Nova Zembla is a popular, tough evergreen rhododendron, known for its clusters of red flowers in late spring. Like all rhododendrons, though, how well it does depends less on the plant than on what happens around its roots. Most care comes down to a few habits that keep the root zone acidic, moist and airy, plus timing pruning and feeding to the plant’s yearly flower cycle. The shrub is also poisonous to pets and livestock, which matters when choosing where to plant it.1PubMed. Grayanotoxin poisoning in animals following the ingestion of Ericaceae plants
Soil and Planting Site
Rhododendrons grow best in acidic soil with a pH, a measure of acidity, between 4.5 and 6.0.2American Rhododendron Society Web Site. Plant Culture and Care – Soil Conditions In soil that is too alkaline, iron and manganese get locked up and the plant can’t absorb them.3Indiana Yard and Garden – Purdue Consumer Horticulture. Interveinal Chlorosis on Azaleas and Rhododendron So test your soil before planting, using a home kit or your local extension service.
The roots are shallow and fine, like hair.4American Rhododendron Society Web Site. Plant Culture and Care – Soil Conditions They need moist soil but cannot tolerate waterlogged ground.5American Rhododendron Society Web Site. Plant Culture and Care – Soil Conditions Heavy clay and compacted soil often drain poorly and stay wet.6American Rhododendron Society Web Site. Plant Culture and Care – Soil Conditions Low spots and the areas under downspouts or near foundations are common trouble areas. If that describes your yard, plant in a raised bed or mound rather than a deep hole.
For planting, dig wide rather than deep, about twice the width of the root ball. Before planting, fill the hole with water and watch how fast it drains. If water is still standing in the test hole after several hours, drainage is poor. Set the top of the root ball level with, or slightly above, the surrounding soil, and mix acidic organic matter such as peat moss or composted bark into the backfill. Then water in thoroughly.
For light, the usual garden recommendation for Nova Zembla is morning sun with afternoon shade. Rhododendron leaves adjust to their conditions, growing smaller and thicker in strong sun.7Plant Ecology. Photosynthetic acclimation of rhododendrons to light intensity in relation to leaf water-related traits Too much shade tends to mean fewer flowers, while harsh afternoon sun can scorch the leaves.
Mulching to Protect the Roots
Good mulches include pine needles, oak leaves, pine bark chips or shredded pine bark, and evergreen branches, because these stay loose and are acidic.8Valley Forge Chapter of the American Rhododendron Society. Valley Forge ARS Newsletter January / February 2021 Pine bark mulch has a pH of about 3.5 to 4.5 and can gradually acidify the soil when used year after year.9Rutgers Agricultural Experiment Station. Nova Zembla Rhododendron Care: A Complete Guide A yearly top-up of pine bark can help keep the root zone acidic, though soil testing is the way to confirm it.
Spread about 2 to 3 inches over the whole root zone, out past the branch tips, and keep the mulch a few inches away from the main stem. Never dig or hoe around the plant, since the roots sit just below the surface. Refresh the layer as it breaks down, usually each spring.
Some materials work against you. Fine ones like grass clippings mat down and cut off air. Thick layers of sawdust or fresh wood chips can pack and hold too much water. Stone or rubber mulches heat up and can cook the shallow roots.
Watering
Because the roots are shallow, the top few inches of soil dry out fastest. In dry weather, water deeply about once a week rather than giving daily sprinkles. To check, push a finger an inch or two into the soil and water if it feels dry. New plants need closer attention through their first two summers. In fall, water before the ground freezes so the plant enters winter well hydrated.
Container-grown plants dry out much faster and may need water daily in summer. Use an acidic potting mix and a pot with drainage holes. Roots in pots are more exposed to cold, so shelter containers in winter.
Drooping on a hot afternoon that recovers by evening is usually fine. Feel the soil before adding more water.
