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Friday - January 23, 2015

From: Lakeway, TX
Region: Southwest
Topic: Seeds and Seeding, Herbs/Forbs, Wildflowers
Title: Germinating Milkweed Seeds
Answered by: Anne Van Nest

QUESTION:

When is the best time to plant milkweed seeds outside? I was told when the overnight temperature hits 70 F. (our garden is in Lakeway, Texas). So around late May? Is this when the seeds germinate in nature? And what tips do you have for a successful planting? We have bought seeds from a supplier and a process called cold stratification is recommended. Do you know if this is successful?

ANSWER:

The Monarch Watch website has a very good information page for you about propagating  milkweeds. They say that milkweeds can be propagated from seeds, cuttings, and, in some cases, from root divisions. So once you get some milkweeds started from seed, you can look into taking cuttings or dividing the roots to increase plant quantities. Cold stratification is recommended for most milkweed species (except the non-native tropical ones such as Asclepias curassavica) and will significantly help increase the germination rate. Cold stratification (vernalization) does require several weeks of preparation before sowing the seed, so timing is very important. Your young seedlings can safely be planted outside after the soil temperature reaches 75 F. Monitor your soil temperature during March and April to see when you reach this target.
In addition, if you are trying to grow Texas native milkweed species, they often have long tap roots and should be transplanted outdoors after the stratification process so the roots can grow unimpeded. Take a look at the Texas Butterfly Ranch website for some additional information specifically about raising Texas native milkweed from seed.
Here’s some general information from the Monarch Watch website about starting milkweed from seed. Milkweed seeds can be planted in prepared beds outdoors or started indoors in flats. They recommend the latter approach since germination rates are generally higher indoors and it is easier to establish your milkweeds with transplanted seedlings that are well-rooted and therefore more resistant to weather extremes and pests.
Starting Seeds
Milkweed seedlings can be started indoors in a greenhouse or under artificial lighting and then transplanted outdoors after the average date of last frost. If seeds are started indoors, allow 4-8 weeks growing time before transplanting. Plastic flats can be used to start the seeds. Fill the flats with a soil mix suitable for seedlings (most potting mixes are), thoroughly soak the soil, and let the excess water drain. Sow the seeds by scattering them on the soil surface 1/4-1/2 inch apart, and then cover with about 1/4 inch of additional soil mix. Gently mist the soil surface with water to dampen the additional soil mix that has been added. In an effort to improve germination rates, many gardeners place the seeds in packets made from paper towels and soak them in warm water for 24 hours prior to planting. This method seems to work especially well for seeds of species that require vernalization (see below).
After the seeds are sown in the flats, cover each flat with a clear plastic cover or a plastic bag to keep the seeds from drying out while germinating. Then, place the flat under grow lights, in a warm sunny window, or in a greenhouse. Most seeds will germinate in 7-10 days if the flats are maintained at 75˚F. After the seeds have germinated, remove the plastic covering from the flats. Once the seedlings have emerged, the soil should be kept moist by watering the flat from the bottom. You can water from the bottom by placing the flat in a sink or a larger flat filled with 2 inches of water until moisture appears on the soil surface. The soil should be kept moist but some care is needed to keep the seedlings from getting too wet - such conditions contribute to fungal growth that can kill the young seedlings (“damping off”). Thinning can reduce damping off.
Vernalization
Seeds of most temperate plants need to be vernalized, which is a fancy way of saying that they need cold treatment. The best way to give the required vernalization is through stratification. To stratify seeds place them in cold, moist potting soil (sterilized soil is best but is not required) in a dark place for several weeks or months. Since most people prefer not to place potting soil in their refrigerators, an alternative is to place the seeds between moist paper towels in a plastic bag. This procedure works well, in part because there are fewer fungi and bacteria available to attack the seeds. After a vernalization period of 3-6 weeks, the seeds can be planted in warm (70˚F), moist soil. Without vernalization/stratification, the percentage of seeds that germinate is usually low. Seeds from the tropical milkweed, Asclepias curassavica (and other tropical milkweed species) do not require this treatment. “Shocking” seeds that have been refrigerated by soaking them in warm water for 24 hours also seems to improve germination rates.
Scarification
Even after vernalization/stratification, seeds of many plant species will not germinate. In these cases, the seed coats appear to require action by physical or chemical agents to break down or abrade the seed coat. “Scarification” with some type of physical abrasion that breaks the seed coat usually works and can be accomplished by placing the seeds in a container with coarse sand and shaking the container for 30 seconds or so. Scarification may be required for some milkweeds (e.g., A. viridiflora and A. latifolia) and might improve the germination rates of other species.

 

From the Image Gallery


Engelmann's milkweed
Asclepias engelmanniana

Swamp milkweed
Asclepias incarnata

Fewflower milkweed
Asclepias lanceolata

Broadleaf milkweed
Asclepias latifolia

Slim milkweed
Asclepias linearis

Longleaf milkweed
Asclepias longifolia

Zizotes milkweed
Asclepias oenotheroides

Aquatic milkweed
Asclepias perennis

Purple milkweed
Asclepias purpurascens

Showy milkweed
Asclepias speciosa

Texas milkweed
Asclepias texana

Butterflyweed
Asclepias tuberosa

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