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Sweet alyssum

Lobularia maritima · brassicas

At a glance

Light
Part sun
Frost
Half hardy (estimate)
Days to maturity
50–70 (estimate)
Lifespan
Annual
On this page
No photo of this plant yet. We add one once we have a picture we can credit.

When do I plant this? › Plan this in your garden ›

The numbers

Mature height
8–25 cm3–10 in
Mature spread
15–30 cm6–12 in
Days to maturity
50–70 (estimate)
Light
Part sun
Feeding
Light feeder (estimate)
Root depth
Shallow (estimate)
Frost tolerance
Half hardy (estimate)
Lifespan
Annual
Habit
Groundcover
Value to pollinators
High (estimate)

USDA PLANTS profile

Before you sow

What the seed asks for before it will come up.

Light to germinate
needs light, so sow shallow and barely cover the seed
The source

hgic.clemson.edu

Sweet Alyssum Clemson HGIC

Numbers we’re less sure of

Many of our estimates come from an extension page we’ve read that gets close without pinning the number down. Others are still our own best guess, and on some the sources disagree. Treat them all as a starting point.

Rules that apply to sweet alyssum

Do not plant brassicas in ground that held brassicas within 4 years (extension recommends 4-7, and even that is not a cure). Promising

Do not plant brassicas in ground that held brassicas within 4 years (extension recommends 4-7, and even that is not a cure).

Promising

If you skip this, you lose real yield. The planner warns you.

How it works: Plasmodiophora brassicae forms thick-walled resting spores that survive in soil more than 10 years. They germinate in wet, acidic soil (pH < 7); liming above ~7.2 suppresses infection. Spores outlast any garden rotation, so a host-free break reduces but does not clear inoculum.

Evidence: checked against 3 sources in the research-based literature.

The 3 sources

content.ces.ncsu.edu

Clubroot of Brassicas NC State Extension
Infested fields should be rotated away from brassica for a minimum of 4-7 years, but this is not guaranteed to prevent disease

vegetables.cornell.edu

Clubroot of Crucifers Cornell Vegetables
… rotation of at least 7 years out of susceptible crucifers may be effective.

www.umass.edu

Brassicas, Clubroot UMass Extension
Long (5-7 years) rotations between brassica crops
Remedy

Plant brassicas in different ground, or wait at least 4 years (extension recommends 4 to 7). Rotation alone is not a cure, because the resting spores outlast it. Raising soil pH above ~7.2 suppresses infection.

Read the full rule › · open in the planner ›

On ground that has carried brassicas within the rotation interval, a measured soil pH below about 7.2 leaves clubroot's door open; raising it is the lever every clubroot guide we read names, and the direction is all we give. Promising

On ground that has carried brassicas within the rotation interval, a measured soil pH below about 7.2 leaves clubroot's door open; raising it is the lever every clubroot guide we read names, and the direction is all we give.

Promising

If you skip this, it still grows, just less well. The planner suggests it.

How it works: Plasmodiophora brassicae's resting spores germinate in wet soil below about pH 7.0, and alkaline soil with high calcium suppresses the infection; liming to 7.1 to 7.5 slowly reduces severity but is not reliable once an infestation is established.

Evidence: checked against 4 sources in the research-based literature.

The 4 sources

content.ces.ncsu.edu

Clubroot of Brassicas NC State Extension
The pathogen germinates in wet soils when the soil temperature is between 18 and 25 ºC (64 to 77 F), and the soil pH is acidic (pH less than 7.0)

umass.edu

Brassicas, Clubroot UMass Extension
The disease is favored by soils with a pH less than 7.0

ipm.cahnr.uconn.edu

Clubroot of Crucifers UConn IPM
Maintain soil pH at 7.0-7.2 where clubroot is a concern. Lime well ahead of planting and incorporate thoroughly

vegetables.cornell.edu

Clubroot of Crucifers Cornell Vegetables
It is essential that hydrated lime be added each year unless the soil pH exceeds 7.5
Remedy

Raise this ground's pH toward 7.2 with lime well ahead of planting, using calcitic lime unless your magnesium is low (then dolomitic), and watch for boron shortage on coarse soils once the pH is up. Your lab gives the amount of lime, from a lime requirement test of your soil, and we don't. Rotation stays the first lever; this is the second.

