In short
- Forms a silvery, evergreen to semi-evergreen mat 15-30 cm tall; a single plant spreads to 25-100 cm and, left unchecked, keeps going.
- Flowers white from May to June, with blooms 1.2-2 cm across and petals notched to 1/3 to 1/2 of their length, which makes them look doubled.
- Requires full sun and lean, free-draining soil with a neutral to alkaline reaction - in wet and shady conditions it rots.
- Very expansive: the stems root at the nodes and the plant self-seeds abundantly. A bed edging and shearing after flowering are a duty, not an option.
- Hardy down to zone 3a, but short-lived - after 3-4 years the centre of the clump goes bare and it has to be divided or renewed from cuttings.
Botanical data
- Family
- Caryophyllaceae (Caryophyllaceae)
- Height
- 0.15–0.3 m
- Width
- 0.25–1 m
- Habit
- Creeping
- Growth rate
- Fast
- Position
- Full sun
- Soil
- Sandy, Chalky
- pH reaction
- pH 6.5–7.8
- Moisture
- Dry, Moderate
- Bloom
- May–June
- Hardiness
- USDA 3a–7b
- Propagation
- By division, From cuttings, From seed
Characteristics
A creeping perennial that forms a low, dense mat: the stems lie on the ground and root at the nodes, so the clump widens sideways and quickly closes over the surface. The leaf layer itself is about 10-15 cm tall, while the erect flowering stems raise the whole plant to 15-30 cm. A single plant spreads to 25-100 cm across, and an unchecked mat keeps going - sources give a spread ranging from 25-45 cm for a clump a few years old to 1-1.5 m for a freely growing carpet, and that discrepancy in itself tells you about the pace of the species. The leaves are narrow, oblong-lanceolate, up to about 3 cm long and a few millimetres wide, arranged in opposite pairs and covered in dense white tomentum - it is this that gives the plant its silvery-grey shade. The foliage persists throughout the season and usually through winter too, but its evergreen nature is conditional here: after harsh, snowless winters and a muggy summer the mat thins out and behaves more as semi-evergreen, and some sources even classify the species as deciduous. The flowers are white, star-shaped, 1.2-2 cm across, gathered in groups of several to a dozen or more on stalks raised above the mat. There are five petals, but each is notched at the tip to 1/3 to 1/2 of its length, so the flower gives the impression of having ten petals. Flowering falls in May and June and can be so profuse that the white almost covers the silver of the leaves. The fruit is a capsule that opens with teeth at the top; the seeds are small, and the plant self-seeds spontaneously and abundantly. The site must be in full sun, and the soil lean, sandy or stony and absolutely free-draining, with a neutral to alkaline reaction - snow-in-summer tolerates lime very well. Drought does it no harm, whereas standing water, heavy clay and lingering wet snow lead to rotting of the roots and the base of the stems. It tolerates light shade, but in partial shade the stems stretch, the mat loses its density and silver bloom, and in humid air and too deep shade young seedlings and fresh cuttings more often fall victim to damping-off. It handles frost superbly (zones 3a-7b), much worse hot and muggy summers, after which brown patches appear in the mat. It is short-lived: after 3-4 years the oldest part of the clump dies off and needs dividing or renewing from cuttings.
Popular cultivars
- ‘Silberteppich’
- The most commonly encountered garden cultivar, also sold under the name Silver Carpet. It spreads noticeably more slowly than the species, forming tighter, lower cushions with intensely silver-grey hairiness. The default choice for smaller rock gardens and edgings where the typical species would be too expansive. It is sometimes assigned to the lower and more compact geographical variety var. columnae, under which it also reaches the trade.
- ‘Yoyo’
- A selection with slower spread and a compact, dome-shaped habit, flowering very abundantly. Easier to keep in check than the species, which makes it suitable for compositions with smaller rock perennials and for containers.
Growing and care
Watering
Once established it practically requires no watering - water is needed only during long drought and in containers. Far more dangerous than drying out is excess moisture: in compacted, wet substrate the plant rots from the roots.
Fertilizing
Snow-in-summer comes from stony screes and grows best on poor soil. Nitrogen feeding produces loose, sprawling shoots, weaker silver colouring and even stronger growth - on average garden soil a complete lack of feeding is enough.
Planting
Lean, sandy or stony, absolutely free-draining soil in full sun; on heavier sites mix the top layer with sand and gravel and raise the level of the bed. It is worth sinking an edging 10-15 cm deep right away, because limiting the spread after the fact is much harder.
Pruning
Shear the whole mat back by roughly half with shears - together with the spent flowering stems. The plant then sends out fresh, densely leafy shoots and by winter forms a dense, silver turf. Cut the edges of the clump with a spade once or twice a season.
Companion plants
Good companions
Ta sama sucha, przepuszczalna i wapnolubna gleba w pełnym słońcu. Zdrewniała lawenda stoi wysoko ponad darnią i nie daje się zagłuszyć, a srebrne liście obu roślin tworzą spójne zestawienie.
