The North Carolina Agricultural Research Service tested selected tulip and daffodil (Narcissus spp.) cultivars for four years. Trials were conducted in three climate zones so that results could be extrapolated to most of the United States. A few hyacinths were also tested on a limited basis.

Of the tulip, daffodil, and hyacinth cultivars tested, a number were found to be satisfactory in at least one of the three climatic zones represented. The number of successful cultivars increased from the warmer zone 9 to the cooler zone 7 as indicated in Table 1.

Successful cultivars of tulips and daffodils represent most of the groups and divisions tested. Where a group or division fails to be represented by a high performing cultivar, fewer than four cultivars from that classification were tested. The single exception was the Kaufmanniana group of tulips. None of the seven Kaufmanniana cultivars tested performed well in zones 7 and 8.

Interpreting the Results

Any attempt to test cultivars of spring-flowering bulbs for suitability to a given region is, of course, subject to wide variability. An unseasonably cool spring in a normally warm climate favors bulb survival. A rainy period or unusually humid conditions can increase losses to diseases that are not usually problems. Considerable confidence can be placed in the trials reported here which extended over four years at each site. The winter and spring of 1982-83 were among the wettest on record, and the winters of 1983-84 and 1984-85 were abnormally short and provided insufficient cooling in zones 8 and 9. Given a different set of conditions, some cultivars rated unsatisfactory in this study might perform well in these same zones, while others might do poorly.

Drainage Is Crucial

The most important finding of the cultivar trials was that a well-drained planting bed is the critical factor in determining whether any cultivar will survive a second or third season. Amending heavy clay soils with organic matter, planting on a slope, adding drainage tile, and constructing raised beds are all methods that will improve drainage. If the bed is not prepared properly, no cultivar will perennialize.

“Perennial” Is Three Years

The trials reported here covered a four-year period. Although daffodil cultivars can be expected to survive for many years, tulip cultivars are generally considered successful perennials in the southern states if they survive for three years.

Fertilizer

The bulbs tested in the trials were fertilized each fall with the slow release fertilizer Bulb Booster™(9-9-6) at 4 pounds per 100 square feet. An alternative fertilization treatment consisting of the standard garden fertilizers applied in the fall and at emergence was also tested. Fertilized bulbs performed much better than non-fertilized bulbs, but the method of fertilization made no difference.

Tulips originated in the moderately cool climates of the eastern Mediterranean and central Asia. With a little special attention, however, they can also be grown successfully in humid climates that warm rapidly in the spring. The best results come from following three steps:

1. Preparing a well-drained site for the bulbs;
2. Selecting the best cultivars for the hardiness zone;
3. Fertilizing the bulbs twice a year, or using slow release fertilizer once a year.

When these steps are followed, tulips and hyacinths will probably return for at least three seasons; daffodils for much longer.

Good Drainage

A well drained site is the single most essential factor for successful tulip and hyacinth cultivation. It is also beneficial for daffodils. Bulbs do best in deep, well-drained loam soil or sandy soil. If high clay content causes poor soil drainage, locate the bed on a slope if possible. Adding compost, composted bark, or peat moss will further improve drainage and let air into the soil.

To open up a heavy clay soil, use enough organic matter to equal one third to one half of the volume of the soil in the bed. Coarse concrete-grade sand can be used (at a greater rate) as a substitute for organic matter to lighten clay soil, but the soil-sand mix will not hold nutrients or water as well as soils amended with organic matter.

Beds prepared to a depth of 12 to 14 inches (30 to 36 cm) offer good conditions for root growth. If the bed is not this deep, the water that accumulates on the undisturbed clay layer can prevent oxygen from reaching the roots and carbon dioxide from escaping, so that root injury becomes more likely. When bulbs must be planted in shallow beds, plant shallower to allow space for root growth above any clay layer.

Raised Beds

On flat sites, raised beds will improve drainage. Organic matter can be plowed into the entire garden. The extra soil in the aisles is then pulled up onto the beds to add an extra 4 to 6 inches (10 to 15 cm) of depth.

Drainage Tile

Some sites do not provide a slope or lend themselves to raised beds. When a bulb bed must be prepared in flat, heavy clay soils, drainage tile can be used to keep water from perching on the undisturbed clay subsoil and turning the bed into a tub. To install drainage tile, cut a trench into the undisturbed subsoil along the center of the bed. The drainage tile should be completely embedded in gravel within this trench. The tile must lead to a low point where the water can drain away freely so that aeration in the bed will be improved.

Planting Depth

The Dutch bulb producers and exporters recommend planting bulbs 8 inches (20 cm) deep to the base of the bulb. Bulbs planted at shallower depths will experience wider temperature shifts in the winter and earlier warming in the spring. Early spring warming can lead to smaller bulbs and poorer performance the following spring. The bulbs tested in these trials were planted at a 6-inch (15-cm) depth at the two test sites with heavy clay soils.

Mulch

Mulches are good for the bulb bed because they keep soil temperatures more uniform during winter and delay warming of soil in the spring. Coarse bark and pine needles are both suitable. Straw and other mulches that decompose rapidly and plug soil pores should be avoided.

Table 1 lists, by zone, the cultivars that performed well during the three or four years of testing. Within each zone, cultivars are listed by bloom time, and within bloom time, by color. A complete list of all cultivars tested can be found in The Cultivars Trials section below. Information on failures is as important in successful selection of cultivars as information on successes. Selection of cultivars based on the information in these two lists will contribute to successful bulb beds in warmer climates.

Performance ratings were made on a scale of 0 (total bulb loss) to 5 (excellent) based on the bulbs in fertilized beds. The cultivars listed in Table 1 are those rated 3 or better. Bulbs were rated on the size, form, and brilliance of blooms and on the color and size of foliage.

Bulbs become active with the cooling of the soil in late September in the North and early November in the South. Root growth continues throughout much of the winter. During mid or late winter, stem elongation occurs within the bulbs, and by late winter or early spring shoots appear above ground. While daughter bulbs have been slowly enlarging throughout the winter, most of their growth will occur at and shortly after flowering in the spring. The result of these events is an increase in total weight and number of bulbs and an increase in the size of the nutrient pool within these bulbs.

The increased mass of the daughter bulbs exceeds the parent bulb’s capacity to pass along nutrients. While decomposition of organic matter and soil weathering will supply part of the nutrient demands, part of the need must be met from fertilizer.

What and When

Follow a fertilization program using nitrogen, phosphorus, and potassium in the fall at planting time. Apply nitrogen again after emergence six to eight weeks before bloom. An alternative is to use slow release fertilizer at planting and each fall thereafter.

No benefits are derived from nutrients applied at or after flowering. As the soil warms up after the flowering period, nitrogen can stimulate development of the fungal disease Fusarium spp. which is a major cause of bulb loss. Fusarium is easily identified by the sour smell given off by the rotting bulbs.

Slow-release fertilizer (Bulb Booster 9-9-6) was used in all the cultivar trials of this project. This fertilizer is used once each year at the rate of 4 pounds per 100 square feet. The first year it is incorporated into the soil at planting. In subsequent years slow-release fertilizer is broadcast over the bed surface in the fall shortly after customary planting time, and is a convenient way to supply all fertilizer needs with one application.

pH is Important
A pH level of 5.4 to 7.3 was found to be acceptable for tulips. The range for daffodils is probably similar. A range of 6.0 to 6.8 would be safe. The bulb bed soil should be tested annually. The soil testing laboratory will recommend the amount of limestone necessary to bring the pH level into the desired range. Test the soil pH before planting and again each fall to insure adequate calcium and magnesium. Using dolomitic limestone to adjust pH will supply both calcium and magnesium. Follow the schedule shown in Table 2.

Bulbs can suffer from calcium deficiency when planted in very acid soils (pH below 5.4). Tulip foliage becomes small and pale green. Tulip flower stalks (scapes) and daffodil stems may topple over at about the time the flower opens. In extreme cases, buds form, but flowers do not develop.

Very alkaline soil (pH above 7.3) can cause deficiencies of phosphorus, iron, manganese, zinc, copper, and boron. Small pale leaves may also be a symptom of high pH.

Trials were conducted at three sites across North Carolina (see map). With sites in zones 7, 8, and 9, it should be safe to extrapolate findings at the test sites to comparable situations in these plant hardiness zones.

Bed Preparation

Soil was rototilled to a depth of 9 inches (23 cm). Sufficient dolomitic limestone to achieve a pH level of 6.0 to 6.5 was applied along with a 4-inch (10-cm) layer of composted pine bark. This was rototilled into the loosened soil. Pine bark was applied to the clay soils of zones 7 and 8 but was omitted in the sandy soil of zone 9. Omission in zone 9 was based on a lack of response to pine bark in an earlier experiment. Finally, soil in the aisles was transferred to the beds to increase the bed depth by 3 inches (8 cm). The final amended soil depth was approximately 12 inches (30 cm). All beds were mulched year-round with pine needles. Bulbs were left in the ground during the summers, and cover crops were not used.