Read the full rule › · open in the planner ›

Pests of one plant family spread more easily through a solid block of it A good hunch

Pests of one plant family spread more easily through a solid block of it

A good hunch

If you skip this, it still grows, just less well. The planner suggests it.

How it works: Resource concentration. A specialist locates and holds its host more efficiently at higher host density and continuity, so an unbroken run of one family across adjacent beds is an easier target than the same family separated by a non-host (associational resistance).

Evidence: checked against 2 sources in the research-based literature.

The 2 sources

Organization of a plant-arthropod association in simple and diverse habitats: the fauna of collards (Brassica oleracea) Root 1973
… the resource concentration hypothesis, which states that herbivores are more likely to find and remain on hosts that are growing in dense or nearly pure stands; that the most specialized species frequently attain higher relative densities in simple environments.

Flea beetle movement in a broccoli monoculture and diculture Elmstrom, Andow & Barclay 1988
… strong, direct support for the hypothesis that host plants are harder to find and easier to lose in vegetationally diverse habitats than in monocultures.
Remedy

Where one family fills adjacent beds in a single season, breaking the run with a non-host family between them lengthens the distance the specialist must travel to the next host. The app discloses this and never docks a score. A continuous single-family block can be the right call anyway (a wind-pollinated crop like corn sets better massed), so the app names the trade-off and leaves the choice to the gardener.

Read the full rule › · open in the planner ›

Cold-hardy annuals can be sown or transplanted before the last spring frost (hardy crops about three weeks before, half-hardy about two weeks before). Tender crops wait for the frost itself. Promising

Cold-hardy annuals can be sown or transplanted before the last spring frost (hardy crops about three weeks before, half-hardy about two weeks before). Tender crops wait for the frost itself.

Promising

If you skip this, it still grows, just less well. The planner suggests it.

How it works: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.

Evidence: checked against 2 sources in the research-based literature.

The 2 sources

extension.illinois.edu

Time Your Vegetable Plantings by Cold Hardiness University of Illinois Extension
… withstand freezing temperatures and hard frosts without injury

extension.colostate.edu

Vegetable Planting Guide Colorado State Univ. Extension, GardenNotes #720
As early as 2-4 weeks before the date of the average last spring frost

Plus 2 working notes on how the sources were read.

Read the full rule › · open in the planner ›

A few kinds of seed need light to germinate, and burying them stops them coming up. Sow these on the surface, pressed into firm, moist soil and left uncovered or barely dusted over. Don't sow them at the depth a larger seed would take. Promising

A few kinds of seed need light to germinate, and burying them stops them coming up. Sow these on the surface, pressed into firm, moist soil and left uncovered or barely dusted over. Don't sow them at the depth a larger seed would take.

Promising

If you skip this, it still grows, just less well. The planner suggests it.

How it works: These seeds carry the light-sensing pigment phytochrome, which must catch light to switch on the hormone cascade that releases the seed to germinate, so a seed buried past a few millimeters sits in the dark with that switch never thrown and stays dormant.

Evidence: checked against 1 source in the research-based literature.

The source

content.ces.ncsu.edu

Propagation North Carolina Extension Gardener Handbook, ch. 13
When sowing light-requiring seeds, sow them on the soil surface
Remedy

Press light-requiring seed onto firmed, moist soil or medium and leave it uncovered or only barely dusted, then keep the surface damp. A seed sitting on top dries out fast, and a fine mist keeps it in place better than a heavy watering. The plant's page says whether it needs light; a seed that needs darkness is the opposite and is covered as usual.