Znosi te same suche, jałowe warunki, tworzy sztywną kępę wystarczająco mocną, by nie zostać przerośniętą, i przejmuje kwitnienie późnym latem, gdy rogownica jest już tylko srebrnym dywanem.
Klasyczne połączenie: cebule przebijają się przez darń wiosną, a rozrastające się srebrne pędy zakrywają latem ich zasychające liście, których nie wolno usuwać ani zaplatać, dopóki same nie zżółkną i nie zaschną — zwykle około sześciu tygodni po przekwitnięciu.
Bad companions
Dwie rośliny darniowe o identycznych wymaganiach i tym samym piętrze — rogownica rośnie szybciej, wchodzi płożącymi pędami w poduchę floksa i po dwóch–trzech sezonach ją wypiera.
Niska, wolniej rosnąca okrywowa bylina skalna; zostaje przerośnięta przez pędy rogownicy i zacieniona, a przez to wypada z kompozycji.
Rozety rojnika leżą tuż przy ziemi i zostają przykryte darnią rogownicy — pod nią gniją, bo tracą powietrze i światło.
The evidence level indicates whether the relationship is backed by research, observation, or gardening tradition.
Diseases and pests
The disease develops in overly wet, poorly aerated substrate and attacks the roots and the base of the stem. The roots turn brown, rot and lose their bark, which slips off easily, and a dark, watery rot appears at the stem base. The above-ground part wilts, yellows and dies back despite moist soil – a characteristic symptom, because the damaged roots no longer take up water. Especially at risk are houseplants, succulents and cacti (from overwatering), and in the ground conifers, rhododendrons, ericaceous plants and strawberries on waterlogged sites. The process begins inconspicuously at the root hairs and root tips, which darken and disappear, after which the rot advances towards the framework roots and the root collar. Healthy, pale, active roots waste away — what remains is a dark, rusty-brown or blackish, watery mass, and the outer layer of the root bark slips off like a glove, exposing the discoloured central cylinder. In trees and shrubs, infection of the collar and the base of the trunk (stem-base rot) shows as a dark, often streaking patch, sometimes with oozing, and the bark there is moist and sunken; in conifers and rhododendrons, browning and dieback are seen from the bottom or from the centre of the crown while the soil is still moist. Characteristic is the nested (patchy) occurrence of symptoms in the bed — where water and the pathogen accumulate locally. It is worth distinguishing the two causal oomycete genera: Pythium mainly attacks seedlings and fine roots, causing damping-off and root rot in cool, wet substrate, whereas Phytophthora also attacks older, woody plants, framework roots and the collar, more strongly in warmer, water-saturated soil. Key for diagnosis is the contradiction of the symptoms: the plant wilts and looks 'dry', although the substrate is wet — this results from the destruction of the roots, not from a shortage of water.
Damping-off of seedlings (black leg)Seedlings and young transplants suddenly wilt, topple over and die, often in patches. At the base of the stem, just above the soil surface, the root collar turns black, softens and constricts, so the plant loses its support and falls over. Seeds may also rot before emergence, which shows as sparse, uneven emergence of vegetable transplants, annual flowers and herbs raised from seed. Two phases are distinguished: pre-emergence damping-off (rotting of the germinating seeds, absence or gaps in emergence) and post-emergence damping-off (toppling of seedlings that have already emerged). The exact picture depends on the pathogen. With infection by oomycetes (Pythium) the rot is watery, translucent and soft — the constricted stem looks as if 'cut with a knife', and the seedling falls flat, especially in cold and excess water. Rhizoctonia solani produces a brown to rusty-red constriction, the tissue is drier and more pinched than watery, the seedling may remain standing for some time, and a delicate, brown, cobweb-like mycelium is sometimes visible on the surface of the medium and on the collar. Fusarium causes wilting with yellowing and discoloration at the base and of the roots. The focus spreads in a circular, patchy way from the first infected seedling, because the pathogen travels through the moist medium and water; the roots of diseased seedlings are brown, poorly developed, with sloughing bark.
Toxicity
| For whom | Level | Notes |
|---|---|---|
| Humans | None | It is not recorded as a poisonous plant, nor is it an edible or medicinal plant - it is treated exclusively as ornamental. |
| Dogs | None | — |
| Cats | None | — |
History and origin
In the wild, snow-in-summer grows on stony slopes and screes of the mountains of central and southern Italy, on Sicily and Corsica. The species was described by Carl Linnaeus in his work Species Plantarum of 1753, and the epithet tomentosum refers to the dense tomentum covering the leaves and stems. As an ornamental plant it spread throughout Europe and North America and in cooler regions it has naturalised - it persists on railway embankments, walls, slopes and roadsides completely outside cultivation, which is the best testimony to its toughness and expansiveness. In Polish gardens it belongs to the classics of edgings, rock gardens, low walls and grave plantings: it is cheap, practically indestructible and covers barren, drying-out spots where most perennials fail. In the horticultural trade the species is sometimes confused with Crimean Cerastium biebersteinii - both form a similar silver mat and cross with each other, so under one name the trade offers plants belonging to the other species, and North American reports of C. biebersteinii mostly refer to C. tomentosum. Breeding has gone above all in the direction of compact, slower-spreading forms, because it is excessive expansion, not a lack of beauty, that is the main charge against the species.
