Ask the Master Gardener: Heat can be harmful to hostas

Leaves damaged by high temperatures can be pruned, but wait until it's cooler outside.

A hosta plant with heat-damaged leaves. Photo by Jennifer Knutson

Dear Master Gardener: This 90 degree heat has damaged some of my hostas. The edges turned white and are curled. Did my hostas get stressed from the high temperatures? Should I prune the damaged leaves?

Answer: It has been very hard to keep plants hydrated during this bout of 90-plus degree temperatures! Damage from heat stress can develop in susceptible plants and your description sounds like heat stress. Some hosta cultivars are not heat-tolerant and it appears you have one of them. When susceptible hostas are exposed to sudden and extreme heat and low humidity, the leaves can be damaged in less than a day. The searing heat dries the tissue so quickly that most of the leaf retains its normal color but a white line is created separating the healthy tissue from the damaged edges which get crispy, dry and curl. If the leaves look really bad you can prune them, but wait until it is cooler. Pruning plants in high heat will stress them even more. I have a few hostas that got stressed from the high heat and I’m going to just let them be.

Related: Ask the Master Gardener: How you can keep animals from grazing on your garden No plant or fencing can be said to be completely animal proof, but there are varieties and barriers that can help keep them at bay.
Dear Master Gardener: I wrote in last week about an animal eating my cabbage. I caught the culprit — it was a rabbit. The type of fencing I have around my vegetable garden is the plastic kind with small squares. The rabbit chewed the fencing near one corner and got in that way and chewed the fencing in the opposite corner and escaped that way. I have now repaired the fencing and all is well.

Answer: Well, it was between a squirrel, deer, rabbit, or woodchuck. Mystery solved — rascally rabbits!

Related: Master Gardener: Add spectacular color to gardens Phlox subulata are great additions to rock gardens, another plant is underused as a fabulous addition to a sunny garden, how to keep rhubarb healthy and productive and do marigolds really repel rabbits and bugs -- are all part of this week's Master Gardener column.
Dear Master Gardener: It’s been so dry for so long. I’m watering my flowers all the time. Should I be watering my shrubs and big trees too?


Answer: Probably. We didn’t have much snow to melt and what we did have melted so fast most of it drained away instead of soaking in. The big rain storms seem to dry up before they get to us, and we haven’t had any long, slow soaking rains that get deep into the soil. Most plantings require at least one inch of water per week — either as rainfall or watering. When did we last have a 1-inch rainfall?

Roots, even for really big trees, rarely go deeper than 12-18 inches. It’s important that the roots have water that deep to maximize the water available to the leaves. A soil probe can be useful to see how far down your soil is wet. Take something like a really long screwdriver or shish kabob skewer and poke it into the ground. It should go into damp soil easily and will stop when it reaches dry soil. Grasp it at ground level when it stops, pull straight up and measure the distance to the tip. Ideally it will be at least 12 inches. If it’s less, it’s time to get out the hose!

Related: Ask the Master Gardener: To identify poison ivy, follow the old adage: Leaves of 3, let it be Poison ivy is a plant that is important to identify, so it can be avoided. It can be distinguished from other plants by its leaves, which are always divided into three leaflets.
Above all, don’t just lightly sprinkle. If the soil is only wet up near the surface, then that’s where the roots will go. In hot weather, those top inches dry out quickly and the plant wilts, putting stress on it. Because of our extended dry spell, all the shrubs and trees need additional water. Long, slow watering, with no runoff is what you want. Soaker hoses are perfect because they can cover the whole drip line under the leaf canopy. Even milk jugs or five-gallon pails with a couple nail holes punched in the bottom, placed in the drip line and refilled several times, will effectively water trees and shrubs.

Related: Ask the Master Gardener: Can potted lilies be planted in the garden? If you received Oriental lilies go ahead and plant them, but just know that they may or may not come back as they are not as hardy as Asiatic lilies. 
Dear Master Gardener: I’m growing garlic for the first time. It’s growing well, but how do I know when it’s ready to harvest?

Answer: Hard neck garlic is an easy crop to grow in Minnesota. Garlic requires really rich, well-drained soil — work a lot of rotted manure and compost into your sunny plot well before planting. In late fall, separate the cloves of a head of garlic (not grocery store garlic — that’s usually a California variety that won’t do well here) and bury each clove a couple inches deep, 6 inches apart, pointed part up. Cover immediately with a thick layer of leaf mulch, pine needles, or straw. The roots will grow until the ground freezes and early in the spring you will see green sprouts appear. Pull back the mulch as the weather warms, but leaving it between the plants will help keep down weeds. Water an inch per week. In June, graceful curved scapes will appear — they will turn into flowers, but you don’t want that, so snip them off and add them to stir fry or your scrambled eggs for a very mild garlic flavor. In July the plant leaves will start to yellow and fall over, signaling it’s time to harvest. Pull out the entire plant, shake off excess dirt, and hang the whole thing in a warm, dry place (the garage is perfect) for about three weeks while the papery skins form. At that point, trim off the roots and cut the stem to about 1 inch. Store the heads in a cool, dark place. Save some of the biggest cloves to plant in the fall and you’re ready for next year’s crop!

Related: Ask the Master Gardener: Adding ornamental grasses and more Recommendations for grasses that do well in the lakes area, tips on annuals and perennials, restoring winter-killed grass and how to add a pop of chartreuse to a perennial bed are all part of this week's questions with the master gardeners.

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 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.
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