What and how to divide?
Carol, Carols Garden
The easiest way to propagate from one parent plant is by division. It is a form of vegetative propagation ie propagating from one parent plant only as opposed to seed which may be from mixed parentage.
Why do we divide plants?
It’s a quick, easy and relatively reliable way to increase the number of plants
Most perennials benefit from regular division, usually every 2-4 years, to keep them vigorous and healthy
It is a good way to get plants which are genetically identical to the parent. In practice, that means you can select the best example from a batch of plants and divide it to give you many more of the same plant. For example, I have selected and multiplied the best achillea plants from a mixed batch grown from seed.
What is a division?
Most herbaceous perennials spread by some kind of root or underground system, to produce many over-ground shoots. This means you can usually take off sections of these underground root systems, with shoots (or buds which will become shoots) and they will produce new plants. That’s the key - you want a combination of root with shoot. You will need to be able to differentiate between roots and underground systems (including rhizomes, tubers) and shoots or buds (or ‘eyes’) which will will grow into stems to produce leaves. These shoots will be less obvious when a plant is dormant.
If you just take root without a stem shoot or bud, then that is effectively a root cutting and not all plants will generate shoots from a root cutting.
If you are taking shoots without a root, that is a stem cutting and may or may not generate roots.
If you take a section, however big or small, with roots and stems, then this is a division. This will have the greatest and quickest potential to grow away into a new plant.
Most herbaceous perennials build this kind of root system and can be propagated by division. A few shrubs can also be divided, especially if they sucker or produce runners eg most forms of Rubus, some Spirea.
A few perennials take a long time to recover from being divided and so are best left to bulk up eg Peonies, Hellebores.
How to divide
This depends a bit on how many plants you want. The main methods are:
Lift, split and replant: If you have a large clump and want to make maybe three to four more, then you need to dig up the whole plant and clear some of the soil. Look for identifiable gaps or small clumps that you can separate out. You want to end up with good sized clumps, with plenty of vigorous new shoots. Depending on the size of clump, type of roots, and how precious the plant is, you can then just cut it up with a spade or prise apart with forks, just pull it apart, or use a knife or secateurs. The aim is to make good sized new clumps which will be big enough to establish and to quickly flower again. Throw away any old or woody parts from the centre of the original clump and tidy up any dead or damage stems. The divisions can then be immediately replanted either in the same site or a new one. Take the opportunity to prepare the soil before replanting with organic matter and maybe some bone meal to help root establishment. Water in well. If you aren't ready to replant, they can usually be stored for a short while in crates, or heeled in elsewhere. Just be careful they don't dry out or freeze as they are much more vulnerable when out of the ground.
Take small divisions to grow on in pots: If you want to make many smaller plants, you can lift a parent plant, wash it clean and then divide it into many small divisions. So long as they each have a section of root and shoot, they will probably grow, but they will take longer to regrow and to reach flowering size. These are best started in pots, possibly with some protection, until they are established and big enough to plant out.
Sometimes you can take plantlets or small divisions form the edge of the parent plant without even lifting the parent so that it can grow away unaffected. Some plants produce runners or running stems with roots and shoots. or you can just cut down with a trowel or knife and cut away small sections form the edge of the parent, without lifting it.
When to divide
In general, plants are always best divided whilst they are entering their dormant period, or just starting to regrow, in spring. There will be a strong permanent root network, but no leafy shoots to support. Lifting and dividing causes a shock to the plant and it will take some time for the roots to establish and for it start growing again. It will do this best when the soil is warm and moist, but when the plant does not have a full leaf canopy to support - so usually in autumn or spring, although mild periods in winter are also fine. It is possible to divide a plant in full leaf, but the divisions will need more care - watering and shading - to enable them to grow away. Never divide while the soil is frozen, as you will cause damage and the roots will not be able to access water or to grow.
The broad rule of thumb that I was taught is to divide early flowering perennials in autumn (so they have longer to establish before they start to flower), and late flowerers in spring. If conditions are good, most plants seem to respond best to division in spring, when they are just showing signs of new growth. In reality, I do it when it suits me, or when the ground conditions are suitable, although there are a few specific exceptions:
Peonies and Hellebores form their flowering buds in late summer, so move or divide them in early autumn if you need to - so you should still get flowers the following year
Grasses almost all sulk and may rot if you divide in autumn or winter, so only divide in spring once they are actively showing signs of growth. I would say the same for any mediterranean type plants eg Salvias.
Very early spring flowerers should be divided just after they have finished flowering - eg Primulas, Epimediums
Astrantias are best divided when they are in active growth in spring, or even after their first flush of flowers in summer, if you can keep them well watered while they recover. I've only discovered this from experience and comparing notes with other growers.
Dahlias - A special case
Dahlias are a herbaceous perennial, which form a tuberous root system and many shoots. They are effectively just the same as any herbaceous perennial, but neither the stems nor the root sytems/tubers will withstand being frozen. They can be propagated by division or by cuttings (coming next...)
The same principles apply as with all divisions:
You need both a root (or a tuber, in this case) as well as a shoot or a bud which will grow into a shoot (usually called an eye on tubers) A root (or tuber) without a bud will not grow. A stem without a root is a cutting and must be treated differently.
The smaller the division you make, the smaller the plant will be for that season. If you have excellent growing conditions - water, nutrient, warmth and good light, then the plants will grow away quickly. Dahlias have phenomenal growth rates in good conditions. In the UK, we have a relatively short dahlia season and so you will get a smaller plant from a single tuber (or from a rooted cutting) than from a division with 2, 3, or 4 tubers and buds. The smaller ones often do not catch up in a normal growing season, unless you can provide these perfect conditions.
Dahlia tubers are usually lifted in autumn, around the time of the last frosts, dried and stored in cold, but frost-free conditions over winter. They should then be divided if you wish, and started into growth in spring, to plant out after your last frost. You would divide for the same reasons as any perennial - to rejuvenate large congested clumps and to generate new plants. They become congested quite quickly and you will see the old, spent tubers in the centre of the clump. You can choose not to lift and store or divide tubers, but the risks are:
If tubers become frozen, they will rot and die. This risk is reduced in warmer climates, light soils, drier sites, if they are planted quite deep, or if you cover them with insulation (eg straw) and rain protection. Temperatures are more moderate in the ground than at the surface, but many of the new tubers are created close to the surface of the ground.
Plants left in the ground quickly become congested and bloom quality may suffer as a result.
Plants in the ground in spring (especially if under an insulating layer) may shoot before the risk of frost is over and will be at risk from pests as well as frosts.
It can be hard to spot disease on the tuber, e.g crown gall, especially if they are heavily mulched.
But...leaving them in the ground is much less work if you think you can get away with it!
How to divide...Claire has done a handy you tube tutorial on this:
When to divide dahlias?
Divide in autumn, winter or spring: You can do it any time, but spring is most usual because:
You can tell which tubers are strong and healthy after winter, discard any soft or rotten ones.
You can plant up and start them into growth straight away.
You will have smaller plants to plant up - saving on compost.
There is still time to order more if some have failed to make it through the winter.
Plants will grow away quickly and there is less time for rots to enter through the cuts you make when dividing.
The disadvantages are that you're likely to be busier in spring, and that you end up storing large undivided tubers which take up lots of space.
How to store your dahlias?
There are as many methods as there are flower farmers, but here is Claire’s tried and tested, simple method: