Soil types

The soil provides the basic conditions for plants to grow. Soil consists of solid particles, differing considerably in size, interspersed with air and water. Soil type, structure and fertilisation combine to determine the growing capacity of bulbs and other plants. While most plants and bulbs will grow in any medium, they will only thrive in the type of soil that is right for them. This explains why soil type and soil characteristics are important starting points when drawing up a planting scheme.

It is therefore important that any soil to be used for planting meets the following requirements:

  • good structure
  • good drainage
  • disease-free
  • sufficiently low salt (EC) level
  • the acidity level (pH) should range between 5.5 and 7, although there are exceptions

A comparison of the three most common soil types should identify which type is most suitable for flowering bulbs:

Clay
Clay soil consists of tiny particles (< 0.002 mm), closely packed together. This means that clay soil is heavy, with a solid structure. Compared to sand, clay soils are relatively impermeable. In dry weather they will retain water much longer, while in wet periods the plants quickly become waterlogged. If clay soil can be supplemented with organic materials like compost the upper layers will remain aerated, allowing the roots of bulbs and other plants to get sufficient oxygen. A lack of oxygen will cause suffocation, with bulbs failing to come up and plants dying off. With clay soils, flushing is less of a problem than with sandy soils. Clay soils retain plant nutrients better and are generally more nutritious.

Sand
Sandy soil consists of relatively large, loosely packed particles. Plants growing on sandy soil are rarely exposed to oxygen deficiency, but drought may be a problem. It is therefore advisable to incorporate a good quantity of organic material into the ground, and to provide water regularly during dry periods. Note: watering will also flush away nutrients. The application of fertiliser each spring is therefore recommended for healthy growth of bulbs and other plants used in perennial planting schemes. With strongly growing subjects a further light application of fertiliser in May or June is recommended.

Peat
Peat soil consists not of granules but of compressed plant remains. Deposits of half-rotten plant remains filled up marsh land, turning into peat over the course of time. This type of soil is therefore found in what were formerly boggy areas. Peat retains water like a sponge, and peat soil therefore tends to be soggy. The water table may need to be lowered before such soil can successfully be used for planting. If peat soil becomes too dry, however, it may lose its capacity to absorb water. Plants with deeper roots generally experience fewer problems from these sponge-like properties. Surface-rooting plants, including bulbs, will suffer in both wet and dry periods.

Planting time and depth

To get the best results with flower bulbs, it is important to take account of the correct planting time. Early flowering bulbs (January-March) must be planted in September-October, while the later flowering varieties should go in in October-November.

Our rule of thumb to determine the correct planting depth is to take at least twice the height of the bulb, with a minimum of five centimetres. Too shallow planting results in poor root development, with uneven emergence of short and scrawny foliage. Excessively deep planting may result in rot and late emergence.

Planting density per m²

The ideal number of bulbs per m² is strongly dependent on the type of planting. Below we have listed average planting densities for some of the varieties used in each type of planting.

E = Exhibitions and events

P = Public green spaces                                                             

C = Combination planting schemes (in borders)                            

Spring E P C
Allium large flowered 20 5-7 3-5
Allium small flowered 100 20-30 10-20
Anemone blanda 150 50 20
Chionodoxa 200 40-50 20
Crocus large flowered 100-150 40-50 15-20
Crocus botanical 200 50-60 20
Fritillaria meleagris 100 40 20
Fritillaria imperialis 20 10 3-5
Galanthus 120-150 40-50 25
Hyacinthoides hispanica 100 15-20 7-10
Hyacint multiflora 30 15 5-7
Hyacint 50 15-20 5-7
Leucojum 50 15 5-7
Muscari 150 30-40 15
Narcis large flowered 40 15 5-7
Narcis small flowered 60 20 7-10
Puschkinia 150 50-60 25
Scilla 150 50-60 25
Tulp longstemmed 50 20-25 8-10
Tulp botanical 80 30 15
       
Summer E P C
Anemone coronaria 150 50 20
Canna 9 3 1
Crocosmia 60-70 20-30 10
Dahlia up to 50 cm high 15-20 8 4-5
Dahlia 50 to 50 cm high 9 3-4 1-2
Eucomis 9 3-5 1-3
Gladiolus large flowered 50 20 7
Gladiolus small flowered 60-70 20-30 10
Lilium asiatictypes 40 10 3--5
Lilium orientalis 20-30 5-8 3-5
Lilium longiflorum 20 5 3-5
Zantedeschia 20-30 5-8 3-5

 

Additional inspirational formulae

The Spring Meadow

The Spring Meadow is a formula that is put into practice at the Keukenhof each year, to the delight of thousands of visitors.

At a number of sunny spots in the full sun, wave-shaped areas are cut out to a depth of around 10 cm. These areas are filled with sharp sand and the upper and lower soil layers are mixed, to create an excellent substrate for naturalising bulbs. A mixture of 55,000 bulbs in 35 varieties are spread out on the soil and planted by hand over an area of approximately 500 m2. In this particular case, a mix of herbal meadow plant seeds supplied by the Cruydt-hoeck is sown at the same time. This will cause the bulbs to emerge in a mist of herbal species, which then flower in their turn, guaranteeing weeks of colour after the bulbs have finished flowering. Any bulb varieties suitable for naturalisation and without too much of a "cultivated" appearance can be considered for inclusion into this mixture.

