How do you solve a problem like Lilac suckers?

Lilacs are a great tree for a small garden but does yours leave you fighting a thicket of suckers? Is your trees graft failing or thriving?

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My head insisted on singing this to the tune of “How do you solve a problem like Maria” as I, once again, tackled the forest of shoots that are in a clients garden around her Lilac tree. I know Lilacs aren’t the only tree that have a habit of doing this but it did get me to thinking about the reasons why trees do this and what, if any, are permanent solutions to stopping them from doing so?

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Lilacs are a lovely tree to have in the garden, relatively small, some are highly scented and with judicious pruning can produce a wealth of blooms. Brought from Turkey in the 15th century we now think of them as part of the quintessential English garden. Since the 1700’s they have been forced for the florists market to provide early flowers for the market but in recent years they seem to have disappeared somewhat off the gardening radar.

A couple of lovely varieties to grow are Syringa vulgaris ‘sensation’ an unusual bicoloured bloom of pink edged with white. ‘Congo’ is an old (1890’s) variety but reliable and highly scented, with distinctive flowers the colour of a good Merlot. ‘Madame Lemoine’ is a pure white and another reliable old variety. Bred by the Lemoine nursery in France it has stood the test of time.

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Syringa vulgaris ‘Sensation’

Lilacs are also a good tree for chalky, well-drained soil. Once established they can withstand a summer drought which in these days of climate change is never a predictable thing!

I’ve always been told with suckers the best method of dealing with them is to rip them from the tree, this damages the growing point and prevents them from regrowing. If you cut them off neatly with secateurs it leaves the dormant buds intact and in a short while you’ll have 2 where there was only 1 previously. Like a many headed hydra this can quickly become an out of control beast! If caught early enough you can even just rub the buds out but what happens when the suckers come from the roots?

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Some plants like Lilacs, Wisteria, cherries, roses & witch hazels are grown on root stocks. Whilst this has the advantage of controlling the plants vigour & health it can come with the down side of suckers. Once the rootstock has established shoots it will no longer aid the graft instead preferring to give its energy to its own leaf production, eventually leading to the demise of the scion.

This raises several questions

  1. Why graft in the first place?
  2. Once started how can it be stopped?
  3. What is the cause & how can it be prevented?

1 is for the ease of the nursery producing (in the case of lilacs) it means they can produce plants at a time when the nursery would be quiet and far quicker than by other means of propagation.

2, simply put it can’t really, only controlled, which leads us neatly to…

3, this last question is probably the most pertinent as prevention is always better than cure!

So lets look at a bit of science as to why this happens….

The first and most obvious reason for a sucker is stress. Lets assume, in the case of lilacs & cherries in particular, the suckers are coming from the roots. This is a type of vegetative propagation, the tree is cloning itself through basal shoots from adventitious buds on the roots. Plants are so clever in this respect! If we cut off an arm we couldn’t possibly expect to grow a new us from it but plants can send out new versions of themselves from practically any body part. Each part of a plant, including the roots has the capability to clone itself.

Plants do this in a response to a few things that cause stress, over enthusiastic pruning is one cause or injury to the root system can be another.

In the case of over enthusiastic pruning the plant has developed a root system directly in proportion to the canopy, usually 3 times the canopy’s size. When the canopy is reduced dramatically the plant thinks it’s under attack and will do its utmost to propagate itself.

The more tricky one to deal with in a garden situation is root damage, this can be caused by digging near a tree in a border or even by mowing the grass. For trees that have a shallow root system their roots can often be damaged by close mowing and once this stress response is triggered it can often be almost impossible to prevent a reccurrence of shoots. Better to prevent it than cure it. This of course doesn’t help us once it’s already happened though.

The second reason can be from a failing graft union. This can occur for many reasons

  1. Anatomical mismatch – a failure to line up the rootstock and scion in the initial graft. This usually becomes apparent very quickly.
  2. Damage to the graft union – this can occur when hoeing around the base of the tree or in the case of grass land strimming too close to the base and “ringbarking” (removing the bark from around the base of the tree causing the flow of sap to the scion to be interrupted)
  3. Fungal/bacterial infection – the graft is a weak spot and is always susceptible to infection.

How can you give your tree the best start in life to avoid these things?

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Selecting a plant that looks healthy, vigorous and has a clean healthy graft union is obviously a good start, if you can buy from a reputable nursery they will be happy to explain what to look for. That’s the great thing about buying from nurseries that produce their own stock. The staff actually understand plants and are enthusiastic to share their knowledge with you.

The next step is planting, in the case of lilacs their rootstock graft (if they have one) is often privet ligustrum ovalifolium. Now the RHS will tell you to plant with the graft joint above soil level. This will prevent suckering of the lilac itself but it can also have the disadvantage of preventing the scion to from forming its own roots and if the graft fails, which it likely will over time, the plant is lost. However advice on lilacs from the Arnoldia arboretum and from Chris Lane(owner of the Witch Hazel nursery in Kent who has a wealth of knowledge and who’s opinion I greatly respect) differs, they suggest that the scion be allowed to develop its own rootstock by initially planting level or slightly deeper and then mulching for the first year. Their observations on lilac grafted on various rootstocks go as far as to suggest that the graft is doomed to failure after 4 – 5 years and encouraging the scion to develop its own roots is necessary. If you’d like to find out more have a look at this paper. I’m inclined to  go with their advice myself despite the risk of developing suckers in later years but as with all cases of differing advice the choice is yours of course which you listen to.

Back to the problem of suckers though!

Lets assume you’ve given the plant the best start in life you can but despite your best efforts it’s started developing suckers or that you’ve inherited a plant that is already creating a mini forest of clones around its base. What then?

Dont ever be tempted to spray these suckers with weedkiller! Their “blood supply” is the same one that nourishes your beloved tree and will kill that too… you’d be surprised how many times people have done this and then wondered why the tree has died!

The best way as already mentioned is to tear/rip the sucker off as close to the root or stem as possible. If this just isn’t possible I find a sharp spade does the trick. The key is always persistence, once a plant starts to sucker it will continue. Your job is to make sure the plant stays as happy and healthy as possible. In autumn feed with well-rotted manure mulched around the base. In dry periods, like we’ve been experiencing in the last month or so even established trees will appreciate a bit of water on their roots and continue to remove the suckers as and when they appear and your lilac trees will continue to have happy healthy lives!

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