PLANTING & CARING FOR MILKWEEDS
While there are numerous milkweed planting guides, there are none that have specifically been prepared for the highly variable conditions in Arizona, nor have many of the native milkweeds in Arizona been domesticated or grown in home gardens. Gardening can be a challenge in Arizona due to low soil fertility, extreme day/night temperature fluctuations, unpredictable and generally low rainfall, and highly variable microsites.
Here are our suggestions for transplanting and culture of milkweeds. These are based on several years of experience and input from many Master gardeners.
Always keep track of the labels that come with your plants so you can correctly identify individual species. Depending on your taxonomic source, there are as many as 40 species of milkweeds in Arizona. It is a good idea to create a sketch map of the layout of your milkweed garden in case the labels get lost.
Milkweeds can easily be grown in native soil with little fertilizer. A little additional phosphorus (P) added as triple phosphate, rock phosphate or bone meal may be helpful since P is commonly limiting in Arizona soils. Should you choose to add P, 1 tablespoon mixed into the soil at planting time will suffice. Do not be afraid to adjust fertilizer if there is evidence of nutrient deficiency during the growing season.
A soil pH of 6.5- 7.5 is probably optimal, but we have no experimental data to provide a specific recommendation on the optimum pH for milkweeds. As a practical matter it is likely that species of milkweeds that are native to desert areas could tolerate or even perform better with a soil pH of 7.5 to 8.5.
Generally, milkweeds prefer well-drained soil, but there are a few species like swamp milkweed (Asclepias incarnata), tropical milkweed (Asclepias curassavica) and showy milkweed (Asclepias speciosa) that grow best with abundant moisture.
We know milkweeds can grow under the rainfall conditions that exist in Arizona, but a little water to get plants started may be a good idea. Gardeners who water during the first season have had good survival. In general, an established native plant should only be watered when there is a below normal rainfall period and the amount of water should represent enough water to return the amount the plant receives back to what is normal for your area. Adding more than the normal rainfall may increase the suitability of your plants as fodder for monarchs. In addition, milkweeds grown in pots require regular watering. Always use your judgement and do not be afraid to vary these suggestions based on your experience or local conditions.
Most milkweeds grow well in full sun but can tolerate a little shade.
Mulching around milkweeds that prefer wetter soil conditions or occur at high elevation may be beneficial. Lower elevation plantings or milkweed species from desert areas should probably not be mulched.
Time to first flowering for milkweeds varies by species. Some will flower in the first year after planting such as Arizona milkweed (Asclepias angustifolia) and others such as spider milkweed (Asclepias asperula) may not flower for several years.
Milkweed seed pods open slowly when mature and it is best to harvest the entire pod when the pod just begins to split open. You can press the pod and it will pop open if it is ripe and seeds are mature. The mature seed is dark brown. If seeds are partially brown they often are mature and can be harvested. Do not harvest pods if the seeds are still white. Put the pods in a small paper bag and label with the species. A wooden clothes pin or binder clip works well to close the bag. Keep pods in a dry cool place but not in the refrigerator. If the pods are ripe they will split open in a day or two. The seeds will gradually blow away from the pod over several days so if you check your plants every few days you very likely be able to collect some seed.
Want to know how Monarch Watch separates the seeds? Watch this amazing and innovative video!
The question is simple, but the answer is complex, especially in a hot climate like Arizona. It is important to develop growing recommendations for a range of milkweed species in central and northern Arizona. The following are some of the important factors to keep in mind when trying to decide how much water to give to your plants: soil texture, age of plant, planting location and weather.
Soil texture refers to the relative proportions of sand, silt and clay particles in the soil. A sandy soil will allow water to move quickly through the soil where a clay soil will drain slowly. In contrast, the nutrient holding capacity of a sandy soil is generally lower than that of a clay soil. A quick test to determine soil texture and the rate that water will pass through your soil is to dig a 1 ft wide hole about 2 ft deep in your garden. Fill the hole with water and determine how long it takes for the water to drain away. If the water drains away in 15 minutes or less the soil is sandy. If it takes a couple of hours it is a clay soil. Both soil types occur in central and northern AZ. If you were to put a plastic ring on the soil surface and apply 1 inch of water it would penetrate the soil to a depth of 12 inches in sandy soil and 6 inches in clay soil. You can water a milkweed in sandy soil about twice as much as a milkweed in clay soil.
Milkweeds should be watered to a minimum depth of 1 ft, so about 1-2 inches of water. Following a complete watering that penetrates to the desired depth, it is best to wait until the soil is dry at a depth of 1 inch before re-watering. One gallon of water will irrigate 1 cubic foot of soil. A general guideline would be 1-2 gallons per week depending on whether the soil is clay or sand under normal conditions when the milkweed plant is recently planted.
A young seedling needs more water than a mature, established plant. For most of the milkweeds, establishment takes 4-6 weeks. You might water a seedling 2-3 times a week for the first 2 weeks then gradually cut back to 2 X per week and finally to 1 x per week after a month. Most native plants that are well established and in their second season of growth should grow with no supplemental water or if the weather is dry perhaps once every 2-3 weeks. Always water deep and infrequent rather than shallow and frequent. A fully mature plant may have roots 3-4 ft deep so even 3-4 inches of irrigation during a very dry period would be an appropriate amount but only very infrequently.
The amount of water used by a plant will vary greatly based on where the plant is located. In central AZ when temperatures are hot and it is windy a plant can use 2-3 times as much water as at a higher location in northern AZ. The weather (seasonal variation in climate) is an obvious factor in water needs of plants, but native plants are adapted to local climate. A helpful rule is to only water adapted plants during dry weather when the deviation from normal rainfall is at least 1 inch, then water only to the amount of the deviation.
Finally, a few more tips. Under-watered plants wilt; over-watered plants turn yellow. If a plant wilts, water immediately and often it will recover. If a wilted plant does not recover, stop watering until the plant begins new growth. If you have been watering daily and the plant starts to turn yellow, stop watering. Do not water plants when they are dormant, such as over the winter.
Dead or Dormant?
Milkweeds are perennial plants that normally go dormant over the winter and re-sprout the next spring if there is a correct match between the planting climate and the milkweed species. Sometimes when there is not a perfect match between climate and milkweed species or the milkweed is planted at the wrong time the plant can go into a growing season dormancy. A drought can also cause the milkweed plant to go dormant in the middle of the growing season. When the above ground part of the plant drops its leaves or appears to die it may just be going into a growing season dormancy. Milkweeds tend to grow deep roots before they begin above ground growth. Seedlings produced by the AZ Milkweeds for Monarchs Project will have extensive roots and a good ratio of roots to shoots. A milkweed plant that drops its’ leaves or gets clipped off by an herbivore may not be dead.
We have now seen several examples of milkweeds that have been planted when night temperatures are still cool drop their leaves at temperatures several degrees above freezing. Swamp milkweed does this. We have also seen plants whose above ground foliage has been eaten but the plant re-sprouts (many species do this). A dead top does not necessarily mean the roots are dead. If the top of the milkweed plant dies or disappears it may take several weeks for the plant to re-sprout. It may even remain dormant until the following spring. If your milkweed appears to have died or lost above ground growth the best thing is to stop or dramatically reduce the amount of water it receives. Then wait to see if it re-sprouts before you begin to water again. Leave the roots in place and undisturbed.
Credit: Dr. Mike Wagner
30 minute presentation powerpoint (created by Wagner, Hofstetter and Hensen Feb 2021)
Terrior Seeds is partnering with AZ Milkweed for Monarchs to grow and distribute 11 unique milkweeds.
These milkweed seeds are the highest quality available anywhere because they are hand-grown across central and northern Arizona, then hand-harvested, hand-cleaned, and hand-packed. Over the past five years, they have been test-grown in different elevations to verify their vigor and adaptability to different conditions.
This is the first time several of these species have been available for sale, for the simple reason that they can’t be commercially grown and machine harvested.
If you are a gardener who cherishes the appearance of the Monarch butterfly and want to attract them into your garden, this is your best chance to plant the finest milkweed seed before the word gets out and we sell out!
Credit: Mary Barnes