Fertilizing for Green Leaves and Flowers
Rhododendrons absorb nitrogen best as ammonium; the nitrate form is taken up less readily.10Journal of the American Rhododendron Society. Phosphorus and Nitrogen Nutrition of Rhododendrons Nitrate can trigger an iron shortage, shown as yellow leaves and weak growth, unless extra iron is supplied.11Journal of the American Rhododendron Society. Phosphorus and Nitrogen Nutrition of Rhododendrons Nitrate feeding also raises soil pH, while ammonium lowers it.12Journal of Plant Nutrition. Influence of Nitrate:Ammonium Ratios on Growth and Elemental Concentration in Two Azalea Cultivars13Journal of Plant Nutrition. Influence of Nitrate:Ammonium Ratios on Growth and Elemental Concentration in Two Azalea Cultivars So choose a fertilizer labelled for acid-loving plants with mostly ammonium nitrogen, such as ammonium sulfate.
Applying slow-release fertilizer right after blooming is a good choice, because flower buds for next year form mainly in June, July and August.14Journal of American Rhododendron Society. Correlation Between Weather Conditions and Rhododendron Flower Bud Formation Don’t overdo it, though: too much nitrogen increases water stress as much as too little does.15HortScience. Effects of Irrigation Frequency and Nitrogen Fertilizer Rate on Water Stress, Nitrogen Uptake, and Plant Growth of Container-grown Rhododendron Follow label rates.
On the label, look for “acid-loving plants,” “azalea” or “rhododendron” formulas. Check that the nitrogen is listed as ammonium or urea rather than mostly nitrate. Slow-release or organic products such as blood meal feed gradually.16Journal of the American Rhododendron Society. Phosphorus and Nitrogen Nutrition of Rhododendrons
Late feeding can push soft new growth that winter cold then damages, so skip late-summer feeding. Established plants in good soil may need little or no fertilizer at all.
Pruning and Deadheading
Prune rhododendrons just after they bloom.17American Rhododendron Society. Pruning and Spent Flower Removal They set next year’s flower buds by mid to late summer, so pruning after that removes next year’s flowers.18American Rhododendron Society. Pruning and Spent Flower Removal
Remove the withered flower clusters after bloom; it improves looks, cuts down on fungus and stops the plant spending energy on seed.19American Rhododendron Society. Pruning and Spent Flower Removal Push the cluster sideways with your thumb and it breaks off, or cut it with hand pruners.20American Rhododendron Society. Pruning and Spent Flower Removal Avoid damaging the new shoots and growth buds just below the cluster.21American Rhododendron Society. Pruning and Spent Flower Removal Skipping deadheading isn’t very harmful, but it can mean fewer flowers next year.22American Rhododendron Society. Pruning and Spent Flower Removal
To reshape an overgrown plant, cut just above a leaf whorl or visible bud so new shoots sprout there. Remove dead or crossing branches any time you see them. If the plant needs hard renewal pruning, spread it over two or three years, and expect fewer flowers the year after heavy cutting.
In late summer you can tell the two bud types apart. Flower buds are fat and rounded at the branch tips, while leaf buds are slim and pointed.
Why the Leaves Turn Yellow
Iron shortage causes yellowing between the veins while the veins stay green.23Indiana Yard and Garden – Purdue Consumer Horticulture. Interveinal Chlorosis on Azaleas and Rhododendron The leaf looks like a grid of green veins over yellow.24Journal of American Rhododendron Society. Micronutrients and Plant Nutrition It appears first on the youngest leaves.25Indiana Yard and Garden – Purdue Consumer Horticulture. Interveinal Chlorosis on Azaleas and Rhododendron Manganese shortage looks similar, and treating it with iron makes it worse.26Indiana Yard and Garden – Purdue Consumer Horticulture. Interveinal Chlorosis on Azaleas and Rhododendron
These shortages usually come from soil that is too alkaline, not from missing nutrients.27Indiana Yard and Garden – Purdue Consumer Horticulture. Interveinal Chlorosis on Azaleas and Rhododendron The lasting fix is to make the soil more acidic so existing nutrients are released.28Indiana Yard and Garden – Purdue Consumer Horticulture. Interveinal Chlorosis on Azaleas and Rhododendron For small areas, add acidic organic matter like peat moss; for larger ones, use elemental sulfur, iron sulfate or aluminum sulfate.29Indiana Yard and Garden – Purdue Consumer Horticulture. Interveinal Chlorosis on Azaleas and Rhododendron Switching to ammonium-based fertilizer helps too.30Journal of Plant Nutrition. Influence of Nitrate:Ammonium Ratios on Growth and Elemental Concentration in Two Azalea Cultivars
Not all yellowing is a nutrient problem. Older inner leaves yellowing and dropping in fall is normal shedding. Pale yellow across all leaves can mean too little nitrogen. Yellowing with wilting in wet soil points to drowned roots.
