Dear Master Gardener: My friend wanted to collect the “orange seeds” from my Asclepias tuberosa (butterfly weed) to see if he could start some new plants. I told him those were the fallen petals — the seeds form later in pods and then pop open and float around on their fluff. Am I right? If I give him some of the pods, what does he need to do to grow new plants for the monarchs?

Answer: You are right! After the flowers fade on the plant, Asclepias tuberosa get grayish-green seed pods that are three to 6 inches long. When the seed pods turn brown, they will break open and the hundreds of seeds, which have puffs of cotton attached to them, will get dispersed by the wind. To collect the seeds, let the seed pods dry on the plant, but collect them before the pods break open. The seeds are flat, brown, and shaped like a teardrop. Plant the seeds in the fall (November is a good time) as they need cold stratification; making them perfect candidates for winter sowing. It will take two to three years to see flowers on the plants.

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Dear Master Gardener: I recently planted some shrubs and would like to mulch them with some of my many white pine needles. My wife, however, says that pine needles will make the soil acidic and I shouldn’t use them. Is she right? If so, what should I use?

Answer: It is a longstanding myth that pine needles acidify soil. University tests show that there is little, if any, acidifying of soil from pine needles. Pines thrive in acidic soil but do not significantly acidify it themselves. Go ahead and mulch with them. Pine needles are plentiful and free if you have pine trees. Other mulch options are wood chips, bark chips, shredded wood chips, cocoa bean hulls (which emit a mild and delicious chocolate aroma but tend to get moldy and shouldn’t be used if you have dogs), and wild rice hulls. Mulch has many benefits. It conserves moisture in the root zone of plants, prevents weed growth, stabilizes soil temperatures, prevents the spread of soil-borne diseases, increases the fertility of soil as it decomposes, and gives a tidy appearance. In the winter mulch does not prevent the soil from freezing but it minimizes the heaving up of plant crowns from alternate freezing and thawing. Rock and stone mulches are attractive but are not weed-free, as many assume. They tend to compact soil, and are extremely difficult to remove.

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Dear Master Gardener: I planted new shrubs this spring and they seem to be struggling. Should I water them every day during this heat and drought or fertilize them to help them along?

Answer: This prolonged heat wave and drought has really caused stress to plants and the gardeners who are spending inordinate amounts of time watering them to keep them alive! Shrubs vary in their water requirements, but as a general rule, younger plants typically need more water than more established ones. The best way to tell if your shrubs need water is by feeling the soil. When the top two inches begin to dry out, it is time to water. Light sprinklings do little for any plant. Each time you water saturate the soil around the base of the plant. The typical rule of thumb is to give established shrubs one inch of water per week. Your shrubs are newly planted, so frequent and deep watering is very important during the first two years to encourage strong root growth. You have probably heard that overhead watering is bad for plants, but during weather like this it can discourage spider mites (which thrive in hot, dry weather). Also, during high heat, overhead watering will cool your shrubs down, which is especially important for azaleas. The best time to do overhead watering is in the morning to give the foliage time to dry off before nightfall. Again, light sprinklings may do more harm than good. You may need to run a sprinkler for several hours to get water deep enough into the soil to be effective.

It is very common for gardeners to think that fertilizing stressed plants is the boost they need; however, it is best to avoid fertilizing shrubs (and trees) when they are under heat and drought stress. In our cold climate, shrubs usually have more growth in the spring and that is when they need nutrients. Fertilize each spring with a 10-10-10 granular fertilizer, applying it around the base of the plant and watering it in.

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Dear Master Gardener: My Purple Dome Asters have white all over them. What is it and what should I do?

Answer: It sounds like your Aster plants have powdery mildew - a very common disease found on ornamental plants. Powdery mildew is easy to recognize by its characteristic coating of “powder”, which is typically found on the upper sides of leaves. Although it is quite unsightly, it is rarely fatal to the plant. We have had extremely hot, dry weather, which favors this disease. Poor air circulation caused by overcrowded plants or overhead watering also contribute to mildew. Once you see powdery mildew, it is too late to treat it. It is important not to fertilize them any more this season, avoid overhead watering to help reduce the relative humidity, and in the fall clean up all plant debris around the plants. Do not compost infected plant debris because your compost pile may not heat up enough to kill the fungus. If your plants are too crowded, prune them or remove a few to help increase air circulation, which will reduce relative humidity and infection.

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August Gardening Tips

  • Yellow jacket wasps are numerous in August and are attracted to sweet, ripe and rotting food. Be watchful of cans and glasses of beverages, which may harbor yellow jackets you don’t see or expect before your next sip!

  • Blossom end rot is a disorder that is characterized by a black, leathery sunken area at the bottom of tomatoes and peppers. Symptoms usually appear in hot, dry weather such as we’ve been having. To avoid it, water deeply and regularly and don’t fertilize too heavily with nitrogen. Slice off the affected tissue – the rest of the tomato is perfectly good.

  • Raise the height of your lawn mower blades to three inches. As heat builds in the summer the taller grass protects the roots and allows deeper root development. Shorter cut levels lead to shallower root systems, making the plants more susceptible to water and heat stress.

  • Continue to dead-head perennial flowers to prevent seed set and enable the plant to retain more energy reserves for next year’s bloom.

  • Watch for sphinx moths in the early evening. They look very much like hummingbirds. They are about the same size, also flap their wings rapidly and hover, and they suck nectar from impatiens and other flowers with their long proboscises.

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You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A master gardener will return your call. Or, emailing me at umnmastergardener@gmail.com and I will answer you in the column if space allows.
University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.