....at Het Loo Palace, Apeldoorn, Netherlands
The Dutch royal lineage known as the House of Orange, or Orange-Nassau, began in 1544 when William I inherited the title Prince of Orange. Although the male line was broken on the deaths of the childless William & Mary, the present Dutch monarchs still consider themselves part of the House of Orange. The title originates from the town of Orange in France, but it is clear that the monarchy became associated with the colour orange and took a special interest in the 'orange trees' newly introduced into Europe.
The Palace at Het Loo was built between 1684 and 1686 and was a residence of the House of Orange-Nassau from the 17th century until the death of Queen Wilhelmina in 1962. The building was renovated between 1976 and 1982. Since 1984, the palace has been a state museum open for the general public, showing interiors with original furniture, objects and paintings of the House of Orange-Nassau. The magnificent Dutch baroque gardens, which had fallen into disrepair,  were fully restored starting in 1970.
Het Loo gardens panorama
A stitched-together panorama of the restored gardens behind the palace of Het Loo. Note that this image is geometrically distorted.

page modified 1st October 2009

In September 2009, I was shown around the gardens by the curator, Ben Groen. I was taken to the Queen's Garden, a separate 'secret' garden to one side of the palace. It is here that the historic citrus collection is on display in the summer months.
Queens Garden Het Loo
The majority of the trees in the Het Loo collection are Bitter Orange, Citrus aurantium, probably grown from seeds as no graft lines are visible. Ben Groen has tasted fruit from each tree and says that they have a range of sourness from extreme to almost sweet, and that this tends to correlate with the width of the petiole blade - the wider the petiole the more sour the fruit.
The very first citrus trees to be grown at Het Loo were apparently sold off in 1640, so the earliest ones now date from some ten years later. Many of these trees still survive. They are grown in large circular pots constructed of teak, then painted and covered with a dark-green plastic coating. Strangely, all the citrus trees are in round containers - other species are in the traditional square 'Versailles Planters' usually associated with citrus collections.
Each tree has a nicely prepared display label stating species, variety, introduction date and collection number. A hunt among the foliage also reveals the original, diamond-shaped, pressed lead tags.
vrucht bitter label
vrucht bitter-sweet label
Trees are sprayed weekly against insect pests - so the fruit are perhaps not too healthy to eat! They are fertilised with modern mineral fertilisers, rather than regularly removing and replacing the top few inches of soil. Ben Groen pointed out that the older method results over the years in the trees slowly sinking down in the pot - an effect visible in some of the citrus trees at Twickel
old lead tag
Bitter Orange with bitter fruit
Bitter Orange with bitter-sweet fruit

Links to official web-sites:
Here are just some of the citrus varieties on display at Het Loo Palace
Horned Bitter Orange
Variegated form of the Horned Bitter Orange

Willowleaf Bitter Orange
Furrowed Bitter Orange
The garden curator standing in front of a magnificent potted specimen of Citrus aurantium myrtifolia - also known as Chinotto, or the myrtle-leaved bitter orange. The tree is about 4m (12') tall. Ben Groen has noted two different forms in the collection. This one is mainly tip-bearing, whereas the other produces fruit along a long section of the branches. There is also a beautiful variegated form of Chinotto. See my web-page about  the Dutch National Citrus Collection at Twickel.
Bitter Orange 'Adams-beet' is Dutch for 'Adam's Bite'. Apparently refers to the circular ring which appears at one end of the fruit. Just visible here on the lower right immature fruitlet.
Variegated Bitter Orange. This is the form with yellow variegation. There is also a white variegated type.
Viewed from the terrace, you can see here the tops of the potted trees in the Queens Garden, on the eastern side of the palace.
The  Dutch National Citrus Collection contained 223 specimens when surveyed in 2008.  About 80% of these are Citrus aurantium, the sour or bitter orange, but there are also a few sweet oranges, lemons, citrons, grapefruits, pummelos and calamondins. The following named varieties of Citrus aurantium are included:-
'Adamsbeet'    'Salicifolia'   ' Corniculata'    'Canaliculata'   'Crispifolia'   'Horridus'   'Variegata'   'Corniculata Variegata'   'Myrtifolia-small fruit'  'Myrtifolia-large fruit'  'Myrtifolia Variegata'
Update 2014: The all-seeing Google camera has been visiting the gardens of Het Loo. Click Here to see what it found!

Google Link added 30th May 2014