Read the full rule › · open in the planner ›

Frost can form on a night the forecast low is a few degrees above freezing. Promising

Frost can form on a night the forecast low is a few degrees above freezing.

Promising

If you skip this, the planting fails. The planner warns you.

How it works: On a clear, still night leaves lose heat to the open sky, so a leaf can fall to freezing and ice can form in it while the thermometer, a couple of meters up, still reads a few degrees above.

Evidence: checked against 7 sources in the research-based literature.

The 7 sources

www.weather.gov

Frost and Freeze Information NWS Tallahassee
Issued up to 24 hours when temperature is forecast to be between 32° and 36° on nights with calm winds

www.weather.gov

Freeze Watches and Warnings National Weather Service
… temperatures below 28°F for an extended period of time can kill most types of commercial crops and residential plants.

ipm.ucanr.edu

Freezing and Frost UC Statewide IPM Program
Frost occurs when air is warmer than 32°F but plant tissues drop to 32°F or below because plants radiate (lose) heat into the atmosphere, especially during cool, clear nights.

yardandgarden.extension.iastate.edu

Fall Planting of Vegetables Iowa State University Yard and Garden, Aaron Steil and Kim Hilgers
Tender crops will be damaged or destroyed by a light frost

extension.sdstate.edu

Fall Frost Tolerance of Common Vegetables SDSU Extension, Rhoda Burrows
Crops from more-temperate regions can withstand light freezes (28 to 32 degrees Fahrenheit):

extension.umn.edu

A Minnesota guide to garden timing UMN Extension, Robin Trott
A light frost (32°F) can damage tender plants, while a hard frost (28°F or colder) will kill most warm-season crops.

extension.colostate.edu

Frost Protection and Extending the Growing Season CSU Extension GardenNotes #722, Whiting, O'Meara and Wilson
Radiation frosts occur on calm clear nights that lack cloud cover to hold in heat.
Remedy

The evening before, cover the crops at risk with a sheet or frost cloth, never plastic, reaching the ground and loose at the bottom so the soil's warmth can rise under it, and take it off in the morning. A cover helps least in a long, hard freeze.

Read the full rule › · open in the planner ›

A cover buys cool-season crops a week or two at the ends of the season. A good hunch

A cover buys cool-season crops a week or two at the ends of the season.

A good hunch

If you skip this, it still grows, just less well. The planner suggests it.

How it works: Soil warmed by the sun by day is the heat source at night; a covering low to the ground traps that heat around the plants on calm, clear nights, keeping them a few degrees above the open air through the light frosts at each end of the season.

Evidence: checked against 6 sources in the research-based literature.

The 6 sources

yardandgarden.extension.iastate.edu

All About Cold Frames Iowa State University Extension and Outreach, Aaron Steil
The ground and frame of the cold frame are heated by the sun during the day and radiate that heat during the night, keeping the inside warmer. They are used to extend the growing season by two to four weeks in both the fall and the spring, allowing you to start plants outdoors earlier and continue growing them later in the season.

extension.colostate.edu

Frost Protection and Extending the Growing Season CSU Extension GardenNotes #722, Whiting, O'Meara and Wilson
In trials in Fort Collins, Colorado, a plastic cover on a frame typically provides 3°F to over 6°F of frost protection. It works well for cool season crops that are somewhat tolerant of frosty nights and adds two to six weeks or more on both ends of the growing season.

class.ucanr.edu

Frost Protection and Extending the Growing Season CSU Extension GardenNotes #722, Whiting, O'Meara and Wilson
Soil warmed by the sun in the daytime, is the source of heat for frost protection at night. Moist, smooth soil absorbs more heat. To trap heat from the soil around young vegetables at night, place a covering that is low to the ground and spreading. To recharge the heat source for the next night, any covering must allow sunlight to shine through to the soil or must be removed in the daytime.