Fungicide treatment: Soil in the first trial was fumigated with methyl bromide at the rate of 3 pounds per 100 square feet (150 g/m²) and bulbs in the trial were dipped for 30 minutes in a solution of 26 ounces Banrot™ fungicide per 100 gallon (2.0 g/liter). Note: The fumigant and fungicide were not used in Trials Two and Three in order to better simulate home garden conditions.

Disease and weed control: Plants were sprayed as needed for Botrytis spp. (“fire”) control with either captan or benomyl (Benlate™) at the rate of 1 pound per 100 gallons (1.2 g/liter). Weeds were controlled during the summer and early fall, when bulb foliage was not present, by applications of glyphosate (Roundup™, Kleen-Up™, etc.) at the rate of 2 pounds active ingredient per acre (2.2 k/ha).

Fertilizer: One set of cultivar plots was fertilized with the slow release fertilizer Bulb Booster™ 9-9-6 at the rate of 4 pounds per 100 square feet (200 g/meter) rototilled into soil just prior to planting and applied to the soil surface each fall shortly after the beginning of root development. The other set of cultivar plots was not fertilized.

Spacing: Bulbs were spaced 4 x 6 inches (10 x 15 cm) and planted at a depth of 6 inches (15 cm) to their base. The small bulb cultivars were planted 4 inches (10 cm) deep.

Trial One (1981-1985)
Two 25-bulb plots of each cultivar as follows:
Tulips: 133 cultivars representing 14 groups
Daffodils: 41 cultivars representing 10 divisions

Trial Two (1982-1986)
Replication of Trial One with 3 additional tulip cultivars and 8 hyacinth cultivars.

Trial Three (1983-1986)
An additional 23 tulip cultivars and 8 daffodil cultivars not included in previous trials were tested.

Map of trials conducted across North Carolina.

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Map of trials conducted across North Carolina.

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Causes of Failure

In zone 7, Botrytis spp. infection of foliage, also known as “fire,” was the main problem (see Figure 3c and Figure 3d). Dew on the foliage most mornings in the spring fostered development of this fungal disease and made fungicide protection impossible. The cultivars surviving in zone 7 are rather resistant to Botrytis spp.

In zone 8, some discrepancies between rating and number of surviving bulbs occur for the tulip cultivars Diplomate, Oxford, Holland's Glorie, Beaut of Apeldoorn, Parade, Golden Oxford, and Rosy Wings. In these cases, the surviving plants are floriferous and of excellent quality; however, a large number of plants were lost to Fusarium spp. The soil of zone 8 contained a heavy component of clay and was consequently poorly drained in spite of the added organic matter.

In zone 9, warm spring temperatures resulting in smaller daughter bulbs over the years apparently played a major role in bulb decline.

Flowering Period

While the number of successful cultivars of daffodils and tulips declined from zone 7 to zone 9, the period over which flowering occurred increased (see Table 3). Successful tulip cultivars came into bloom in zones 7, 8, and 9 over 21, 25, and 34 day periods respectively, and flowers were aesthetically pleasing for an additional 7 to 10 days. By careful selection of cultivars, tulips can be enjoyed for approximately 31 days in zone 7, 35 days in zone 8, and 44 days in zone 9.

Successful daffodil cultivars came into flower over a period of 30 to 41 days depending on zone. With a flower life of 7 to 10 days, daffodils can be enjoyed for 6 to 7 weeks in the spring.

Figure 3a. Tulip bulb attacked by Fusarium fungal disease during summer storage. This bulb weighs very little and has pale tan and white areas.

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Figure 3a. Tulip bulb attacked by Fusarium fungal disease during summer storage. This bulb weighs very little and has pale tan and white areas.

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Figure 3b. Tulip plants showing progressively more destruction by Fusarium from left to right.

Courtesy of International Flower-bulb Center, Hillegom, The Netherlands

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Figure 3b. Tulip plants showing progressively more destruction by Fusarium from left to right.

