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Dividing Perennials. Why, When and How.

Why divide perennials – When to divide perennials and How to divide perennials.

Many people are put off the idea of perennial gardening because of the supposed need to dig up and divide your perennials each year, so let's put the record straight about this.

 First and foremost is the fact that most perennials do not actually NEED to be divided each year. Many perennials are happy to go through their entire life, without the discomfort of being dug up each damp, cold autumn and cut into chunks then re planted. It is us – the gardeners – that NEED to divide the perennials – generally for aesthetic or propagation reasons.

The three reasons - perceived or otherwise - are for plant health, tidiness, and propagating extra plants.

Why Divide Perennials

So why should we divide these plants which are supposed to make gardening almost maintenance free? Think a while how different plants live!

Trees have a single stem or trunk, so have to spread their canopy aloft – preferably above all those below. The roots generally spread out to the edges of the canopy

Shrubs are the next ‘group’ down in size, so are many-stemmed or branched, in order to spread their ‘canopy’ wings at a lower level. Again, the roots generally reflect the spread of the canopy.

Perennials are lower down the size scale, and whilst they too, can spread their foliage canopy, they are subject to more local competition, so also have to elbow their way sideways at ground level. Their roots are confined to the spread of the canopy - a small area in relation to the above-ground growth.

 The only way they can expand into new territory, is by spreading their root system outwards. The plant then follows the outward spread. Most herbaceous perennials in particular are multi-stemmed – sending up new stems each year in an ever increasing circle of growth – either searching for new nutrition sources or simply following the roots.

 In doing this, the centre of the plant normally depletes, as the new shoots and stems find new food resources on the outer perimeter of growth.

 The perennial plant as a whole is happy with this way of life as it gradually expands into new territory. We – the gardeners – are not! In this growing process, we are left with a perennial clump which is bare in the middle, and starts to look untidy!

The solution is to lift the mature perennial clump and roots, discard the mature and seemingly lifeless central portion, whilst keeping the outer parts by way of dividing the perennial roots into ‘good’ and ‘bad’ sections. The ‘good’ new outer sections are re-planted, and the whole cycle starts again – normally lasting around three years!

Typically, if you were to plant a single herbaceous perennial – say a Michaelmas daisy – alone in the center of a bed of 1m across, it would die down each autumn, and then reappear each spring in an increased circumference. After three years, there would be a central area devoid of growth, but the outer perimeter would be healthy and happy in its new surroundings.  So we step in, and divide the outer part of the plant and plant anew.

However, if the plant were left to its own devices, it would then also start to send new growth back into the central barren part. (It would spread both inwards and outwards from the perimeter.) Eventually, the entire bed would be full of plant! Problem being that most of us do not have the space to allow this to happen in our border, and in any event, the ‘dead’ middle of the plant for several tyears would be unacceptable to us!

It is mainly the herbaceous perennials that spread this way. Evergreen perennials – of which there are fewer – tend to be slower growing, and are generally happy for many years with their allotted root space.

When to Divide Perennials

Perennials have generally been divided in the autumn, but early spring is just as good if not better! The autumn divisions were practiced in the large garden estates, and this became the norm. the reason simply being, that there were too many other gardening tasks of importance in the Spring.  Also, always having to look forward in gardening – especially in large gardens – if this task is left until the spring, and the season turns out to be very wet and the ground boggy, then there is likelihood that the job would not get done. Plan for it in the autumn, and if the weather precludes, then do it in the spring.

Evergreen perennials that need to be divided for whatever reasons are best tackled as growth starts in mid spring, or even late spring. Evergreen perennials divided in the autumn – going into dormancy - will at least suffer a huge setback – if not death.

How to Divide Perennials

Good exercise is this!

Dig the entire root system fo the plant out of the ground and shake off or otherwise remove the attached soil if necessary. If you are blessed with a light soil, then there may be no need for this, and in any event, some perennials do not have too much of a fibrous root system (Most do!)  Water from hose is a good way to clean the clump - but don’t get your underfoot area soggy.

Cut the outer parts of the roots into sections. If this is carried out after the first three years, then probably divide the clump into four sections. These sections are your new plants.

The new ‘plants’ can be planted immediately, or potted up into containers for selling at the local boot fair. You can also make a lot of friends by offering then round. Maybe even set up a division ring of likewise minded gardeners, who have different plants to you.  (The garden centre managers will love that bit of advice!) 

You do not have to be a mathematician to work out that if you start with a modest collection of five perennials, in six years you could end up with a hundred or so plants!

List of Perennials that we might need to divide. (To follow).


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