Philadelphus fundamentals
This is one of a new series of posts based on real questions I have been asked. Feel free to add to the answers or include your ideas and experiences
Sometimes the number of questions I get about a plant is related to difficulty of cultivation but, in others, I am sure it is due to them being popular. I get more questions about camellias, clematis, roses, hydrangeas and wisteria than other plants. But problems with philadelphus are also common, and are usually related to lack of flowers.
Typical was one that simply stated ‘Our three year old mock orange has lots of growth but still no flowers.’
It would be easier to answer this if there was more information: Was it pruned? Has it actually grown? Is it in a pot? Is it in shade or sun?
When I was on the ‘Roadshow’ and we had an audience asking the questions it was possible to get more details. When someone complained of non-flowering hydrangeas I would bait them with ‘and when did you prune it?’ and as soon as the person opened their mouth I would reel them in and know the answer to the problem!
But non-flowering philadelpus is not quite so simple. I will assume, for the sake of brevity, that it is planted in the ground and the site is at least partially sunny – philadelphus prefer a sunny spot to bloom well. What can be frustrating, but perfectly normal, is for any shrub, bought in bloom, and then planted out, to have a year or two when it will not bloom. This is because the shrub, with its roots surrounded by lots of fresh soil, puts on strong growth at the expense of blooming. This is actually a good thing and shows the plant is getting established and that the roots are expanding into the soil. Potbound shrubs, where the ‘tight’ roots keep going round in circles, even after planting, tend to bloom but not make good growth and they eventually die.
Pruning is the next problem. Philadelphus do not bloom on the new growth and, if that is vigorous, not even on the sideshoots off this and it can be three years for flowers to appear on the upright, straight stems that grow from the base. These shoots are more common if the plants are pruned hard. The way to prune philadelphus is to cut out a few of the oldest stems, near the base, every year, after flowering. Regular hard pruning of the whole plant or light clipping over the shrub will reduce flowering.
It is unfortunate that the most commonly available philadelphus is ‘Virginal’ which is very tall and upright when young. It then spreads and can be a large shrub. It is often much bigger than the planter imagines. Even ‘Belle Etoile’ (photo above) is quite a large shrub. If space is limited it is worth planting naturally smaller kinds such as ‘Manteau d’Hermine’ or one of the more modern dwarf kinds.
This may make it sound as if philadelphus are tricky to please but nothing is further from the truth; they are hardy and will grow in most soils, including heavy clay. What they will not tolerate, as I have found to my cost, is waterlogging. I planted a ‘Minnesota Snowflake’ in a bed that was not really prepared and was wet in winter and, after a summer of good growth it failed to sprout the following spring because of waterlogging killing the roots. I won’t make the mistake again. In the meantime I have planted six others, in various places and look forward to their fragrant flowers in June and July. So far I am still waiting, apart from on P. microphyllus, which is small and pineapple-scented. But I am glad they are growing well and I will leave the secateurs alone for now.
The Biking Gardener 
Pruning is a common problem for MANY plants, whether because it is too aggressive, or more likely, because it is not aggressive enough. Many plants are indiscriminately shorn, so that if they bloom at all, the bloom is not displayed well. The most common problem with roses is that they are not pruned adequately or properly, so bloom only mediocrely. Well, anyway, I could get too carried away on this topic.
Pruning puzzles many people while others just hack away and don’t think about why they are pruning. I don’t think things will ever change. Although I hang my head in my hands at the vast number of new ‘dwarf’ cultivars, perhaps it is the answer.
While I was still working, I found that so-called ‘gardeners’ attacked even dwarf cultivars. There was not much that they did not attack.
people often prune out of habit and without thinking why they are actually doing it!
For some, it is merely a means to contain vegetation.
Few shrubs match philadelphus for fragrance and I love their season in the garden. We were in Mary Keenan’s garden two years ago and were completely taken by the fragrance from P. microphyllus – small flowers but powerful aroma. We have a plant now in the garden, growing very well and it should flower this year.
I am glad it is growing well. I have long been a fan of this one and although it is often said to need more sun and heat than others I have never found that a problem. The perfume really is rather special.
I have it in a very open spot, best sunshine possible, and looking forward to the flowers.