Courtesy of International Flower-bulb Center, Hillegom, The Netherlands

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Figure 3c. Botrytis ("Fire") infects the susceptible culticar "Halcro" shown between two resistant cultivars, "Holland's Glory" (left) and "Gudoshnik" (right).

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Figure 3c. Botrytis ("Fire") infects the susceptible culticar "Halcro" shown between two resistant cultivars, "Holland's Glory" (left) and "Gudoshnik" (right).

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Figure 3d. Botrytis fungal damage on tulip.

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Figure 3d. Botrytis fungal damage on tulip.

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Figure 4. Calcium deficiency of tulip results in "toppling," or collapse of the flower scape.

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Figure 4. Calcium deficiency of tulip results in "toppling," or collapse of the flower scape.

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Table 4 reports the results of all cultivars tested in all zones. The abbreviations used are as follows:

CL -- classification

Tulip Groups

1 – Single early
2 – Double early
3 – Triumph
4 – Darwin hybrid
5 – Single late
6 – Lily flowered (pointed, reflexed petals)
7 – Fringed (petals edged with crystal-shaped fringes)
8 – Vridiflora (partly green perianth)
9 – Rembrandt (broken or striped color)
10 – Parrot (laciniate flowers, generally late)
11 – Double late
12 – Kaufmanniana (very early, sometimes mottled leaves)
13 – Fosteriana (large early flower, sometimes mottled or striped leaves)
14 – Greigii (always mottled or striped leaves, later than Kaufmanniana)
15 – Other species

Tulip color

Tulip colors are coded by one or two letters indicating the predominant color of the exterior of the flower. Colors are abbreviated as follows:

B – blue
R – red
G – green
RO – rose
O – orange
V – violet
P – pink
W – white
PU – purple
Y – yellow

A second color is indicated by a hyphen followed by the color and pattern type: edge (E), feather or flame (F), and stripe (S) (see Figure 1). For example, the midseason tulip Spring Song is coded R-EW, indicating red flowers edged with white.

Daffodil Groups

See Figure 2 for botanical terms.

1 – Trumpet daffodils of garden origin. One flower to a stem; trumpet or corona as long or longer than the perianth segments.
2 – Long-cupped daffodils of garden origin. One flower to a stem; cup or corona not more than one-third, but less than equal to the length of the perianth segments.
3 – Short-cupped daffodils of garden origin. One flower to a stem; cup or corona not more than one-third the length of the perianth segments.
4 – Double daffodils of garden origin. Double flowers.
5 – Triandrus daffodils of garden origin.
6 – Cyclamineus daffodils of garden origin.
7 – Jonquilla daffodils of garden origin.
8 – Tazetta daffodils of garden origin.
9 – Poeticus daffodils of garden origin.
10 - Species and wild forms and wild hybrids. Double forms of these varieties are included.
11 – Split-coronoa daffodils of garden origin. Corona split for at least one-third of its length.
12 – Miscellaneous daffodils. All daffodils not falling into any one of the foregoing divisions.

Daffodil color

The first letter in the color code for daffodils indicates the color of the perianth, while the three letters after the hyphen indicate the colors of the eye-zone, mid-zone, and edge or rim of the corona tube. If the three zones of the corona are the same color, then only one letter appears after the hyphen.

Ratings – RT

Ratings are based on evaluation of the bulbs surviving the fourth season of flowering (cultivars marked * were evaluated in the third season of flowering). The rating scale ranges from 0 (indicating total bulb loss) to 5 (indicating excellent condition). Only cultivars with a score of 3.0 or better were considered satisfactory for that hardiness zone. Bulbs were rated on the size, form, and brilliance of flowers and on the color and size of foliage.

Number of Bulbs – NB

The number of flowering bulbs or bulb clusters persisting from the original 25 bulbs after four years.

Flowers per Cluster – FL

The average number of flowers per remaining bulb cluster during the final flower season.

Height – HT

The average height in inches over the four years. In tulips measured from soil to base of flower. In daffodils measured from soil to crook in stem below flower.

First Bloom – FB

The average date of first bloom over all seasons.

Half Bloom - HB

The average date when half of the buds were in bloom over all flowering seasons. Note that Julian dates (counting continuously from January 1) are used for the bloom dates, e.g., 60 is March 1, 91 is April 1, and 121 is May 1 except in leap year.

Figure 1. Second color patterns in tulips.

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Figure 1. Second color patterns in tulips.

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Figure 2. Flower parts used in daffodil classification.

