We're only human, and gardening isn't an exact science - so follow our advice at your own risk!
I bought a pack of bare roots labelled 'sea holly'.
In our experience the only eryngiums sold as generic 'sea holly' - either here or in the USA - are Eryngium planum. The same goes for plants in pots labelled 'sea holly': they'll be planum. You can find tips on growing Eryngium planum on our 'Selected Species' page.
My eryngium's leaves are going yellow.
The most likely explanation is that they have been attacked by sap-sucking insects. Check them carefully for aphids and whitefly.
Here in North Yorkshire, aphids are our number one problem, with whitefly generally only being an issue in conservatory- or greenhouse-grown plants. Both kinds of insect find eryngiums irresistible, and can transmit plant viruses that cause the symptoms in question.
Ideally natural predators - which in the case of aphids include ladybirds, lacewings and hover fly larvae - would keep these annoying visitors under control. Unfortunately in springtime, when seedlings and new growth are especially vulnerable, the sap suckers tend to turn up a few weeks in advance of their natural enemies.
Mature plants usually recover, but a serious infestation can prove fatal to seedlings and young plants and will need to be treated appropriately.
My sea holly died this winter - what did I do wrong?
Firstly, are you sure the plant in question was a perennial? Most eryngiums are, but some species aren't - and garden centres and nurseries don't always make this clear.
Eryngium leavenworthii, for example, is an annual and hence needs to be grown anew from seed every year. Eryngium giganteum is biennial; it produces a rosette of leaves in its first year, flowers in its second, then dies. Other species, such as Eryngium spinalba are monocarpic: they may spend several years without flowering, but when they do flower, they die.
Secondly, check your plant is actually dead! Some species die back above ground over winter and don't send up new foliage until mid or late spring. The harsher the winter the more completely they vanish. Even supposedly 'evergreen' 'New World' species such as Eryngium ebracteatum and Eryngium yuccifolium can do this.
If your sea holly really has kicked the bucket during winter, it's probably because it was waterlogged, exposed to cold winds, or both. Species commonly grown in gardens can generally cope with wet in spring and summer, and they can often cope with freezing winter conditions providing it's dry. What they won't appreciate is being wet and frozen at the same time.
So you can do one - or all - of three things.
(1) Plant your eryngiums somewhere with good drainage. Dig grit into heavy soils, and maybe position your plants on a mound or an incline. Put a collar of grit around the base of the stem.
(2) Shelter your plants from the worst of the rain and snow with temporary makeshift cloches. Glass or polycarbonate sheets propped on bricks will do the job.
3) Keep them cosy during cold snaps with horticultural fleece or simply by putting an inverted plastic dustbin over them on frosty nights or during icy gales.
Should I fertilise - or mulch - my eryngiums?
That depends on what species they are - and, of course, on your soil. If your eryngiums are included our Selected Species page, you'll find outline guidance on soil requirements there.
As a rule of thumb, most 'Old World' eryngiums are happy on poor to moderate soils. Some of the 'New World' ones, on the other hand, can benefit from slightly richer soil. Unfortunately there is no comprehensive, reliable reference guide for gardeners - hence we're still experimenting with different planting and feeding regimes!
Even if you decide your eryngiums need added nutrients, we would advise using slow-release granules or liquid feed rather than mulching. Eryngiums are prone to rot at the point where the stem meets the soil, and mulching anywhere near the crown is asking for trouble. What's more, mulch attracts slugs and snails, who regard the growing tips of sea hollies as a prime springtime delicacy.
Can I transplant my eryngium?
Moving established eryngiums from one part of the garden to another is discouraged by most authorities. Nonetheless it is possible - although inconfident gardeners will be in for an emotional rollercoaster. Expect your plants to be unhappy for up to a year. And the older they are, the more resentful they will be.
Mid-to-late spring is the best time to do the deed.
Most eryngiums have long roots which inevitably get broken in the process of digging them out. You need to compensate for this by lopping off a large percentage of the top growth. And by large we mean 80%-plus.
With 'Old World' types this means taking off all but the smallest, youngest leaf shoots. With 'New World' types that means shortening all the leaves on the basal rosette to just 3-5cm from the stem. That's right - even a rosette the size of a dustbin lid can be trimmed to the size of a drinks coaster. You'll feel guilty as hell, but those leaves would have died back anyway. And in a month or so, you should see fresh, young growth at the centre.
How do I divide my eryngium?
With difficulty. And a bread knife.
Most established garden eryngiums produce offsets that seem to be just begging to be split off from the parent plant. But take it from us - because it's a mistake we've made several times - that if you stick in a spade between the pup and the parent you'll end up with a sad handful of leaves with no root attached. It's a bit like clumps of Arabis or Dianthus: they seem to have rooted over a substantial area, but it turns out they're only attached at a single point.
Dividing eryngiums is best done as a transplant (see 'Can I transplant my eryngium?' above) plus a bit of extra hacking. And, like transplanting, it's best done in spring.
Dig up the whole plant, then use a serrated blade, such as an old bread knife or a hacksaw to divide the root mass. Ruthlessly trim the top growth, as per our instructions for transplanting. Plant out the scalped pieces and cross your fingers. Don't be surprised if it takes quite a while for your divisions to establish themselves.
Why won't my eryngium seeds germinate?
One possibility is that the seed isn't viable. There are at least three reasons why this might be the case.
Firstly, hybrid eryngiums such as Eryngium x tripartitum, Eryngium x oliverianum and Eryngium x zabelii seldom if ever set viable seed. This inconvenient fact doesn't stop online sellers or seed swaps offering what they claim are seeds of these plants.
Secondly, many 'New World' eryngiums are sub-tropical plants that struggle to set viable seed in the short window of warm, bright weather provided by the average North European (and, in particular, British) summer. Species like Eryngium pandanifolium, Eryngium ebracteatum and Eryngium leavenworthii often don't bloom until late summer, and the process of seed formation can be cut short by early frosts.
Thirdly, the seed of some species needs to be sown fresh. Old or incorrectly stored seed may have deteriorated so far as to have become useless.
Another possibility is that you need to try a different germination regime. Many 'Old World' eryngiums are sub-alpine plants whose seeds are programmed to watch out for a period of cold followed by a rise in temperature to alert them that spring is on its way. You can replicate this seasonal temperature shift using a domestic fridge, or simply by leaving your pots of seeds outdoors over winter.
Subtropical species, on the other hand, may need extra warmth to break dormancy.
My Eryngium planum 'Jade Frost' no longer has variegated leaves
Unfortunately this is a common experience.
It appears that the genetic information that creates the attractive cream-and-pink markings is confined to the overwintering growth tip which is at, or just below, ground level. During a harsh winter this tip can be frosted and killed. Although the plant will happily regrow from from its rootstock, the variegation will be lost forever and you'll be left with a standard-issue Eryngium planum.
So if you buy 'Jade Frost', be prepared either to give it protection if temperatures fall much below -5C, or to replace it following a harsh winter.