34: Autumn 2014

Author: Jennifer Harmer

The Cultivation of Phlox

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The Cultivation of Phlox
Jennifer Harmer

Phlox are happy in open borders in sun or very light shade and the soil needs to be moisture retentive but not waterlogged. Phlox paniculata itself and cultivars close to the original species such as P.p. ‘Alba Grandiflora’ seem to grow very happily on the lighter chalk soils for many years without disturbance but most of the other cultivars do require a deeper, heavier soil and regular division. If phlox are grown in deep shade they are much more difficult to establish and do not always flower so well.

Phlox should be fed in spring with an organic fertiliser such as dried pelleted chicken manure. If they are grown in a mixed border make sure that other neighbours do not swamp the emerging shoots. If they get over-shadowed by larger plants they will not flourish and very often do not flower at all.

Established plants can be restricted to say 4 to 6 heads, the smaller the number of flower heads the larger the size of the panicles. I personally don’t restrict my plants but I do cut the front stems down to about 9”. This prolongs the flowering period and helps to keep the plant looking tidy. You can cut the back stems down when they have flowered and those in front will grow up to cover to them. The timing of the cutback depends rather on the season and location, end of April to end of May. Phlox should have lower growing plants surrounding their feet as no matter how well grown the plants are, the bottom 9” or so always tends to look rather tatty. Phlox do not normally require staking unless grown in a very windy site. Plants should be deadheaded to encourage re-flowering and to prevent self-sown seeds getting into a group of plants, thus losing the original named cultivar. Deadheading also helps prolong the season.

The roots are fibrous and shallow, they therefore need a good mulch in autumn which also helps with the moisture retention the following summer. Phlox are gross feeders and plants should be split every three year although if it is a very slow growing cultivar, the plant can be left for four years or even longer. If plants are left too long whilst growing strongly, the plant can seem healthy, but it can suddenly start to die back in the centre and quite often does not re-appear the following spring. When a plant is weak it becomes susceptible to disease. It is far better to have two or three smaller plants making up a clump than one very large plant. If, due to lack of space, it is necessary to replant in the same place always use fresh compost around the roots. Plants left growing on the same site without replenishment deteriorate and become susceptible to disease. When moving phlox to a different part of the garden try to remove all the roots, otherwise you will find the original plant may be difficult to eradicate. P.p. ‘Fujiyama’ is very vigorous and when moved tends to leave a trail of plants behind, as does P.p. ‘Franz Schubert’. Another plant which often causes confusion is P.p. ‘Norah Leigh’. If this cultivar is moved, any plants coming from roots left in the ground lose their variegation but the white flowers with a lilac eye are very pretty against the plain green leaves!

Phlox are not easy to grow well in containers, therefore if you are buying plants later in the season they may well have lost leaves and look rather sorry for themselves. However, as long as a plant is basically healthy it can be cut down, re-potted and fed with a liquid fertiliser and the plant should grow away well. It can then either be planted in the garden or over-wintered in a cold frame or greenhouse and planted out in the spring. I garden on cold heavy clay and never plant any herbaceous plants in the autumn.


Phlox paniculata can be propagated in a number of ways:

Root cuttings taken from September to spring are the safest way to ensure healthy plants and a large number of plants can be raised in this way. However, it does take two years at least to achieve a good flowering plant.

To take root cuttings – dig up the plant and shake the soil from the roots. Cut the strongest roots into approximately 2” lengths, and ensure that there is at least one bud. If you gently run your fingers along the root, you should feel a small bud which is the ‘eye’ which forms the new plant. Lay the cuttings lengthways on the potting compost, in either trays or wide flowerpots and cover with a grit/potting compost mixture or vermiculite. Alternatively, plant a single root vertically in a small pot. Water and cover the pot with polythene. Place in a garden frame or cold greenhouse. A little bottom heat can speed up the process but is not essential.

‘Irishman’s cuttings’ and soft tip cuttings taken from February through to mid-summer are a quick and easy method but you must be sure that the parent plant is absolutely free from disease. If there is the slightest doubt then only root cuttings should be used.

‘Irishman’s cuttings’ are taken early in the season and are small shoots which when removed from the parent plant already have roots. They just require to be potted up. Phlox root and grow quickly, so plants must be regularly potted-on to ensure large, healthy plants.

To make a tip cutting remove a non-flowering shoot no longer than 2” long, just below a leaf joint. Trim away the lower leaves and insert the cutting in gritty compost. I use 50% Perlite and 50% potting compost. Water and cover with a plastic dome. Hormone rooting powder can be used but I have found that these very small cuttings root very easily without it. I usually remove the cover after about three weeks, but it does rather depend on the weather conditions. I keep mine on a shaded bench in the cold greenhouse, they must not be allowed to dry out for they will very quickly shrivel. If these cuttings are taken late in the season, they must be showing a bud at the base of the stem before autumn or it is unlikely that they will survive the winter, although I do find that now I am only taking the very tip of the shoot they seem much stronger and come through the winter much better than the cuttings I used to take which were about 3-4” long.

Splitting established clumps in small sections is also very easy. Again the plant must be healthy. This can either be done in September after flowering or in the spring, depending upon the soil conditions. If plants are split regularly they can very easily be cut with a large carving knife or small saw but if the plant has been left too long and the centre is woody the old fashioned method of two forks back-to-back is the best way. The young outside parts of the plant should be re-planted and the old woody centre discarded. Do not be tempted to try and revive the centre, thriving new shoots will not be formed and a weak plant is liable to attract diseases.

Whilst named cultivars can only be reproduced vegetatively, phlox can, of course, also be grown from seed.

Pests and diseases

If not well grown Phlox paniculata is prone to mildew but if the moisture content of the soil is well maintained in the growing season mildew is not normally a problem until after flowering. The blue and purple cultivars are usually the first to show the signs but I try and give one or two preventative sprays in July and August when spraying the roses and hellebores. A good mulch helps preserve water but if it becomes necessary to water artificially, direct the hose to the roots and avoid wetting the leaves if at all possible. Artificial watering seems to encourage mildew but in a naturally wet season like the summer of 2012 plants were very happy. If the season has an early cool wet spell followed by a hot dry one, splitting of the stems can occur which weakens them and makes them rather brittle but provided they are not broken off they will flower quite happily.

Eelworm was the scourge of phlox in the past but nurseries are now selling clean stock and the old plants which are still growing very well in the gardens appear to be resistant.

Virus can also cause wilt and either a stem or whole plant just withers and dies but the same effect can be caused by lack of water, particularly in the large dense clumps with woody centres.

Greenfly and other aphids, particularly on pot grown plants, can also cause distorted leaves and they must be sprayed at the first signs in order to prevent diseases being spread.

Although generally, slugs and snails do not seem to be particularly partial to phlox they do attack some of the slow growing, pale pink forms such as ‘Rapture’, ‘Rosa Pastell’ and the ‘Duchess of York’.

An abridged version of an article first published in the Hampshire Group Newsletter Summer 2013
and subsequently in Cornucopia Issue 34.
© Copyright for this article: Jennifer Harmer

This article was taken from a copy of Cornucopia that was published in 2014. You could be reading these articles as they are published to a national audience, by subscribing to Cornucopia.

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