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Figure 2. Flower parts used in daffodil classification.

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Nutrient accumulation begins within 48 hours of root emergence and continues up to the stage of root senescence which begins shortly after flowering. Six to eight weeks after flowering (June), the roots are gone, all nutrient uptake has stopped, and the bulbs enter an apparent resting period. Nutrients applied after flowering do no good and may do harm by enhancing Fusarium spp.

Bulbs need a modest level of nutrients supplied at planting time in the fall and maintained throughout the winter. When plants emerge in late winter or early spring, they need an additional supply for the extensive nutrient uptake during the period of rapid above-ground growth from early spring to flowering.

Nutrients

Nitrogen

Nitrogen is the primary nutrient needed for tulip fertilization. In nearly all situations nitrogen must be supplied from the time of planting until flowering. Repeated applications are necessary unless a slow-release source is used. Nitrogen remains readily available in the soils of moist climates for only 6 to 8 weeks before being leached away.

In the first year, failure to fertilize tulips with nitrogen results in lighter green color, smaller leaves and a large reduction in the mass of daughter bulbs produced. These effects carry into second and subsequent years, leading to small flowers and unattractive plants. Some cultivars manage to maintain a reasonable number of plants without nitrogen fertilizer, but others diminish rapidly.

Daffodils are not as sensitive as tulips to nitrogen deficiency. Effects are not noted during the first year. During the second year, leaves may be lighter green and slightly smaller. In later years these symptoms become more pronounced.

Nitrogen should be applied to tulips twice each year. The first application is needed at planting time and the second shortly after emergence in late winter or early spring approximately six to eight weeks before bloom, when shoots are 1 to 2 inches (3 to 5 cm) tall. At this stage, leaves form a closed spike, which prevents any fertilizer from lodging inside leaves where it can cause a bum. Nitrogen should be applied at the rate of 5 oz nitrogen (N) per 100 square feet of bed (15 g/m²) at each application.

Calcium

Bulbs need calcium throughout the growth period. Limestone is a readily available, inexpensive source of calcium. Applying limestone at rates to maintain a desired soil pH level will assure an adequate supply of calcium. Use of dolomitic limestone instead of calcitic limestone will guarantee an adequate supply of magnesium as well.

Potassium

Potassium deficiency did not occur in either the greenhouse or field studies of tulips. Tulips grown in the field on low potassium soil for two years without application of any potassium showed no deficiency. However, the potassium content of successive generations of daughter bulbs declined. It would be advisable to apply potassium annually to keep the potassium content in the bulb constant over the years.

An adequate level for application is 5 ounces of potassium (K₂0) per 100 square feet of bulb bed (15 g/m²) once per year at the normal time for bulb planting. Multiple applications are not necessary during the year because potassium carries a positive electrical charge. Since soil has a net negative electrical charge, potassium is attracted and held to soil against the forces of leaching. Potassium held in this manner is available to plants over time.

Phosphorus

Tulip bulbs are extremely efficient accumulators of phosphorus as well as potassium. They can persist for a few years without phosphorus fertilizer application. However, this should not be a practice because successive generations of daughter bulbs may have a declining phosphorus content. The year that bulbs are planted, phosphorus can be mixed into the soil in quantities sufficient to last for the four-year life expectancy of the bed. In this situation 16 ounces of phosphorus will be required per 100 square feet (48 g/m²). Alternatively, an application rate of 4 ounces phosphorus per 100 square feet of bed per year (12 g/m²) will also maintain the phosphorus content in future generations of tulip and daffodil bulbs.

Phosphorus released from any source is rapidly fixed on to the soil surfaces so that it is not subject to leaching and will slowly feed back into the soil solution from which it can be accumulated by plants over a period of years. Any source of phosphorus will serve well when mixed into the soil. However, if phosphorus is omitted during bed preparation or only enough is added to last one year, then applications will have to be made to the bed surface annually in the fall when roots develop. Use a water soluble source such as single or triple superphosphate or a mostly soluble source such as a complete fertilizer (10-10-10, 8-8-8, etc.). The phosphorus incomplete fertilizers can range from 50 to 100 percent water soluble.

Bonemeal should not be used for topdressing because phosphorus in this source is mostly insoluble. Use bonemeal only when it can be incorporated into the root zone.

