Giant St. John’s Wort – Facts, Identification, Grow and Care
January 30, 2020
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.
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.
Hardy in USDA Zones 3-7, check your USDA zone here.
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.
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.
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).
In late fall, a seed capsule will form where each flower was and turn brown. There will be hundreds of seeds inside each capsule.
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
Giant St. John’s Wort, Great St. John’s Wort, Tall St. John’s Wort
Hypericum ascyron / Hypericum pyramidatum
Approximately two to three weeks
Individual Blooms are about 2” diameter (5 cm) with 5 petals. Blooms are yellow in color.
The top of the upper stems will terminate/branch into 1-5 short stems, each with a single flower.
3-5’ (0.9 – 1.5 m)
3-4’ spread (0.6-1.2 m)
Full sun to partial shade.
Sandy Loam / Loam / Clay
Well drained, medium to moist
Cut off seed heads to prevent spread. Pull unwanted volunteers in Spring
Bumble bees pollinate it. Several moth and butterfly caterpillars feed on foliage, as well as other insects.
Gray hairstreak butterfly,
Moths: Common Hyppa, Gray Half-spot, Black Arches, Common Pug, Oecophorid
60 Days cold/moist stratification
USDA Zones 5-9
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.
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.
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.
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.
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.
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.
JOIN OUR FREE NEWSLETTER HERE. Get our new content sent to your inbox. (No worries, we won’t spam you.)
Be sure to check out these other articles, I think you would find useful, as well: