Bulbs, corms, rhizomes, tubers


Last updated March 2018

Here’s a quick guide to the basics you need when gardening with nature’s pre-packs. They can be expensive, so you need to get it right.
What’s the difference? – Bulbs, corms, rhizomes and tubers
When to plant, with tips for successful planting
Feeding for best results
Splitting up congested clumps
Which bulbs, corms, rhizomes and tubers to plant
Where to buy

What’s the difference? – Bulbs, corms, rhizomes, tubers

Although often grouped together and referred to for simplicity as ‘bulbs’, botanically-wise, there are four main types, which you need to know about because planting and storing methods vary. However, when shopping online or on the High St, it is better to look for the plant name itself to ensure you find it, since confusion over terms (such as corms being listed as tubers) makes searching frustrating.

A good example of a true bulb is an onion or a daffodil, with their many fleshy layers, and a growing point in the depths of the base, although the new shoot emerges from the top. In contrast, a corm (e.g. crocus, freesia or crocosmia) looks and feels more solid, although growth is similar, arising from the base and appearing at the top, but the right way up is not always so obvious as a bulb.

Rhizomes and tubers do not have a protective papery layer, and grow from many points so can be divided up before planting. Also there is no right way up.

Rhizomes are swollen underground stems that grow horizontally (e.g. iris). Tubers are swollen underground stems with ‘eyes’ that grow into roots or shoots (think potato). But…tuberous roots are swollen roots that only sprout from one end, so cannot be cut up (e.g. dahlia, begonia).

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When to plant, with tips for successful planting of bulbs, corms, rhizomes, tubers

  • Plant spring-flowering types in autumn.
  • Plant summer-flowering types in spring.
  • Water is the key to a fabulous display from these underground treasures.
  • Plant as soon as possible after buying.Bulbs, corms, rhizomes and tubers are normally bought when dormant. They would not naturally ever be out of the soil, and are susceptible to drying out, so must be planted as soon as possible after purchase so that they can absorb moisture again.
  • Soak corms. Corms may be very hard on buying. If so, soak them overnight in shallow dish of water (don’t cover them totally because you will restrict oxygen/air and may kill them). Soaking is not necessary or advisable for bulbs, rhizomes and tubers.
  • Plant at the correct depth. Read Thompson & Morgan’s general guide, and check instructions for specific plants via the individual listings
  • Avoid dry ground. These types of plant need plenty of moisture once planted so do not place too close to walls or buildings where the foundations make the soil dry. If this cannot be avoided, be sure to soak the area thoroughly in early spring and when leaves are present.
  • Avoid underplanting thirsty trees. Similarly don’t place them beneath large trees or conifer hedges, where the soil is always dry.
  • Water the ground. If the soil is dry at planting time, soak it with a trickle from a hosepipe until the ground is well saturated.
  • Water the plants. Watering is also essential if there’s little or no rain during the critical stage of flower development. Lack of water can lead to bulbs being blind (ie not having any flowers) in the following year.
  • Be patient. Many bulbs fail to do much in the first year, but grow the following year, so don’t give up hope.
  • Avoid damage. Don’t hoe where bulbs are planted, and don’t use weedkillers containing paraquat (e.g. Weedol) as they will damage the bulbs and any growth below ground.
  • Snowdrops. Planting bulbs while in leaf is termed ‘in the green’ and this is the most reliable way of establishing snowdrops. If you cannot buy in the green, beg some plants from your friends.
  • Remember where you’ve planted. Put markers in the ground to show where you’ve planted – otherwise in following years you’ll find yourself spearing the bulbs when cultivating or when digging a planting hole for something else. If you can’t make the markers stylish and attractive, then keep them low key and unobtrusive – for your eyes only.

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Feeding for best results

  • Use fertiliser. Bulbs, corms, rhizomes and tubers store energy, and additionally contain the embryo flowerbud(s) for the following year. The leaves make the food necessary for this process so it is important to allow the leaves to die down naturally and to apply a general fertiliser (from Thompson and Morgan). Use GrowmoreVitax Q4 or bonemeal bulb fertiliser around the plants while the leaves are still green and active.
  • Don’t damage leaves. Don’t tie the leaves up, knot them or cut them off – this stops them making food.
  • Don’t mow off too soon. If they’re in a lawn, mow around them for several weeks until the leaves have died naturally. You can mow daffodil leaves 6 weeks after flowering at the earliest. Any sooner reduces the number of flowers the following year.

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Splitting up congested clumps

After a few years an overcrowded clump may develop, which results in poorer flowering.

How to split?
Split up a clump of bulbs or corms by first digging around and under the whole clump, taking care to avoid slicing or piercing them. Then carefully prise them apart. Sometimes it’s easier to dunk the bulbs or corms in some water for a few minutes to release them from the soil. Once split into individual units, you can replant with space between each. You might find new bulbs growing alongside their parent bulb – you can carefully pull off these offsets (you might have to cut through the join at the basal plate) and plant these bulblets individually too. Corms produce cormlets (or cormels) which can also be detached to make new plants. In both cases, they will take a few years to develop flowers.

When to split?
With smaller plants, the general rule is to do this just after flowering, before the leaves have died back, so that you can see where to dig.

However, rhizomatous plants vary considerably in their growth and response to splitting, so this is only a very general guide. Where rhizomes grow outwards each year, they leave a central unproductive area. If this is the case, you should dig up the whole plant, pull apart each thriving outer fleshy ‘finger’, then cut into sections, ensuring there is a growth bud on each, and replant these. Discard the very woody central section.

Plants with very short rhizomes are better split up using two forks, back to back.

Bearded irises are probably the commonest rhizomatous plants that gardeners grow. These benefit from dividing up every 3-4 years, and this is best done after flowering has finished, but leaving enough time for the replanted rhizomes to establish before winter. As rhizomes are only half planted, leaving the top exposed, divisions are susceptible to being dislodged, so cut back top growth if necessary so the wind can’t move them.

Other plants may have different requirements, so check on Thompson & Morgan’s A-Z guide to plants.

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Which to plant

More than daffodils

Think of bulbs and a lot of us automatically think of tall showy spring-flowering daffodils, irises and tulips. These big hybrids are fine in their place en masse, but many other daintier versions (usually species) make attractive clumps for small gardens, rockeries, by patios, in containers and so on. Species may not flower over such a long period as hybrids, but they are little gems that light up different areas of the garden when not much else is in bloom. A plus point is that as they are usually quite short, they are not so vulnerable to wind and rain as the hybrids.

More than spring flowers

Remember too that there are bulbs for every season, not just spring, although some summer flowerers are not winter hardy, so you’ll have to be prepared to lift them and store them (the easiest way is to grow them in permanent pots, so you can keep them in a sheltered place in winter, and plunge (bury) the pots in the garden once frosts have finished) or re-plant each year.

Some suggestions

Look out for species tulips, daffodils, Iris, Erythronium, Fritillaria, squills, Hyacinth, Crocus, skunk cabbage, lilies, Dahlia, Gladiolus (these last two are not winter hardy), Montbretia (now called Crocosmia), Chionodoxa, grape hyacinths, wood sorrels, Cyclamen, Anemone, and winter aconites. Ornamental onions (Allium) are also spectacular, but be warned that some types self seed everywhere so check for how invasive the type is before buying, and beware gardeners bearing gifts of spare plants from their garden. There could be a good reason that they have so many….

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Where to buy

Specialist mail order suppliers are the best source of a wide range of common and unusual bulbs – shops and garden centres tend to stock only the most common, and the display conditions are not good for the majority of bulbs.

Unless in pots, availability will only be during your choice’s planting season, although you might be able to pre-order to ensure you don’t miss out. Online suppliers include Blooming Direct, Gardening DirectJersey Plants Direct and Thompson & Morgan.

On a conservation note, be sure to buy from suppliers who bulk up their stock on nurseries rather than depleting the wild. If you’re not sure, ask them before you buy.

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