How to Grow Series
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Clematis have a very similar feeding schedule as roses and require the same type of fertilizer as well, except for the first year. I prefer to use organic granular, slow release fertilizers as they are better for the plants in the long run and it's more difficult to over fertilize. Liquid fertilizers like compost tea or alfalfa tea are great for a quick boost of energy to plants just before they start to bloom and they feed the helpful organisms in the soil as well. I avoid products like Miracle Grow because they do nothing to improve the soil and are basically junk food for plants.
The first year, you want to encourage root and foliage development, so a fertilizer higher in Nitrogen is recommended. Nitrogen is the first number in the sequence of three on the fertilizer package (N-P-K; Nitrogen - Phosphorous - Potassium). I use Espoma products, so Plant Tone would be perfect for the first year of your clematis. Follow the instructions on the package based on size of your plant. You will want to fertilize once at planting time, then again about a month later, but don't fertilize after August 1st for northern climates and September 1st for southern climates. You want the plant to slow down it's growth so that it can wind down for winter.
Once the Clematis has been in the ground through at least one winter, you want to switch to a fertilizer that is higher in Phosphorous, or the second number on the fertilizer package (N-P-K; Nitrogen - Phosphorous - Potassium). Rose Tone by Espoma is perfect for established clematis. Again, follow the package instructions for the time of year and size of the plant for the correct amount of fertilizer needed.
Time Line for Fertilizing Clematis
Early Spring - Snow has melted, ground is thawing and daffodils have started to bloom. At this time you will start to see some new buds forming on the stems of your clematis. Pull back the existing mulch, spread about an inch of compost or composted manure around the base of the plant and apply your higher Phosphorous fertilizer around the plant. Scratch the compost and fertilizer into the top inch or so of the existing soil. Water well, replace the mulch, then water again.
4-6 Weeks Later - just before flower buds begin to form, you will want to pull back the mulch, apply another dose of granular fertilizer, water well, replace the mulch and water again.
When buds have swelled - At this time I like to use a liquid fertilizer such as compost tea or alfalfa tea to give the plant a quicker boost of fertilizer just as they start to bloom. I generally apply this to my entire gardens at this time. At this point, I don't fertilize again until after they have finished blooming. Fertilizing during the bloom cycle accelerates the blooming and they will be done blooming sooner. If your soil is full of compost and manure and you have been diligent about applying fertilizer twice before, you can skip this step with no harm coming to your clematis or to it's blooms.
After blooming - Apply another dose of granular fertilizer and this is usually the last one of the year. You want to stop fertilizing during extreme hot spells and at least two months before your first frost in the fall.
Spring Blooming Clematis (Group 1 or A) should be pruned right after they bloom. If you prune them before they bloom you will be removing the flower buds. They also don't need a lot of pruning. Just lightly prune the vine to shape it and keep it in check.
Continuous Blooming Clematis (Group 2 or B) don't require a lot of pruning either. All you need to do is lightly prune the stems after each flush of bloom, as needed to tidy up the plant.
Summer & Fall Blooming Clematis (Group 3 or C) need to be pruned to about 18" high in early spring just before they start to bud out. I usually do this at the same time I fertilize for the first time. Pruning now helps prevent dead looking stems at the base of the plant and to encourage more sprouts from the base of the plant. This will also give you a chance to remove any excess stems as these bloomers tend to get very large and tangled over time without pruning. Group 3 clematis don't need pruning again until the following Spring.
Clematis are pretty disease and pest resistant but they can become susceptible to Clematis Wilt from time to time, though I have rarely had this problem. Large flowering varieties are more prone then small flowering varieties.
Clematis Wilt is actually a fungal disease which affects the leaves and stems but does not affect the roots, so plants rarely die from this. Clematis Wilt is more common during hot, humid, wet weather and is generally seen in mid spring to early summer when clematis are growing rapidly.
At first sign of the disease, cut the affected stems right down to ground level and discard the stems in the trash (do not compost them). Remove and discard any fallen leaves on the ground around the plant. When watering plants water at the base to avoid getting the stems and leaves wet.
Since this disease can over winter in the soil, it is best to cut the remaining stems down to the ground in fall to help prevent the disease from spreading. Always disinfect your pruning shears with a diluted bleach solution between cuts (10 parts water to one part bleach).
Prevent Clematis Wilt by making sure it is planted in an ideal situation first with good compost rich soil in full sun with their roots shaded. Planting at least two leaf nodes below the surface is very important. Any stems that might get damaged from wind should be pruned away as soon as possible. A plant that is not stressed or damaged is less likely to get any disease.
If Clematis Wilt tends to be a problem on a yearly basis, consider spraying the plant with compost tea or alfalfa tea monthly as these fertilizers also help plants defend against diseases. If this does not work, you may want to replace the plant with a variety that is not as susceptible to Clematis Wilt.
See photos of clematis from my garden here.