A soluble phosphorus source such as superphosphate (not bonemeal) and heavy initial irrigation with 1 inch (2.5 cm) of water are necessary for top-dress application of phosphorus in gardens. The initial wetting of the top-dressed phosphorus fertilizer must be sufficient to carry it to the root zone 6 to 10 inches (15-25 cm) below. Phosphorus will be fixed onto soil surfaces as it is carried through the soil. At whatever point it stops, the remainder will be quickly fixed. In field crop situations phosphorus is not thought to move in the soil profile from the point of application. However, phosphorus movement is possible in heavily amended and very porous garden soils.

Bonemeal

Bonemeal by itself is not sufficient fertilizer for spring-flowering bulbs. It is primarily a source of phosphorus. Our studies have demonstrated that the major nutrient these plants need is nitrogen. Although bonemeal provides some nitrogen, it is not enough.

Some years ago bonemeal was made from bone, cartilage and other tissue scraps. At that time, it was a source of nitrogen -- along with phosphorus and the micronutrients. Today, bonemeal consists only of bones that have had their nitrogen-rich marrow steamed out to make gelatin byproducts. Much of the nitrogen and micronutrients are gone; however, the phosphorus content is higher. Therefore bonemeal should only be considered as a phosphorus source today and not as a total source of nutrients.

Fertilizer Plan for Large Landscapes

It is more economical to fertilize large bulb plantings with separate nutrient sources. Such sources might include ammonium nitrate for nitrogen, potassium sulfate for potassium and superphosphate for phosphorus. These sources supply only the necessary nutrients without the waste associated with excess phosphorus and potassium in 10-10-10 or 8-8-8.

The schematic in Table 5 details the particular nutrients and quantities recommended for beds of tulips and daffodils for large-scale plantings. Particular fertilizers and amounts that can be used to provide the quantities of phosphorus, nitrogen, and potassium recommended in Table 5 can be found in Tables 6 and 7.

Follow recommendations of soil test and adjust pH during bed preparation to insure adequate calcium and magnesium. Use dolofiitic limestone to raise pH. All phosphorus sources can persist in the soil for a year or more. The nitrogen and potassium sources recommended in the tables release these nutrients immediately, however, the potassium sources are adequate for one year.

Classified list and international register of tulip names. 1987. Royal General Bulbgrowers Assoc., Parklaan 5, Hillegom, Holland. 250p.

Daffodils to show and grow, and abridged classified list of daffodil names. 1985. 3rd Ed. American Daffodil Society, Inc., Hernando, MS 38632.

De Hertogh, A. A. 1982. Holland bulb garden guide. Intl. Flower-Bulb Centre, Hillegom, Holland. Available from: Netherlands Flowerbulb Information Centre, 250 West 57th Street. Suite 629, New York, NY 10019.

De Hertogh, A. A. 1985. Holland bulb forcers guide. 3rd ed. Intl. Flower-Bulb Centre, Hillegom, Holland. 284 p. Available from: Netherlands Flowerbulb Information Centre, 250 West 57^th Street. Suite 629, New York, NY 10019.

De Hertogh, A. A. (ed.) 1986. Spring-flowering bulbs. Horticolor, B. P. 69200 Venissieux, France. 159 p. Available from: Langeveld Bulb Co., PO Box 69, Park Ridge, NJ 07656.

De Hertogh, A. A., L. H. Aung, and M. Benschop. 1983. The tulip: botany, usage, growth, and development. Horticultural Reviews 5:45-125. Available from: AVI Publishing Co. Inc., Westport, Ct.

Fanning, J., S. Gilbert, F. McGourty, and M. E. B. Joyner. (eds.). 1981. Handbook on bulbs. Brooklyn Botanic Garden Record–Plants and Gardens 37(3):1-64. Available from: Brooklyn Botanic Garden, 1000 Washington Ave. Brooklyn, NY 11225.

Genders, R. 1973. Bulbs: a complete handbook. Robert Hale & Co., London. 622 p.

Horton, A., and J. McNair. 1986. All about bulbs. Ortho Books. San Francisco. 96 p.

Lodewijk, Tom. 1979. The Book of Tulips. R. Buchan (ed.) English translation by The Vendome Press. New York. 128p. Distributed in the United States by The Viking Press. Distributed in Canada by Penguin Books. Out of print, but well worth searching for.

Rees, A.R. 1972. The growth of bulbs. Academic Press, NY 311 p.

Scott, G. H. Bulbs–How to select, grow and enjoy. H.P. Books. Tucson, AZ 85703. 160 p.