Uses
Snow-in-summer does well anywhere it is dry, sunny and lean: on rock gardens, in the pockets and crevices of retaining walls, on slopes hard to mow, in gravel gardens, as a bed edging and as a cover over bulbs - daffodils and squills - whose withering leaves disappear beneath the silver mat. It looks good in the company of plants with a stronger structure and contrasting colour: lavender, catmint, tall stonecrops, grasses with glaucous leaves. It is planted 25-35 cm apart; at this spacing it closes over the surface in one, at most two, seasons. The key decision, however, is about the place, not the neighbours: this is a plant for surfaces that can be overgrown entirely, not for delicate rock compositions with small species. The creeping stems root at the nodes and move in under neighbours, and abundant self-seeding carries it into paving crevices and adjacent beds. That is why it is worth closing the bed off from the start with an edging sunk 10-15 cm deep, cutting the edges of the mat with a spade during the season, and after flowering shearing the whole plant - a single operation removes the seeds and at the same time rejuvenates the foliage. It is not suitable for walked-on surfaces or as a lawn substitute: the stems break and do not regenerate. Nor does it tolerate shade under tree canopies or waterlogged low-lying ground. In containers, boxes and wall tops it grows well provided the substrate is very free-draining and there is no saucer of water, but even there it needs renewing after 2-3 years. In tight spots it is better to reach for slower-spreading cultivars than the species.
Trivia
- There are five petals, but each is notched at the tip to 1/3 to 1/2 of its length, which makes the flower look ten-petalled - a trait common in the pink family, among others in stitchworts and catchflies, and exceptionally pronounced in snow-in-summer.
- The silver colour of the leaves does not come from a pigment but from a dense layer of tangled white hairs. This same tomentum limits evaporation and reflects light, which lets the plant grow on hot, dry screes.
- The English name snow-in-summer describes the white flowering, while the German Filziges Hornkraut ('felted hornwort') describes the hairiness of the leaves. Two languages captured two different features of the same plant.
- Snow-in-summer and Crimean Cerastium biebersteinii cross with each other and form an almost identical silver mat, so both names are sometimes used interchangeably in the trade - for the gardener the difference is practically imperceptible.
Frequently asked questions
How do I stop snow-in-summer from spreading?
Three measures at once: an edging sunk 10-15 cm deep around the bed, cutting the edges of the mat with a spade once or twice a season, and shearing the whole plant right after flowering, before it sets seed. Simply pulling out the stems is not enough - every fragment left in the ground roots again at the nodes. In a tight rock garden it is worth choosing a slower-growing cultivar from the start, for example Silberteppich or Yoyo.
When and how do I prune snow-in-summer?
Right after flowering, usually in the second half of June or in July. The whole mat is sheared back by roughly half, together with the spent flowering stems. Cutting prevents self-seeding and prompts the plant to send out fresh, densely leafy shoots that look attractive right up to winter. Without this operation the mat becomes thick, stuffy and goes bare in the middle more quickly.
Why does snow-in-summer go bare in the centre of the clump?
This is the normal course of things for this short-lived perennial: after 3-4 years the oldest part of the mat dies off, especially after a muggy summer or when the stems have grown heavily through each other. The empty spot is raked out and filled with rooted cuttings, or the whole clump is divided in spring. This is prevented by shearing the mat every year after flowering and avoiding nitrogen feeding.
Is snow-in-summer suitable as a lawn substitute?
No. It does not tolerate being walked on - the stems break and do not regrow, and trodden spots rot. It works only as a cover for surfaces that are not walked on: slopes, rock gardens, edgings, gravel gardens and wall crevices. For places people walk through, a different solution has to be chosen.
My snow-in-summer rotted over winter - what went wrong?
Almost always water, not frost. The species overwinters without covering even in zone 3a, but does not tolerate wet, compacted substrate, long-lingering wet snow or a layer of leaves on the mat. The site should be raised and free-draining, with an admixture of gravel, and it is a good idea to spread fine grit over the surface beneath the stems. In autumn do not cover the plant with bark or fleece - under a covering it rots faster than out in the frost.
Sources
- Plants of the World Online (POWO) — Cerastium tomentosum L.Database (GBIF, POWO…)
- RHS — Cerastium tomentosum (snow-in-summer)Institution / botanical garden
- Missouri Botanical Garden Plant Finder — Cerastium tomentosumInstitution / botanical garden
- NC State Extension Gardener Plant Toolbox — Cerastium tomentosumInstitution / botanical garden
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