Test the soil before adding sulfur so you don’t overdo it. Sulfur works slowly, over months. Leaves that are already yellow may not turn green again; the improvement shows in new growth.
Winter Care and Curled Leaves
In frigid weather, hardy rhododendron leaves droop and roll up along the midrib, then unroll when it warms.31PubMed Central. Mechanical basis for thermonastic movements of cold-hardy Rhododendron leaves Rolling cuts the leaf area exposed to winter sun and helps protect it.32PubMed. Infrared thermography of in situ natural freezing and mechanism of winter-thermonasty in Rhododendron maximum Curled leaves in a cold snap are no reason to water or worry.
Drying out is the real danger. In mountain rhododendrons, leaves not fully covered by snow lost much more water and coped with drying worse.33PubMed. Impact of snow cover on photoinhibition and winter dessication in evergreen Rhododendron ferrugineum leaves during subalpine winter Evergreen leaves can likewise dry out in winter wind and sun when the roots can’t draw water from frozen soil.
To guard against it, water deeply in late fall before the ground freezes, and add fresh mulch to insulate the shallow roots. Plant out of strong winter wind and afternoon sun, or put up a burlap screen. Avoid anti-desiccant sprays unless leaves are repeatedly browning.
Brown, crisp edges in spring often mean winter burn. Wait to prune winter-killed tips until you see where new growth starts.
Pests to Watch For
Lace bug young and adults suck chlorophyll from leaves, weakening the plant, so the tops of leaves show fine pale speckles.34ResearchWorks Archive. Optimizing Management Guidelines for the Non-Native Azalea Lace Bug on Rhododendron Species in Western Washington To check, turn leaves over and look for tiny lacy-winged bugs and dark tar-like spots. Hosing the undersides knocks many bugs off, and insecticidal soap sprayed on the undersides works on young bugs. Plants in hot, full sun are usually hit hardest.
Other problems are less common. Notched leaf edges usually mean root weevil adults feeding at night. Root rot in soggy soil causes sudden wilting and decline. Leaf spots are mostly cosmetic and limited by removing old flowers.
Is Nova Zembla Poisonous to Pets
Rhododendrons contain grayanotoxin and are among the plants most often behind animal poisonings.35PubMed. Grayanotoxin poisoning in animals following the ingestion of Ericaceae plants Sheep and goats are poisoned most often, and fresh leaves equal to about 0.1% of their body weight can be toxic.36PubMed. Grayanotoxin poisoning in animals following the ingestion of Ericaceae plants Dogs, cats, rabbits, tortoises and pigs have been poisoned too.37PubMed. Grayanotoxin poisoning in animals following the ingestion of Ericaceae plants Signs in dogs and cats include vomiting, diarrhea, lethargy and loss of appetite.38PubMed Central. Grayanotoxin Poisoning: ‘Mad Honey Disease’ and Beyond
If a pet chews the plant, call your veterinarian or a pet poison hotline right away. Note how much was eaten and when. Don’t induce vomiting unless a vet tells you to. To prevent trouble, fence plants away from puppies and keep prunings out of reach of livestock.