extension.usu.edu

Extending the Garden Season Utah State University Extension, Beddes, Caron, Hansen and Gunnell
Low = Plant 1–2 weeks before suggested planting date; Medium = 2–4 weeks before suggested planting date; High = 4–8 weeks before average planting date

hort.extension.wisc.edu

Floating Row Cover University of Wisconsin-Madison Extension, Wisconsin Horticulture, Susan Mahr
Depending on the weight of the covering you choose, you can gain between 2 and 8 degrees of frost protection or warm the local environment sufficiently to harvest certain crops a week or two early.

uaex.uada.edu

Row covers for vegetable gardening University of Arkansas System Division of Agriculture, Cooperative Extension Service
Clear plastic is useless in providing frost protection because the radiant energy released from the soil on those frosty nights passes right through the covering and never warms the air around the plants.
Remedy

Sow or set out cool-season crops under the cover that many weeks ahead of the open-ground date, and keep autumn crops covered past the usual last call. Vent on sunny days, keep plastic off the leaves, and close the cover before sunset.

Read the full rule › · open in the planner ›

Hardy crops can go in later, counted back from the freeze they can't stand Promising

Hardy crops can go in later, counted back from the freeze they can't stand

Promising

If you skip this, you lose real yield. The planner warns you.

How it works: A hardy or half-hardy crop's tissue survives the ice of a light freeze down to about 28 °F (-2 °C), so its season ends at the first night that cold, which comes after the first 32 °F frost, and later still where winters are mild.

Evidence: not yet confirmed. It is cited to 8 sources, which a person has yet to read.

What this rests on - 8 pointers

agrilifeextension.tamu.edu

Fall Vegetable Gardening Guide for Texas Texas A&M AgriLife Extension, EHT-056

We summarise this source in our notes rather than quote a line from it.

yardandgarden.extension.iastate.edu

Fall Planting of Vegetables Iowa State University Extension and Outreach
Tender crops will be damaged or destroyed by a light frost, semi-hardy crops can tolerate temperatures in the upper 20s°F, and hardy crops will survive temperatures in the mid to low 20s°F.

extension.sdstate.edu

Fall Frost Tolerance of Common Vegetables SDSU Extension
Crops from more-temperate regions can withstand light freezes (28 to 32 degrees Fahrenheit):

vegetables.ces.ncsu.edu

Frost and Freeze Protection for Vegetable Crops NC State Extension
If water within the plant cell or between the plant cells freeze, this can result in damage to plant tissue.

extension.oregonstate.edu

Plant cold-hardy vegetables now for a spring harvest Oregon State University Extension
Semi-hardy crops tolerate light frost (28–32°F), including beets, carrots, parsnip, lettuce, chard, garden pea (before flowering), Chinese cabbage, endive, radicchio, cauliflower, parsley and celery.

extension.umn.edu

A Minnesota guide to garden timing University of Minnesota Extension
A light frost (32°F) can damage tender plants, while a hard frost (28°F or colder) will kill most warm-season crops.

www.weather.gov

Understanding Cold Weather Alerts National Weather Service
This temperature threshold kills some types of commercial crops and residential plants, while temperatures below 28°F for an extended period of time can kill most types of commercial crops and residential plants.

www.ncei.noaa.gov

Normals Calculation Methodology 2020 NOAA NCEI
Frost-freeze probabilities are the likelihood that a given minimum temperature will be experienced at least once during a month or year.

Plus one working note on how the sources were read.

Remedy

Sow or set out hardy crops up to the date the app gives. After it, choose a faster variety or wait for the spring sowing.

Read the full rule › · open in the planner ›

Another 135 rules depend on your own garden, like crop rotation, frost dates, sun, spacing and soil. See them all.

Plant teams that place it

When do I plant sweet alyssum? ›

What grows well with sweet alyssum? · Something wrong with your sweet alyssum?

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Milpa Gardens. “Sweet alyssum (Lobularia maritima).” https://milpa.garden/plants/lobularia-maritima/

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