Giant St. John’s Wort – Facts, Identification, Grow and Care

Giant St. John’s Wort is a yellow native perennial wildflower that grows in Eastern North America.  I first noticed this flower while on a hike.  It was quite tall at about 5′.  And the interesting golden yellow blooms were standing above all other flowers in bloom.Great St. Johns Wort Bloom Hypericum ascyron

The blooms of Great St. John’s Wort are very interesting to examine close up.  I find the hundreds of small stamens sticking up above the petals almost hypnotizing to look at when the plant sways in the breeze.

 Facts

  • Hardy in USDA Zones 3-7, check your USDA zone here.
  • Great St. John’s Wort is listed as threatened or endangered in 7 States
  • The Native Range for Giant St. John’s Wort is from the Northern Kansas/Missouri borders, then North to Minnesota, and straight East to the Atlantic Ocean.  But, it is also native to Eurasia!
  • Has the largest flower size in the St. John’s Wort family Hypericum, with that are 2″ diameter (5 cm).
  • It’s flowers produce no nectar, only pollen.  Making this a great plant for native Bumble Bees, as there is no competition from European Honey Bees.
  • Can be adapted to many ecosystems, from prairies to riversides
  • Extracts from Giant St. John’s Wort have been studied for use in anti-cancer drugs

Scientific Name of Giant St. John’s Wort

An interesting factoid about this plant is that botanically it is known as both Hypericum ascyron and Hypericum pyramidatum.

Great St. John’s Wort Physical Description

This plant will reach a height of about 3-5′ tall (~ 1.5 m) and be 2-4′ wide (60 cm – 1.2 m).  When in bloom they are quite noticeable, as the bright yellow flowers contrast against the dark green foliage.

Stalk / Stem

The stalk/stem are generally quite strong and erect.  Any branching will happen towards the upper end of the stalk.  Stalks are somewhat square, smooth and light-medium green.

Leaves

Along the stalk will be opposite lance-shaped leaves that are 3-5″ long and about 1-2″ wide.  Edges of the leaves are smooth, and the surface is veined.  The underside of the leaf will be lighter green when compared to the upper side.  Leaves will wrap around the stem somewhat, and not have a stem.

Flower

The uppermost portion of the stalk will branch/terminate into 1-5 stems for individual flowers.  Blooms are bright yellow and about 2″ across.  Each flower will have 5 petals, and the petals will be somewhat curled, floppy, or wavy. There are gaps in-between each petal.  There will be a lot of hairs/stems (called stamens) emanating from the center part of the flower (corolla).Hypericum pyramidatum

In late fall, a seed capsule will form where each flower was and turn brown.  There will be hundreds of seeds inside each capsule.

Root

Roots Great St. John’s Wort are rhizomatous in nature, spreading/sprouting new plants in a single colony.

Giant St. John’s Wort Reference Table

Common NameGiant St. John’s Wort, Great St. John’s Wort, Tall St. John’s Wort
Scientific nameHypericum ascyron / Hypericum pyramidatum
Bloom TimeSummer
Bloom DurationApproximately two to three weeks
ColorYellow
Bloom SizeIndividual Blooms are about 2” diameter (5 cm) with 5 petals.  Blooms are yellow in color.
CharacteristicsThe top of the upper stems will terminate/branch into 1-5 short stems, each with a single flower.
Height3-5’  (0.9 – 1.5 m)
Spacing/Spread3-4’ spread (0.6-1.2 m)
Light RequirementsFull sun to partial shade.
Soil TypesSandy Loam / Loam / Clay
MoistureWell drained, medium to moist
MaintenanceCut off seed heads to prevent spread.  Pull unwanted volunteers in Spring
Typical UseWildflower Gardens, Backyard micro prairies, Flower beds.
Fauna AssociationsBumble bees pollinate it.  Several moth and butterfly caterpillars feed on foliage, as well as other insects.
Larval HostGray hairstreak butterfly,

Moths: Common Hyppa, Gray Half-spot, Black Arches, Common Pug, Oecophorid

Sowing DepthSurface sown
Stratification60 Days cold/moist stratification
Native RangeUSDA Zones 5-9
NotesGrowitbuildit.com

Giant St. John’s Wort Growing Conditions

Full sun to partial shade is the preference of this plant.  As well as medium to moist soil that drains well.  I’ve encountered this plant in a small woodland opening, along a hillside prairie, and just out in the open.  Overall it seems to be highly adaptable to many environments, but will grow tallest/best in full sun with medium moist/well-draining soil.

How to care  for

If you plant it in it’s preferred growing conditions, you will not have any trouble growing this flower.  It is tough and relatively disease free.

Maintenance

You may have some unwanted seedlings sprout up from seed or rhizome roots.  Those can be pulled and/or roots cut if needed.  In general though it isn’t that aggressive if other competition is present.Giant St. Johns Wort Hypericum pyramidatum

How to Establish Giant St. John’s Wort from Seed

To grow Great St. John’s Wort from seed, surface sow the seeds in a disturbed area or bare soil in Fall/Winter.  Walk over the seed so that it is pressed into the soil, but not covered.  Seed of Giant St. John’s Wort needs sunlight to germinate.

For starting in pots, winter sow or cold-stratify seed for two months before Spring.  Just sprinkling the seed on-top of the soil.

Garden Uses

Due to its height, Giant St. John’s Wort should be planted in the back of a flower bed garden, or micro-prairie so that it doesn’t shade/crowd out other plants.  Provide it with 2-3′ of space on each side to allow it to reach its full potential.  This can make a great wildflower for any garden.

Fauna

Giant St. John’s wort does not produce nectar, just pollen.  So it’s pollination is heavily dependent on native Bumble Bees.  There are numerous moth caterpillars and some butterfly caterpillars that will feed on the foliage.

Stalks and seed heads of Great St. John’s Wort in Autumn

Pests and diseases

This plant is generally disease free, and most animals will not eat it.  The foliage is bitter, making it deer resistant.  Although powdery mildew can effect the appearance if in a moist area with little air movement.

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