The following species were used in the specific case of the Spring Meadow at the Keukenhof. They are planted at an average density of 150 bulbs to the square meter:

  • Bellevalia pycnantha
  • Chionodoxa forbesii
  • Chionodoxa luciliae
  • Crocus tommasinianus 'Ruby Giant'
  • Crocus tommasinianus 'Whitewell Purple'
  • Leucojum aestivum 'Graveteye Giant'
  • Muscari aucheri 'Blue Magic'
  • Muscari 'Valerie Finnis'
  • Narcis 'Jack Snipe'
  • Narcis 'Jetfire'
  • Narcis poeticus recurvus
  • Narcis 'Topolino'
  • Ornithogalum umbellatum
  • Scilla siberica
  • Tulipa bakeri 'Lilac Wonder'
  • Tulipa clusiana
  • Tulipa clusiana 'Lady Jane'
  • Tulipa linifolia
  • Tulipa tarda
  • Tulipa urumiensis  

A Riot of Colour!

In flowerbeds stocked with seasonal plantings the aim is to achieve a massed colour effect. The result must be festive and luxuriant, and the best way to achieve this with spring bulbs is to plant in two layers, each consisting of a mix of two varieties. The lower layer will flower later. This layer consists of two tulip cultivars growing to around the same height, "Couleur Cardinal" (deep red) and "Garden Party" (pinkish white with a darker pink fringe to the flower petals). 15 Garden Party tulips are then distributed evenly around each square metre of this area, with the remaining area receiving 35 Couleur Cardinals, also evenly spaced. A layer of soil follows, in which the early flowering species are planted, a mix of "Pickwick" crocus (white striped with purple) and "Show Winner" tulipa kaufmanniana (bright red). 30 Show Winner tulips and 100 Pickwick crocuses per m2 are evenly distributed across the entire surface.

Snowdrift

Plantings below trees and shrubs can place spring bulbs in direct competition with the woody plants. Clearly , only the stronger growing varieties will be suitable here, but in favourable conditions (with adequate light in the post-flowering period) they may naturalise. Choosing white-flowered varieties can create a "snowdrift" effect, with an ever-changing display of white flowers set off against the fresh spring green of the trees. The following types are pre-mixed and then planted in groups of varying sizes:

Narcissi, early to late:

'Mount Hood' (5% at 15 bulbs per m2)
'Salome' (5% at 15 bulbs per m2)
'Thalia' (10% at 20 bulbs per m2)
'White Marvel' (5% at 20 bulbs per m2)
'Geranium' (5% at 15 bulbs per m2)
'Pueblo' (10% at 20 bulbs per m2)
'Petrel' (10% at 20 bulbs per m2)
poeticus recurvus (10% at 20 bulbs per m2)
Ornithogalum nutans (20% at 20 bulbs per m2)
Hyacinthoides hispanica 'Alba' (20% at 25 bulbs per m2)

In the most favourable conditions (assumed here) the bulbs are planted into a natural herbaceous layer to draw the eye away from the dying leaves of the bulbs as it grows to maturity.

The herbaceous layer can consist of a few varieties with a balanced pattern of growth:

Aegopodium podagraria 'Variegata'   10%
Geranium nodosum   15%
Lamiastrum galeobdolon 'Herman's Pride'   30%
Vinca minor 'Marie'   20%
Waldsteina ternata   25%

Planting bulbs in exchangeable containers is straightforward and provides instant results when pot-grown bulbs are used. Select varieties with the appropriate flowering period: snowdrops and crocuses in January and February, Scilla and Chionodoxa in March and tulips in April. Combining the forced bulbs with a flowering spring planting of violas, daisies and wallflowers will guarantee several weeks of colour.

Place the plant pots with their bulbs centrally in the containers and surround these with the companion plants, firmly planted. As soon as the bulbs are past their best they can be gently removed and replaced with a new batch, whose colour and habit should be chosen to match the perimeter plants, which will now have grown larger and more profuse. It's sometimes possible to perform this switch twice in a year, until the bulb season comes to an end and the summer flowers take over.

A miniature garden in rose pink and red

Suitable for:  large circular pots or bowls with a minimum diameter of 100cm.

Three small shrubs are planted in a rough triangle in the centre of the container, in this case Viburnum tinus 'Eve Price'. Around 20 pot-grown Christmas Dream tulips are then planted in a triangular strip around the shrubs. The remaining space between the tulips is filled with 30 to 35 small-flowered rose pink and red bicolour violas. The small-flowered violas have the benefit of standing up to poor weather conditions and they will continue to flower through to May. Like the Viburnum, they are used as a fixed planting element in the bowl or pot: the tulips can be replaced two or three times in the interim.

River of pink

Gradients in a lawn or earth embankments providing shelter in a specific area are ideal starting points for a pink river of spring flowers. Meandering strips of contrasting and merging pastel tints as found in a rich assortment of spring violas alternate with strips of tulips following the same alignments. The strips may widen and narrow, or fade out entirely only to reappear on the other side of a rise.

The most attractive results are achieved when both late and early flowering varieties are included, and when the main colour selected carries a large range of associated shades.

For example, pure pink as a main colour may shade off to pinkish white on one side and purplish red on the other. In terms of cultivars this may translate into 'Ollioules', 'Blue Ribbon', 'Shirley', 'Peach Blossom', 'Pink Diamond', 'Uncle Tom' and 'Wildhof'. These are premixed and planted in October, in lines of different lengths, resembling nothing so much as the sagging drips seen in poorly executed paintwork. An average density of 20 bulbs per square metre is used in calculating the number of tulips required.

As soon as the first shoots appear above ground in spring and tulip planting lines have become visible, the violas are carefully planted among them, also in meandering lines of varying length and breadth.