence of such soil, or if the soil is not in very good condition, prepare the site before planting a clematis. If the soil is heavy clay, add coarse horticultural grit or sharp sand to improve it. If the soil is sandy, add as much humus (organic matter) as possible. Well-rotted farmyard or horse manure, garden compost, leaf-mould, mushroom compost, or good-quality, proprietary, soil-based potting compost are excellent materials for improving and enriching the soil.
Although clematis are known to thrive in alkaline soils (pH value more than 7), they also grow satisfactorily on neutral to acid soil (pH value of 7 or below). For better results with extremely acidic soils, add ordinary lime (calcium carbonate), which is both easy to handle and relatively safe to use. Ideally, lime should be dug into the soil far in advance of planting. If using farmyard manure to enrich the soil, refrain from adding lime at the same time, as it reacts with the nitrogen-rich manure to release nitrogen in the form of ammonia, which may damage the plants. Annual liming is not recommended as over-liming may result in deficiencies of other soil nutrients. It is worthwhile, however, to test the soil occasionally, especially light, sandy soils, as rain tends to leach lime from it. Lime-rich mushroom compost also helps to increase soil alkalinity.
When planting a clematis in the garden, dig a hole at least twice as wide as the pot in which the plant is growing and at least twice as deep again. 18 by 18 in. or larger is ideal for accommodating organic matter below and around the root ball, giving the clematis a good start for healthy growth. Loosen the base and sides of the hole with a garden fork. If the soil is heavy clay, place some coarse grit or sharp sand mixed with the soil at the bottom of the hole before placing any organic material in it. This improves drainage and prevents water logging. Place some well-rotted compost or shredded peat at the base of the planting hole to a depth of at least 4 in.
Soak the container in which the plant is growing so that the entire root ball is wet. Make sure the planting hole is moist. This is a very important step before planting because, once planted, the roots will need time to grow into the surrounding soil, absorb water and nutrients, and distribute adequate amounts of water to the rest of the plant. Ease the plant with its cane support out of the container and gently loosen the roots at the bottom of the root ball to encourage quick growth into the surrounding soil. Place the root ball in the prepared planting hole.
Ensure that the surface of the root ball is at least 2in. below the rim of the hole. Deep planting encourages large-flowered cultivars to develop a healthy basal root crown of buds below the soil level as a precaution against sudden wilting. When the plant is in position, fill the area around the root ball with equal parts of good soil and potting compost mixed with the recommended amount of any general-purpose fertilizer. Gently firm the mixture around the root ball. Cover the base of the plant with additional organic matter used at the base of the planting hole, taking care to keep it away from the stems or vines. When mounded, it serves as a mulch to prevent excessive loss of moisture. Water the plant well, allowing at least 1 gallon of water per plant. Finally, attach a label to the plant with the name of the clematis and the date of planting.
To provide a certain amount of shade for the plant's root system and to prevent excessive loss of moisture, plant a low-growing perennial or shrub close to the newly planted clematis. Avoid using slates, slabs, or tiles to shade the clematis roots as these also provide hiding places for slugs, snails, woodlice, and other insect pests.